Pluto

<h2 id="history">History</h2>
<h3>Discovery</h3>
Further information: <a href="/facts/Planets_beyond_Neptune/whs44sNI">Planets beyond Neptune</a>

In the 1840s, <a href="/facts/Urbain_Le_Verrier/mVzvmAPM">Urbain Le Verrier</a> used <a href="/facts/Classical_mechanics/ykG8upsN">Newtonian mechanics</a> to predict the position of the then-undiscovered planet <a href="/facts/Neptune/1eucwHIC">Neptune</a> after analyzing <a href="/facts/Perturbation_(astronomy)/og5hPpjQ">perturbations</a> in the orbit of <a href="/facts/Uranus/x7YwT1ar">Uranus</a>. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune.<a class="footnote-ref" id="fnref:2" href="#fn:2">2</a>
In 1906, <a href="/facts/Percival_Lowell/BBj0lfNg">Percival Lowell</a>—a wealthy <a href="/facts/Boston/exjfn6xf">Bostonian</a> who had founded <a href="/facts/Lowell_Observatory/BKJ1kgLx">Lowell Observatory</a> in <a href="/facts/Flagstaff%2c_Arizona/CYTyVnIW">Flagstaff, Arizona</a>, in 1894—started an extensive project in search of a possible ninth planet, which he termed "<a href="/facts/Planet_X/whs44sNI">Planet X</a>".<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a> By 1909, Lowell and <a href="/facts/William_Henry_Pickering/odcTMgtl">William H. Pickering</a> had suggested several possible celestial coordinates for such a planet.<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> Lowell and his observatory conducted his search, using mathematical calculations made by <a href="/facts/Elizabeth_Langdon_Williams/0jYB8hqn">Elizabeth Williams</a>, until his death in 1916, but to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a><a class="footnote-ref" id="fnref:6" href="#fn:6">6</a> There are fourteen other known <a href="/facts/Precovery/RbTUMt9F">precovery</a> observations, with the earliest made by the <a href="/facts/Yerkes_Observatory/kEpCtkwL">Yerkes Observatory</a> on August 20, 1909.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>

Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory over her husband's legacy, and the search for Planet X did not resume until 1929.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> <a href="/facts/Vesto_Melvin_Slipher/ewBeHwIC">Vesto Melvin Slipher</a>, the observatory director, gave the job of locating Planet X to 23-year-old <a href="/facts/Clyde_Tombaugh/GmgVmmk2">Clyde Tombaugh</a>, who had just arrived at the observatory after Slipher had been impressed by a sample of his astronomical drawings.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a>
Tombaugh's task was to systematically image the night sky in pairs of photographs, then examine each pair and determine whether any objects had shifted position. Using a <a href="/facts/Blink_comparator/0SdkeKPB">blink comparator</a>, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29. A lesser-quality photograph taken on January 21 helped confirm the movement.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the <a href="/facts/Harvard_College_Observatory/VaeaI4U6">Harvard College Observatory</a> on March 13, 1930.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>
One Plutonian year corresponds to 247.94 Earth years;<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> thus, in 2178, Pluto will complete its first orbit since its discovery.

<h3>Name and symbol</h3>
The name Pluto came from the Roman <a href="/facts/Pluto_(mythology)/o89QounF">god of the underworld</a>; and it is also an <a href="/facts/Epithet/uUQxrjCC">epithet</a> for <a href="/facts/Hades/L8jg3utR">Hades</a> (the Greek equivalent of Pluto).
Upon the announcement of the discovery, Lowell Observatory received over a thousand suggestions for names.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a> Three names topped the list: <a href="/facts/Minerva/q4NlHUbb">Minerva</a>, Pluto and <a href="/facts/Cronus/bWszmeSX">Cronus</a>. 'Minerva' was the Lowell staff's first choice<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> but was rejected because it had already been used for <a href="/facts/93_Minerva/qkJcmo4C">an asteroid</a>; Cronus was disfavored because it was promoted by an unpopular and egocentric astronomer, <a href="/facts/Thomas_Jefferson_Jackson_See/qYeHPamT">Thomas Jefferson Jackson See</a>. A vote was then taken and 'Pluto' was the unanimous choice. To make sure the name stuck, and that the planet would not suffer changes in its name as Uranus had, Lowell Observatory proposed the name to the <a href="/facts/American_Astronomical_Society/f5ohaItD">American Astronomical Society</a> and the <a href="/facts/Royal_Astronomical_Society/FY4DbvNW">Royal Astronomical Society</a>; both approved it unanimously.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>: 136 <a class="footnote-ref" id="fnref:16" href="#fn:16">16</a> The name was published on May 1, 1930.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a><a class="footnote-ref" id="fnref:18" href="#fn:18">18</a>
The name Pluto had received some 150 nominations among the letters and telegrams sent to Lowell. The first<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> had been from <a href="/facts/Venetia_Burney/NEs415nE">Venetia Burney</a> (1918–2009), an eleven-year-old schoolgirl in <a href="/facts/Oxford/kHvWDTXW">Oxford</a>, England, who was interested in <a href="/facts/Classical_mythology/143wLJoq">classical mythology</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a><a class="footnote-ref" id="fnref:21" href="#fn:21">21</a> She had suggested it to her grandfather <a href="/facts/Falconer_Madan/ev0eXeUn">Falconer Madan</a> when he read the news of Pluto's discovery to his family over breakfast; Madan passed the suggestion to astronomy professor <a href="/facts/Herbert_Hall_Turner/muGV0cGR">Herbert Hall Turner</a>, who cabled it to colleagues at Lowell on March 16, three days after the announcement.<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a><a class="footnote-ref" id="fnref:23" href="#fn:23">23</a>
The name 'Pluto' was mythologically appropriate: the god Pluto was one of six surviving children of <a href="/facts/Saturn_(mythology)/ur41xX4l">Saturn</a>, and the others had already all been chosen as names of major or minor planets (his brothers <a href="/facts/Jupiter_(mythology)/JPetM2m9">Jupiter</a> and <a href="/facts/Neptune_(mythology)/wc03adCP">Neptune</a>, and his sisters <a href="/facts/Ceres_(mythology)/z8aja7rQ">Ceres</a>, <a href="/facts/Juno_(mythology)/6PvC2hmh">Juno</a> and <a href="/facts/Vesta_(mythology)/r07yXKqc">Vesta</a>). Both the god and the planet inhabited "gloomy" regions, and the god was able to make himself invisible, as the planet had been for so long.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a> 
The choice was further helped by the fact that the first two letters of Pluto were the initials of Percival Lowell; indeed, 'Percival' had been one of the more popular suggestions for a name for the new planet.<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a><a class="footnote-ref" id="fnref:26" href="#fn:26">26</a>

<h3>Symbol</h3>
Once named, Pluto's <a href="/facts/Planetary_symbol/xevvbQvq">planetary symbol</a> ⟨⟩ was then created as a <a href="/facts/Monogram/7SMVEawE">monogram</a> of the letters "PL".<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a> This symbol is rarely used in astronomy anymore,<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a> though it is still common in astrology. However, the most common <a href="/facts/Astrological_symbol/qKVtrCIh">astrological symbol</a> for Pluto, occasionally used in astronomy as well, is an orb (possibly representing Pluto's invisibility cap) over Pluto's <a href="/facts/Bident/XlYVdsSQ">bident</a> ⟨⟩, which dates to the early 1930s.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a><a class="footnote-ref" id="fnref:30" href="#fn:30">30</a>
The name 'Pluto' was soon embraced by wider culture. In 1930, <a href="/facts/Walt_Disney/6a3CPUKT">Walt Disney</a> was apparently inspired by it when he introduced for <a href="/facts/Mickey_Mouse/XHA50EJP">Mickey Mouse</a> a canine companion named <a href="/facts/Pluto_(Disney)/3E3nHX5w">Pluto</a>, although <a href="/facts/Disney/R8kCbAVc">Disney</a> animator <a href="/facts/Ben_Sharpsteen/lFmzMhpc">Ben Sharpsteen</a> could not confirm why the name was given.<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a> In 1941, <a href="/facts/Glenn_T._Seaborg/w1a3mume">Glenn T. Seaborg</a> named the newly created <a href="/facts/Chemical_element/0jducgWD">element</a> <a href="/facts/Plutonium/4bUw9zH3">plutonium</a> after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following <a href="/facts/Uranium/n2sGWBzU">uranium</a>, which was named after Uranus, and <a href="/facts/Neptunium/pwp7Yvds">neptunium</a>, which was named after Neptune.<a class="footnote-ref" id="fnref:32" href="#fn:32">32</a>
Most languages use the name "Pluto" in various transliterations.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a> In Japanese, <a href="/facts/Houei_Nojiri/cbDwKclv">Houei Nojiri</a> suggested the <a href="/facts/Calque/A9M3UtAo">calque</a> Meiōsei (冥王星, "Star of the King (God) of the Underworld"), and this was borrowed into Chinese and Korean. Some <a href="/facts/Languages_of_India/6XDhJrTq">languages of India</a> use the name Pluto, but others, such as <a href="/facts/Hindi/n1QIEeCC">Hindi</a>, use the name of <a href="/facts/Yama/CgClJmer">Yama</a>, the God of Death in <a href="/facts/Hinduism/oxV9YvQh">Hinduism</a>.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a> <a href="/facts/Polynesian_languages/Q0Cm2uk6">Polynesian languages</a> also tend to use the indigenous god of the underworld, as in <a href="/facts/M%25C4%2581ori_language/pVhKfILB">Māori</a> <a href="/facts/Whiro/fL3uKlIu">Whiro</a>.<a class="footnote-ref" id="fnref:35" href="#fn:35">35</a>
Vietnamese does not follow the Chinese usage due to a phonological constraint: the <a href="/facts/Sino-Vietnamese_vocabulary/7hLYq6m0">Sino-Vietnamese</a> word 冥 minh "dark" is homophonous with 明 minh "bright". Instead, Vietnamese uses Yama, which is also a Buddhist deity, in the form of Sao Diêm Vương 星閻王 "Yama's Star", derived from Chinese 閻王 <a href="/facts/Yama_(Buddhism)/71ppqUc0">Yán Wáng / Yìhm Wòhng</a> "King Yama".<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a><a class="footnote-ref" id="fnref:37" href="#fn:37">37</a><a class="footnote-ref" id="fnref:38" href="#fn:38">38</a>

<h3>Planet X disproved</h3>
Once Pluto was found, its faintness and lack of a <a href="/facts/Angular_diameter/p2xv4vcQ">viewable disc</a> cast doubt on the idea that it was Lowell's <a href="/facts/Planets_beyond_Neptune/whs44sNI">Planet X</a>.<a class="footnote-ref" id="fnref:39" href="#fn:39">39</a> Estimates of Pluto's mass were revised downward throughout the 20th century.<a class="footnote-ref" id="fnref:40" href="#fn:40">40</a>

Mass estimates for Pluto<table><tbody><tr><th scope="col">Year</th><th scope="col">Mass</th><th scope="col">Estimate by</th></tr><tr><th scope="row">1915</th><td>7 Earths</td><td><a href="/facts/Percival_Lowell/BBj0lfNg">Lowell</a> (prediction for <a href="/facts/Planet_X/whs44sNI">Planet X</a>)<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a></td></tr><tr><th scope="row">1931</th><td>1 Earth</td><td><a href="/facts/Seth_Barnes_Nicholson/n3pM5Ua5">Nicholson</a> & <a href="/facts/Nicholas_U._Mayall/EAPJY6TD">Mayall</a><a class="footnote-ref" id="fnref:42" href="#fn:42">42</a><a class="footnote-ref" id="fnref:43" href="#fn:43">43</a><a class="footnote-ref" id="fnref:44" href="#fn:44">44</a></td></tr><tr><th scope="row">1948</th><td>0.1 (1/10) Earth</td><td><a href="/facts/Gerard_Kuiper/2VghxPem">Kuiper</a><a class="footnote-ref" id="fnref:45" href="#fn:45">45</a></td></tr><tr><th scope="row">1976</th><td>0.01 (1/100) Earth</td><td><a href="/facts/Dale_Cruikshank/7RL1TaB6">Cruikshank</a>, Pilcher, & <a href="/facts/David_Morrison_(astrophysicist)/3JYwK2Nd">Morrison</a><a class="footnote-ref" id="fnref:46" href="#fn:46">46</a></td></tr><tr><th scope="row">1978</th><td>0.0015 (1/650) Earth</td><td><a href="/facts/James_W._Christy/JEBftHFw">Christy</a> & <a href="/facts/Robert_Sutton_Harrington/U66rpyke">Harrington</a><a class="footnote-ref" id="fnref:47" href="#fn:47">47</a></td></tr><tr><th scope="row">2006</th><td>0.00218 (1/459) Earth</td><td><a href="/facts/Marc_W._Buie/v9s6wooo">Buie</a> et al.<a class="footnote-ref" id="fnref:48" href="#fn:48">48</a></td></tr></tbody></table>
Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931, Pluto was calculated to be roughly the mass of <a href="/facts/Earth/HLLmDfj0">Earth</a>, with further calculations in 1948 bringing the mass down to roughly that of <a href="/facts/Mars/AQGtVvz0">Mars</a>.<a class="footnote-ref" id="fnref:49" href="#fn:49">49</a><a class="footnote-ref" id="fnref:50" href="#fn:50">50</a> In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the <a href="/facts/University_of_Hawai%25CA%25BBi/6nzkEffB">University of Hawaiʻi</a> calculated Pluto's <a href="/facts/Albedo/TJDRdKEB">albedo</a> for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of Earth.<a class="footnote-ref" id="fnref:51" href="#fn:51">51</a> (Pluto's albedo is 1.4–1.9 times that of Earth.<a class="footnote-ref" id="fnref:52" href="#fn:52">52</a>)
In 1978, the discovery of Pluto's moon <a href="/facts/Charon_(moon)/elazjsMp">Charon</a> allowed the measurement of Pluto's mass for the first time: roughly 0.2% that of Earth, and far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by <a href="/facts/Robert_Sutton_Harrington/U66rpyke">Robert Sutton Harrington</a>,<a class="footnote-ref" id="fnref:53" href="#fn:53">53</a> failed. In 1992, <a href="/facts/E._Myles_Standish/YCuDSozs">Myles Standish</a> used data from <a href="/facts/Voyager_2/9FAEaHUr">Voyager 2</a>'s flyby of Neptune in 1989, which had revised the estimates of Neptune's mass downward by 0.5%—an amount comparable to the mass of Mars—to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.<a class="footnote-ref" id="fnref:54" href="#fn:54">54</a> As of 2000 the majority of scientists agree that Planet X, as Lowell defined it, does not exist.<a class="footnote-ref" id="fnref:55" href="#fn:55">55</a> Lowell had made a prediction of Planet X's orbit and position in 1915 that was fairly close to Pluto's actual orbit and its position at that time; <a href="/facts/Ernest_W._Brown/1q20Jtgz">Ernest W. Brown</a> concluded soon after Pluto's discovery that this was a coincidence.<a class="footnote-ref" id="fnref:56" href="#fn:56">56</a>

<h3>Classification</h3>
Further information: <a href="/facts/Definition_of_planet/Nym2zPWF">Definition of planet</a>
From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto is part of a population of objects called the <a href="/facts/Kuiper_belt/IvLzYp5P">Kuiper belt</a>. This made its official status as a planet controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population. Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>. In February 2000 the <a href="/facts/Hayden_Planetarium/xvlAuKMC">Hayden Planetarium</a> in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later.<a class="footnote-ref" id="fnref:57" href="#fn:57">57</a>
<a href="/facts/Ceres_(dwarf_planet)/yIVEMYtG">Ceres</a>, <a href="/facts/2_Pallas/onjUgYtA">Pallas</a>, <a href="/facts/3_Juno/5hQkurE0">Juno</a> and <a href="/facts/4_Vesta/xnQ8acw1">Vesta</a> lost their planet status among most astronomers after the discovery of many other <a href="/facts/Asteroid/g7Ulr1VB">asteroids</a> in the 1840s. On the other hand, planetary geologists often regarded Ceres, and less often Pallas and Vesta, as being different from smaller asteroids because they were large enough to have undergone geological evolution.<a class="footnote-ref" id="fnref:58" href="#fn:58">58</a> Although the first Kuiper belt objects discovered were quite small, objects increasingly closer in size to Pluto were soon discovered, some large enough (like Pluto itself) to satisfy geological but not dynamical ideas of planethood.<a class="footnote-ref" id="fnref:59" href="#fn:59">59</a> In 1998, <a href="/facts/Brian_G._Marsden/uW2IWGIC">Brian G. Marsden</a> of <a href="/facts/Harvard_University/Lhy9L1cx">Harvard University</a>'s <a href="/facts/Minor_Planet_Center/v8T7sdK4">Minor Planet Center</a> suggested that Pluto be given the <a href="/facts/Minor_planet_number/le4Ro9Xj">minor planet number</a> 10000 while still retaining its official position as a planet.<a class="footnote-ref" id="fnref:60" href="#fn:60">60</a><a class="footnote-ref" id="fnref:61" href="#fn:61">61</a> The prospect of Pluto's "demotion" created a public outcry, and in response the <a href="/facts/International_Astronomical_Union/GMFPImXo">International Astronomical Union</a> clarified that it was not at that time proposing to remove Pluto from the planet list.<a class="footnote-ref" id="fnref:62" href="#fn:62">62</a><a class="footnote-ref" id="fnref:63" href="#fn:63">63</a>
On July 29, 2005, the debate became unavoidable when astronomers at <a href="/facts/Caltech/3FPcp7BL">Caltech</a> announced the discovery of a new <a href="/facts/Trans-Neptunian_object/oF5vsbz9">trans-Neptunian object</a>, <a href="/facts/Eris_(dwarf_planet)/vuaR65F7">Eris</a>, which was substantially more massive than Pluto and the most massive object discovered in the Solar System since <a href="/facts/Triton_(moon)/mxxRUygc">Triton</a> in 1846. Its discoverers and the press initially called it the <a href="/facts/Tenth_planet/whs44sNI">tenth planet</a>, although there was no official consensus at the time on whether to call it a planet.<a class="footnote-ref" id="fnref:64" href="#fn:64">64</a> Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.<a class="footnote-ref" id="fnref:65" href="#fn:65">65</a>

<h4>IAU classification</h4>
Main article: <a href="/facts/IAU_definition_of_planet/KeNVLMMt">IAU definition of planet</a>
The debate came to a head in August 2006 during the triennial meeting of the <a href="/facts/International_Astronomical_Union/GMFPImXo">IAU</a>, when <a href="/facts/Uruguay/X6V0apCb">Uruguayan</a> astronomers <a href="/facts/Julio_%25C3%2581ngel_Fern%25C3%25A1ndez/5AntehN8">Julio Ángel Fernández</a> and <a href="/facts/Gonzalo_Tancredi/Vu51nY79">Gonzalo Tancredi</a> first proposed the new definition for the term "planet".<a class="footnote-ref" id="fnref:66" href="#fn:66">66</a><a class="footnote-ref" id="fnref:67" href="#fn:67">67</a> According to their proposal, there are three conditions for an object in the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a> to be considered a planet:

<ul><li>The object must be in orbit around the <a href="/facts/Sun/CadPCVHh">Sun</a>.</li>
<li>The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by <a href="/facts/Hydrostatic_equilibrium/e75gVkh2">hydrostatic equilibrium</a>.</li>
<li>It must have <a href="/facts/Cleared_the_neighborhood/Atswo2Rv">cleared the neighborhood</a> around its orbit.<a class="footnote-ref" id="fnref:68" href="#fn:68">68</a><a class="footnote-ref" id="fnref:69" href="#fn:69">69</a></li></ul>
Pluto fails to meet the third condition.<a class="footnote-ref" id="fnref:70" href="#fn:70">70</a> Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon).<a class="footnote-ref" id="fnref:71" href="#fn:71">71</a><a class="footnote-ref" id="fnref:72" href="#fn:72">72</a> The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called <a href="/facts/Dwarf_planet/IkQoCdwD">dwarf planets</a>. In September 2006, the IAU included Pluto, and Eris and its moon <a href="/facts/Dysnomia_(moon)/g69gHfDT">Dysnomia</a>, in their <a href="/facts/Minor_Planet_Catalogue/hzpMh7gf">Minor Planet Catalogue</a>, giving them the official <a href="/facts/Minor-planet_designation/le4Ro9Xj">minor-planet designations</a> "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia".<a class="footnote-ref" id="fnref:73" href="#fn:73">73</a> Had Pluto been included upon its discovery in 1930, it would have likely been designated 1164, following <a href="/facts/1163_Saga/AALXcadG">1163 Saga</a>, which was discovered a month earlier.<a class="footnote-ref" id="fnref:74" href="#fn:74">74</a>
There has been some resistance within the astronomical community toward the reclassification, and in particular planetary scientists often continue to reject it, considering Pluto, Charon, and Eris to be planets for the same reason they do so for Ceres. In effect, this amounts to accepting only the second clause of the IAU definition.<a class="footnote-ref" id="fnref:75" href="#fn:75">75</a><a class="footnote-ref" id="fnref:76" href="#fn:76">76</a><a class="footnote-ref" id="fnref:77" href="#fn:77">77</a> <a href="/facts/Alan_Stern/kCKGyeIt">Alan Stern</a>, principal investigator with <a href="/facts/NASA/TIMCV0yV">NASA</a>'s New Horizons mission to Pluto, derided the IAU resolution.<a class="footnote-ref" id="fnref:78" href="#fn:78">78</a><a class="footnote-ref" id="fnref:79" href="#fn:79">79</a><a class="footnote-ref" id="fnref:80" href="#fn:80">80</a> He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.<a class="footnote-ref" id="fnref:81" href="#fn:81">81</a> <a href="/facts/Marc_W._Buie/v9s6wooo">Marc W. Buie</a>, then at the Lowell Observatory, petitioned against the definition.<a class="footnote-ref" id="fnref:82" href="#fn:82">82</a> Others have supported the IAU, for example <a href="/facts/Michael_E._Brown/Y5Jn22Tl">Mike Brown</a>, the astronomer who discovered Eris.<a class="footnote-ref" id="fnref:83" href="#fn:83">83</a>
Public reception to the IAU decision was mixed. A resolution introduced in the <a href="/facts/California_State_Assembly/Qz1n4daV">California State Assembly</a> facetiously called the IAU decision a "scientific heresy".<a class="footnote-ref" id="fnref:84" href="#fn:84">84</a> The <a href="/facts/New_Mexico_House_of_Representatives/tfRG9f9z">New Mexico House of Representatives</a> passed a resolution in honor of Clyde Tombaugh, the discoverer of Pluto and a longtime resident of that state, that declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007, was Pluto Planet Day.<a class="footnote-ref" id="fnref:85" href="#fn:85">85</a><a class="footnote-ref" id="fnref:86" href="#fn:86">86</a> The <a href="/facts/Illinois_Senate/Bob2rjkj">Illinois Senate</a> passed a similar resolution in 2009 on the basis that Tombaugh was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU."<a class="footnote-ref" id="fnref:87" href="#fn:87">87</a> Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.<a class="footnote-ref" id="fnref:88" href="#fn:88">88</a> In 2006, in its 17th annual words-of-the-year vote, the <a href="/facts/American_Dialect_Society/0FmXAlN0">American Dialect Society</a> voted <a href="/facts/Plutoed/KeNVLMMt">plutoed</a> as the word of the year. To "pluto" is to "demote or devalue someone or something".<a class="footnote-ref" id="fnref:89" href="#fn:89">89</a> In April 2024, <a href="/facts/Arizona/6ayM3swJ">Arizona</a> (where Pluto was first discovered in 1930) passed a law naming Pluto as the official state planet.<a class="footnote-ref" id="fnref:90" href="#fn:90">90</a>
Researchers on both sides of the debate gathered in August 2008, at the Johns Hopkins University <a href="/facts/Applied_Physics_Laboratory/PN4H87V9">Applied Physics Laboratory</a> for a conference that included back-to-back talks on the IAU definition of a planet.<a class="footnote-ref" id="fnref:91" href="#fn:91">91</a> Entitled "The Great Planet Debate",<a class="footnote-ref" id="fnref:92" href="#fn:92">92</a> the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet.<a class="footnote-ref" id="fnref:93" href="#fn:93">93</a> In June 2008, the IAU had announced in a press release that the term "<a href="/facts/Plutoid/IkQoCdwD">plutoid</a>" would henceforth be used to refer to Pluto and other planetary-mass objects that have an orbital <a href="/facts/Semi-major_axis/l7JES2wo">semi-major axis</a> greater than that of Neptune, though the term has not seen significant use.<a class="footnote-ref" id="fnref:94" href="#fn:94">94</a><a class="footnote-ref" id="fnref:95" href="#fn:95">95</a><a class="footnote-ref" id="fnref:96" href="#fn:96">96</a>

<h2 id="orbit">Orbit</h2>

Pluto's orbital period is about 248 years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference <a href="/facts/Plane_(mathematics)/tAUtRFcn">plane</a> called the <a href="/facts/Ecliptic/cJxXDDDW">ecliptic</a>. In contrast, Pluto's orbit is moderately <a href="/facts/Orbital_inclination/TQCFWyfd">inclined</a> relative to the ecliptic (over 17°) and moderately <a href="/facts/Orbital_eccentricity/WGlB4NBn">eccentric</a> (elliptical). This eccentricity means a small region of Pluto's orbit lies closer to the Sun than Neptune's. The Pluto–Charon barycenter came to <a href="/facts/Apsis/VaL6ow9w">perihelion</a> on September 5, 1989,<a class="footnote-ref" id="fnref:97" href="#fn:97">97</a><a class="footnote-ref" id="fnref:98" href="#fn:98">98</a> and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999.<a class="footnote-ref" id="fnref:99" href="#fn:99">99</a>
Although the 3:2 resonance with Neptune (see below) is maintained, Pluto's inclination and eccentricity behave in a <a href="/facts/Chaos_theory/LTC17vsF">chaotic</a> manner. Computer simulations can be used to predict its position for several million years (both <a href="/facts/Time_reversibility/Jv4CFpSy">forward and backward</a> in time), but after intervals much longer than the <a href="/facts/Lyapunov_time/7DzvRMbS">Lyapunov time</a> of 10–20 million years, calculations become unreliable: Pluto is sensitive to immeasurably small details of the Solar System, hard-to-predict factors that will gradually change Pluto's position in its orbit.<a class="footnote-ref" id="fnref:100" href="#fn:100">100</a><a class="footnote-ref" id="fnref:101" href="#fn:101">101</a>
The <a href="/facts/Semi-major_axis/l7JES2wo">semi-major axis</a> of Pluto's orbit varies between about 39.3 and 39.6 <a href="/facts/Astronomical_unit/gjsbu5xU">AU</a> with a period of about 19,951 years, corresponding to an orbital period varying between 246 and 249 years. The semi-major axis and period are presently getting longer.<a class="footnote-ref" id="fnref:102" href="#fn:102">102</a>

<h3>Relationship with Neptune </h3>

Despite Pluto's orbit appearing to cross that of Neptune when viewed from north or south of the Solar System, the two objects' orbits do not intersect. When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from such a position, it is also the farthest north of Neptune's path. Pluto's orbit passes about 8 <a href="/facts/Astronomical_unit/gjsbu5xU">AU</a> north of that of Neptune, preventing a collision.<a class="footnote-ref" id="fnref:103" href="#fn:103">103</a><a class="footnote-ref" id="fnref:104" href="#fn:104">104</a><a class="footnote-ref" id="fnref:105" href="#fn:105">105</a><a class="footnote-ref" id="fnref:106" href="#fn:106">106</a>
This alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) could alter Pluto's orbit (such as its <a href="/facts/Apsidal_precession/YUWDqXDp">orbital precession</a>) over millions of years so that a collision could happen. However, Pluto is also protected by its 2:3 <a href="/facts/Orbital_resonance/eQLMfBWu">orbital resonance</a> with <a href="/facts/Neptune/1eucwHIC">Neptune</a>: for every two orbits that Pluto makes around the Sun, Neptune makes three, in a frame of reference that rotates at the rate that Pluto's perihelion precesses (about 0.97×10−4 degrees per year<a class="footnote-ref" id="fnref:107" href="#fn:107">107</a>). Each cycle lasts about 495 years. (There are many other objects in this same resonance, called <a href="/facts/Plutino/ptGoJLgF">plutinos</a>.) At present, in each 495-year cycle, the first time Pluto is at <a href="/facts/Perihelion/VaL6ow9w">perihelion</a> (such as in 1989), Neptune is 57° ahead of Pluto. By Pluto's second passage through perihelion, Neptune will have completed a further one and a half of its own orbits, and will be 123° behind Pluto.<a class="footnote-ref" id="fnref:108" href="#fn:108">108</a> Pluto and Neptune's minimum separation is over 17 AU, which is greater than Pluto's minimum separation from Uranus (11 AU).<a class="footnote-ref" id="fnref:109" href="#fn:109">109</a> The minimum separation between Pluto and Neptune actually occurs near the time of Pluto's aphelion.<a class="footnote-ref" id="fnref:110" href="#fn:110">110</a>

The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years.<a class="footnote-ref" id="fnref:111" href="#fn:111">111</a> This prevents their orbits from changing relative to one another, so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide.<a class="footnote-ref" id="fnref:112" href="#fn:112">112</a> When Pluto's period is slightly different from 3/2 of Neptune's, the pattern of its distance from Neptune will drift. Near perihelion Pluto moves interior to Neptune's orbit and is therefore moving faster, so during the first of two orbits in the 495-year cycle, it is approaching Neptune from behind. At present it remains between 50° and 65° behind Neptune for 100 years (e.g. 1937–2036).<a class="footnote-ref" id="fnref:113" href="#fn:113">113</a> The gravitational pull between the two causes <a href="/facts/Angular_momentum/7TuSVFxq">angular momentum</a> to be transferred to Pluto. This situation moves Pluto into a slightly larger orbit, where it has a slightly longer period, according to <a href="/facts/Kepler%2527s_third_law/jYQoIBjM">Kepler's third law</a>. After several such repetitions, Pluto is sufficiently delayed that at the second perihelion of each cycle it will not be far ahead of Neptune coming behind it, and Neptune will start to decrease Pluto's period again. The whole cycle takes about 20,000 years to complete.<a class="footnote-ref" id="fnref:114" href="#fn:114">114</a><a class="footnote-ref" id="fnref:115" href="#fn:115">115</a><a class="footnote-ref" id="fnref:116" href="#fn:116">116</a>

<h4>Other factors</h4>
Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto and Neptune does not change.<a class="footnote-ref" id="fnref:117" href="#fn:117">117</a><a class="footnote-ref" id="fnref:118" href="#fn:118">118</a> There are several other resonances and interactions that enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 2:3 mean-motion resonance).
First, Pluto's <a href="/facts/Argument_of_perihelion/cthncYDb">argument of perihelion</a>, the angle between the point where it crosses the ecliptic (or the <a href="/facts/Invariant_plane/SSoYrjeR">invariant plane</a>) and the point where it is closest to the Sun, <a href="/facts/Libration/LqaRthjF">librates</a> around 90°.<a class="footnote-ref" id="fnref:119" href="#fn:119">119</a> This means that when Pluto is closest to the Sun, it is at its farthest north of the plane of the Solar System, preventing encounters with Neptune. This is a consequence of the <a href="/facts/Kozai_mechanism/KujDlqLG">Kozai mechanism</a>,<a class="footnote-ref" id="fnref:120" href="#fn:120">120</a> which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case, Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (90°–38°). The closest such angular separation occurs every 10,000 years.<a class="footnote-ref" id="fnref:121" href="#fn:121">121</a>
Second, the longitudes of ascending nodes of the two bodies—the points where they cross the <a href="/facts/Invariant_plane/SSoYrjeR">invariant plane</a>—are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and hence it comes closest to the Sun when it is furthest north of Neptune's orbit. This is known as the 1:1 superresonance. All the <a href="/facts/Jovian_planets/PjSfFuUF">Jovian planets</a> (Jupiter, Saturn, Uranus, and Neptune) play a role in the creation of the superresonance.<a class="footnote-ref" id="fnref:122" href="#fn:122">122</a>

<h2 id="rotation">Rotation</h2>

Pluto's <a href="/facts/Rotation_period/vHcCt6cu">rotation period</a>, its day, is equal to 6.387 <a href="/facts/Earth/HLLmDfj0">Earth</a> days.<a class="footnote-ref" id="fnref:123" href="#fn:123">123</a><a class="footnote-ref" id="fnref:124" href="#fn:124">124</a> Like <a href="/facts/Uranus/x7YwT1ar">Uranus</a> and <a href="/facts/2_Pallas/onjUgYtA">2 Pallas</a>, Pluto rotates on its "side" in its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its <a href="/facts/Solstice/3vEDtAhg">solstices</a>, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness.<a class="footnote-ref" id="fnref:125" href="#fn:125">125</a> The reason for this unusual orientation has been debated. Research from the <a href="/facts/University_of_Arizona/ehesIpKx">University of Arizona</a> has suggested that it may be due to the way that a body's spin will always adjust to minimize energy. This could mean a body reorienting itself to put extraneous mass near the equator and regions lacking mass tend towards the poles. This is called <a href="/facts/Polar_wander/FjgbAtsw">polar wander</a>.<a class="footnote-ref" id="fnref:126" href="#fn:126">126</a> According to a paper released from the University of Arizona, this could be caused by masses of frozen <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> building up in shadowed areas of the dwarf planet. These masses would cause the body to reorient itself, leading to its unusual axial tilt of 120°. The buildup of nitrogen is due to Pluto's vast distance from the Sun. At the equator, temperatures can drop to −240 °C (−400.0 °F; 33.1 K), causing nitrogen to freeze as water would freeze on Earth. The same polar wandering effect seen on Pluto would be observed on Earth were the <a href="/facts/Antarctic_ice_sheet/wU3pFebh">Antarctic ice sheet</a> several times larger.<a class="footnote-ref" id="fnref:127" href="#fn:127">127</a>

<h2 id="geology">Geology</h2>
Main articles: <a href="/facts/Geology_of_Pluto/aFoxQplI">Geology of Pluto</a> and <a href="/facts/Geography_of_Pluto/PQstrkU3">Geography of Pluto</a>
<h3>Surface</h3>

The plains on Pluto's surface are composed of more than 98 percent <a href="/facts/Nitrogen_ice/6KF2HYIx">nitrogen ice</a>, with traces of methane and <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a>.<a class="footnote-ref" id="fnref:128" href="#fn:128">128</a> <a href="/facts/Nitrogen/Nf1QpGDz">Nitrogen</a> and carbon monoxide are most abundant on the anti-Charon face of Pluto (around 180° longitude, where <a href="/facts/Tombaugh_Regio/GwHsIdHM">Tombaugh Regio</a>'s western lobe, <a href="/facts/Sputnik_Planitia/b2hHX64U">Sputnik Planitia</a>, is located), whereas methane is most abundant near 300° east.<a class="footnote-ref" id="fnref:129" href="#fn:129">129</a> The mountains are made of water ice.<a class="footnote-ref" id="fnref:130" href="#fn:130">130</a> Pluto's surface is quite varied, with large differences in both brightness and color.<a class="footnote-ref" id="fnref:131" href="#fn:131">131</a> Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as <a href="/facts/Saturn/D54LcGJ9">Saturn</a>'s moon <a href="/facts/Iapetus_(moon)/W9PCus81">Iapetus</a>.<a class="footnote-ref" id="fnref:132" href="#fn:132">132</a> The color varies from charcoal black, to dark orange and white.<a class="footnote-ref" id="fnref:133" href="#fn:133">133</a> Pluto's color is more similar to that of <a href="/facts/Io_(moon)/p3Uc8FmK">Io</a> with slightly more orange and significantly less red than <a href="/facts/Mars/AQGtVvz0">Mars</a>.<a class="footnote-ref" id="fnref:134" href="#fn:134">134</a> <a href="/facts/Geography_of_Pluto/PQstrkU3">Notable geographical features</a> include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), <a href="/facts/Belton_Regio/H7Fbpseq">Belton Regio</a>,<a class="footnote-ref" id="fnref:135" href="#fn:135">135</a> or the "Whale" (a large dark area on the trailing hemisphere), and the "<a href="/facts/Brass_Knuckles_(Pluto)/PQstrkU3">Brass Knuckles</a>" (a series of equatorial dark areas on the leading hemisphere).
Sputnik Planitia, the western lobe of the "Heart", is a 1,000 km-wide basin of frozen nitrogen and carbon monoxide ices, divided into polygonal cells, which are interpreted as <a href="/facts/Convection_cell/4QTnSGww">convection cells</a> that carry floating blocks of water ice crust and <a href="/facts/Sublimation_(phase_transition)/KDoKokMn">sublimation</a> pits towards their margins;<a class="footnote-ref" id="fnref:136" href="#fn:136">136</a><a class="footnote-ref" id="fnref:137" href="#fn:137">137</a><a class="footnote-ref" id="fnref:138" href="#fn:138">138</a> there are obvious signs of glacial flows both into and out of the basin.<a class="footnote-ref" id="fnref:139" href="#fn:139">139</a><a class="footnote-ref" id="fnref:140" href="#fn:140">140</a> It has no craters that were visible to New Horizons, indicating that its surface is less than 10 million years old.<a class="footnote-ref" id="fnref:141" href="#fn:141">141</a> Latest studies have shown that the surface has an age of 180000+90000−40000 years.<a class="footnote-ref" id="fnref:142" href="#fn:142">142</a>
The New Horizons science team summarized initial findings as "Pluto displays a surprisingly wide variety of geological landforms, including those resulting from <a href="/facts/Glaciological/dSElvpIj">glaciological</a> and surface–atmosphere interactions as well as impact, <a href="/facts/Plate_tectonics/RtRG5lY4">tectonic</a>, possible <a href="/facts/Cryovolcanic/YyWhVCJ3">cryovolcanic</a>, and <a href="/facts/Mass_wasting/CKURKaMQ">mass-wasting</a> processes."<a class="footnote-ref" id="fnref:143" href="#fn:143">143</a>
In Western parts of Sputnik Planitia there are fields of <a href="/facts/Transverse_dunes/Hq26h9hh">transverse dunes</a> formed by the winds blowing from the center of Sputnik Planitia in the direction of surrounding mountains. The dune wavelengths are in the range of 0.4–1 km and likely consist of methane particles 200–300 μm in size.<a class="footnote-ref" id="fnref:144" href="#fn:144">144</a>

<h3>Internal structure</h3>
"Life on Pluto" redirects here. For fiction about aliens from Pluto, see <a href="/facts/Life_on_Pluto_in_fiction/XhIb7IX0">Life on Pluto in fiction</a>.

Pluto's density is 1.853±0.004 g/cm3.<a class="footnote-ref" id="fnref:145" href="#fn:145">145</a> Because the decay of radioactive elements would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense <a href="/facts/Core_(geology)/LyQ7DbAl">core</a> surrounded by a <a href="/facts/Mantle_(geology)/RpDdtjCr">mantle</a> of water ice. The pre–New Horizons estimate for the diameter of the core is 1700 km, 70% of Pluto's diameter.<a class="footnote-ref" id="fnref:146" href="#fn:146">146</a> 
It is possible that such heating continues, creating a <a href="/facts/Subsurface_ocean/8W8lYgjD">subsurface ocean</a> of liquid water 100 to 180 km thick at the core–mantle boundary.<a class="footnote-ref" id="fnref:147" href="#fn:147">147</a><a class="footnote-ref" id="fnref:148" href="#fn:148">148</a><a class="footnote-ref" id="fnref:149" href="#fn:149">149</a> In September 2016, scientists at <a href="/facts/Brown_University/o9MRu4jx">Brown University</a> simulated the impact thought to have formed <a href="/facts/Sputnik_Planitia/b2hHX64U">Sputnik Planitia</a>, and showed that it might have been the result of liquid water upweling from below after the collision, implying the existence of a subsurface ocean at least 100 km deep.<a class="footnote-ref" id="fnref:150" href="#fn:150">150</a> In June 2020, astronomers reported evidence that Pluto may have had a <a href="/facts/Extraterrestrial_liquid_water/LYPJSJYj">subsurface ocean</a>, and consequently may have been <a href="/facts/Planetary_habitability/n8A0ehR8">habitable</a>, when it was first formed.<a class="footnote-ref" id="fnref:151" href="#fn:151">151</a><a class="footnote-ref" id="fnref:152" href="#fn:152">152</a> In March 2022, a team of researchers proposed that the mountains <a href="/facts/Wright_Mons/HeXMRNEK">Wright Mons</a> and <a href="/facts/Piccard_Mons/HeXMRNEK">Piccard Mons</a> are actually a merger of many smaller cryovolcanic domes, suggesting a source of heat on the body at levels previously thought not possible.<a class="footnote-ref" id="fnref:153" href="#fn:153">153</a>

<h2 id="mass-and-size">Mass and size</h2>
Pluto's diameter is 2,376.6±3.2 km<a class="footnote-ref" id="fnref:154" href="#fn:154">154</a> and its mass is (1.303±0.003)×1022 kg, 17.7% that of the <a href="/facts/Moon/zKHqEMPb">Moon</a> (0.22% that of Earth).<a class="footnote-ref" id="fnref:155" href="#fn:155">155</a> Its <a href="/facts/Surface_area/WNowzFCw">surface area</a> is 1.774443×107 km2, or just slightly bigger than Russia or <a href="/facts/Antarctica/bEM5Lem5">Antarctica</a> (particularly including the <a href="/facts/Antarctic_sea_ice/YWp6Bunv">Antarctic sea ice</a> during winter). Its <a href="/facts/Surface_gravity/Q5QVbA7e">surface gravity</a> is 0.063 g (compared to 1 g for Earth and 0.17 g for the Moon).<a class="footnote-ref" id="fnref:156" href="#fn:156">156</a> This gives Pluto an escape velocity of 4,363.2 km per hour / 2,711.167 miles per hour (as compared to Earth's 40,270 km per hour / 25,020 miles per hour). Pluto is more than twice the diameter and a dozen times the mass of <a href="/facts/Ceres_(dwarf_planet)/yIVEMYtG">Ceres</a>, the largest object in the <a href="/facts/Asteroid_belt/YHAq2Eh3">asteroid belt</a>. It is less massive than the dwarf planet <a href="/facts/Eris_(dwarf_planet)/vuaR65F7">Eris</a>, a <a href="/facts/Trans-Neptunian_object/oF5vsbz9">trans-Neptunian object</a> discovered in 2005, though Pluto has a larger diameter of 2,376.6 km<a class="footnote-ref" id="fnref:157" href="#fn:157">157</a> compared to Eris's approximate diameter of 2,326 km.<a class="footnote-ref" id="fnref:158" href="#fn:158">158</a>

With less than 0.2 lunar masses, Pluto is much less massive than the <a href="/facts/Terrestrial_planet/jpMC1ULt">terrestrial planets</a>, and also less massive than seven <a href="/facts/Moons/AE4KQKbq">moons</a>: <a href="/facts/Ganymede_(moon)/AmKsF10c">Ganymede</a>, <a href="/facts/Titan_(moon)/lg0l9PCH">Titan</a>, <a href="/facts/Callisto_(moon)/et0eIutu">Callisto</a>, <a href="/facts/Io_(moon)/p3Uc8FmK">Io</a>, the <a href="/facts/Moon/zKHqEMPb">Moon</a>, <a href="/facts/Europa_(moon)/2A9nleK3">Europa</a>, and <a href="/facts/Triton_(moon)/mxxRUygc">Triton</a>. The mass is much less than thought before Charon was discovered.<a class="footnote-ref" id="fnref:159" href="#fn:159">159</a>
The discovery of Pluto's satellite <a href="/facts/Charon_(moon)/elazjsMp">Charon</a> in 1978 enabled a determination of the mass of the Pluto–Charon system by application of <a href="/facts/Kepler%2527s_laws_of_planetary_motion/jYQoIBjM">Newton's formulation of Kepler's third law</a>. Observations of Pluto in <a href="/facts/Occultation/a9Q1KTPe">occultation</a> with Charon allowed scientists to establish Pluto's diameter more accurately, whereas the invention of <a href="/facts/Adaptive_optics/D1vWeHK0">adaptive optics</a> allowed them to determine its shape more accurately.<a class="footnote-ref" id="fnref:160" href="#fn:160">160</a>
Determinations of Pluto's size have been complicated by its atmosphere<a class="footnote-ref" id="fnref:161" href="#fn:161">161</a> and <a href="/facts/Hydrocarbon/Dcg95Ju5">hydrocarbon</a> haze.<a class="footnote-ref" id="fnref:162" href="#fn:162">162</a> In March 2014, Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with a Plutonian diameter greater than 2,360 km, with a "best guess" of 2,368 km.<a class="footnote-ref" id="fnref:163" href="#fn:163">163</a> On July 13, 2015, images from NASA's New Horizons mission <a href="/facts/Long_Range_Reconnaissance_Imager/gnhFt2op">Long Range Reconnaissance Imager</a> (LORRI), along with data from the other instruments, determined Pluto's diameter to be 2,370 km (1,473 mi),<a class="footnote-ref" id="fnref:164" href="#fn:164">164</a><a class="footnote-ref" id="fnref:165" href="#fn:165">165</a> which was later revised to be 2,372 km (1,474 mi) on July 24,<a class="footnote-ref" id="fnref:166" href="#fn:166">166</a> and later to 2374±8 km.<a class="footnote-ref" id="fnref:167" href="#fn:167">167</a> Using <a href="/facts/Radio_occultation/0RFdxflu">radio occultation</a> data from the New Horizons Radio Science Experiment (REX), the diameter was found to be 2,376.6±3.2 km.<a class="footnote-ref" id="fnref:168" href="#fn:168">168</a>

<h2 id="atmosphere">Atmosphere</h2>
Main article: <a href="/facts/Atmosphere_of_Pluto/sBJNJy5x">Atmosphere of Pluto</a>
Pluto has a tenuous <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a> consisting of <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> (N2), <a href="/facts/Methane/kBHneYnh">methane</a> (CH4), and carbon monoxide (CO), which are in <a href="/facts/Equilibrium_vapor_pressure/whrEr96T">equilibrium with their ices</a> on Pluto's surface.<a class="footnote-ref" id="fnref:169" href="#fn:169">169</a><a class="footnote-ref" id="fnref:170" href="#fn:170">170</a> According to the measurements by New Horizons, the surface pressure is about 1 <a href="/facts/Pascal_(unit)/Am1To2Qm">Pa</a> (10 <a href="/facts/%25CE%259Cbar/wwFWTTkT">μbar</a>),<a class="footnote-ref" id="fnref:171" href="#fn:171">171</a> roughly one million to 100,000 times less than Earth's atmospheric pressure. It was initially thought that, as Pluto moves away from the Sun, its atmosphere should gradually freeze onto the surface; studies of New Horizons data and ground-based occultations show that Pluto's atmospheric density increases, and that it likely remains gaseous throughout Pluto's orbit.<a class="footnote-ref" id="fnref:172" href="#fn:172">172</a><a class="footnote-ref" id="fnref:173" href="#fn:173">173</a> New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected.<a class="footnote-ref" id="fnref:174" href="#fn:174">174</a> Alan Stern has contended that even a small increase in Pluto's surface temperature can lead to exponential increases in Pluto's atmospheric density; from 18 hPa to as much as 280 hPa (three times that of Mars to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid.<a class="footnote-ref" id="fnref:175" href="#fn:175">175</a> Just like sweat cools the body as it evaporates from the skin, the <a href="/facts/Sublimation_(phase_transition)/KDoKokMn">sublimation</a> of Pluto's atmosphere cools its surface.<a class="footnote-ref" id="fnref:176" href="#fn:176">176</a> Pluto has no or almost no <a href="/facts/Troposphere/JPDQq8cL">troposphere</a>; observations by New Horizons suggest only a thin tropospheric <a href="/facts/Planetary_boundary_layer/7JavWIVf">boundary layer</a>. Its thickness in the place of measurement was 4 km, and the temperature was 37±3 K. The layer is not continuous.<a class="footnote-ref" id="fnref:177" href="#fn:177">177</a>
In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016.<a class="footnote-ref" id="fnref:178" href="#fn:178">178</a> In 2021, astronomers at the <a href="/facts/Southwest_Research_Institute/8ljrULa4">Southwest Research Institute</a> confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere.<a class="footnote-ref" id="fnref:179" href="#fn:179">179</a>
The presence of methane, a powerful <a href="/facts/Greenhouse_gas/HXaHqrT8">greenhouse gas</a>, in Pluto's atmosphere creates a <a href="/facts/Temperature_inversion/GXdVvy7I">temperature inversion</a>, with the average temperature of its atmosphere tens of degrees warmer than its surface,<a class="footnote-ref" id="fnref:180" href="#fn:180">180</a> though observations by New Horizons have revealed Pluto's upper atmosphere to be far colder than expected (70 K, as opposed to about 100 K).<a class="footnote-ref" id="fnref:181" href="#fn:181">181</a> Pluto's atmosphere is divided into roughly 20 regularly spaced haze layers up to 150 km high,<a class="footnote-ref" id="fnref:182" href="#fn:182">182</a> thought to be the result of pressure waves created by airflow across Pluto's mountains.<a class="footnote-ref" id="fnref:183" href="#fn:183">183</a>

<h2 id="natural-satellites">Natural satellites</h2>
Main article: <a href="/facts/Moons_of_Pluto/g89834mJ">Moons of Pluto</a>

Pluto has five known <a href="/facts/Natural_satellite/AE4KQKbq">natural satellites</a>. The largest and closest to Pluto is <a href="/facts/Charon_(moon)/elazjsMp">Charon</a>. First identified in 1978 by astronomer <a href="/facts/James_Christy/JEBftHFw">James Christy</a>, Charon is the only moon of Pluto that may be in <a href="/facts/Hydrostatic_equilibrium/e75gVkh2">hydrostatic equilibrium</a>. Charon's mass is sufficient to cause the barycenter of the Pluto–Charon system to be outside Pluto. Beyond Charon there are four much smaller <a href="/facts/Circumbinary/3Tpw768E">circumbinary</a> moons. In order of distance from Pluto they are Styx, Nix, Kerberos, and Hydra. <a href="/facts/Nix_(moon)/Qf3qsr7m">Nix</a> and <a href="/facts/Hydra_(moon)/Al9hJ1on">Hydra</a> were both discovered in 2005,<a class="footnote-ref" id="fnref:184" href="#fn:184">184</a> <a href="/facts/Kerberos_(moon)/b0X5ssTS">Kerberos</a> was discovered in 2011,<a class="footnote-ref" id="fnref:185" href="#fn:185">185</a> and <a href="/facts/Styx_(moon)/ssHYXjCc">Styx</a> was discovered in 2012.<a class="footnote-ref" id="fnref:186" href="#fn:186">186</a> The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°),<a class="footnote-ref" id="fnref:187" href="#fn:187">187</a><a class="footnote-ref" id="fnref:188" href="#fn:188">188</a> and therefore tilted approximately 120° relative to Pluto's orbit. The Plutonian system is highly compact: the five known satellites orbit within the inner 3% of the region where <a href="/facts/Prograde_orbit/1HCbwQqy">prograde orbits</a> would be stable.<a class="footnote-ref" id="fnref:189" href="#fn:189">189</a>
The orbital periods of all Pluto's moons are linked in a system of <a href="/facts/Orbital_resonance/eQLMfBWu">orbital resonances</a> and <a href="/facts/Orbital_resonance/eQLMfBWu">near-resonances</a>.<a class="footnote-ref" id="fnref:190" href="#fn:190">190</a><a class="footnote-ref" id="fnref:191" href="#fn:191">191</a> When <a href="/facts/Apsidal_precession/YUWDqXDp">precession</a> is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio.<a class="footnote-ref" id="fnref:192" href="#fn:192">192</a> There is a sequence of approximate ratios, 3:4:5:6, between the periods of Styx, Nix, Kerberos, and Hydra with that of Charon; the ratios become closer to being exact the further out the moons are.<a class="footnote-ref" id="fnref:193" href="#fn:193">193</a><a class="footnote-ref" id="fnref:194" href="#fn:194">194</a>
The Pluto–Charon system is one of the few in the Solar System whose barycenter lies outside the primary body; the <a href="/facts/617_Patroclus/pqwdNRf4">Patroclus–Menoetius</a> system is a smaller example, and the <a href="/facts/Jupiter/95m7XjMT">Sun–Jupiter</a> system is the only larger one.<a class="footnote-ref" id="fnref:195" href="#fn:195">195</a> The similarity in size of Charon and Pluto has prompted some astronomers to call it a <a href="/facts/Double_planet/eGP67rqj">double dwarf planet</a>.<a class="footnote-ref" id="fnref:196" href="#fn:196">196</a> The system is also unusual among planetary systems in that each is <a href="/facts/Tidally_locked/E3W4dEBj">tidally locked</a> to the other, which means that Pluto and Charon always have the same hemisphere facing each other — a property shared by only one other known system, <a href="/facts/Eris_(dwarf_planet)/vuaR65F7">Eris</a> and <a href="/facts/Dysnomia_(moon)/g69gHfDT">Dysnomia</a>.<a class="footnote-ref" id="fnref:197" href="#fn:197">197</a> From any position on either body, the other is always at the same position in the sky, or always obscured.<a class="footnote-ref" id="fnref:198" href="#fn:198">198</a> This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter.<a class="footnote-ref" id="fnref:199" href="#fn:199">199</a>

Pluto's moons are hypothesized to have been formed by a collision between Pluto and a similar-sized body, early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.<a class="footnote-ref" id="fnref:200" href="#fn:200">200</a>
<h3>Quasi-satellite</h3>
In 2012, it was calculated that <a href="/facts/15810_Arawn/pIMoygsP">15810 Arawn</a> could be a <a href="/facts/Quasi-satellite/bPHHLFsj">quasi-satellite</a> of Pluto, a specific type of co-orbital configuration.<a class="footnote-ref" id="fnref:201" href="#fn:201">201</a> According to the calculations, the object would be a quasi-satellite of Pluto for about 350,000 years out of every two-million-year period.<a class="footnote-ref" id="fnref:202" href="#fn:202">202</a><a class="footnote-ref" id="fnref:203" href="#fn:203">203</a> Measurements made by the New Horizons spacecraft in 2015 made it possible to calculate the orbit of Arawn more accurately,<a class="footnote-ref" id="fnref:204" href="#fn:204">204</a> and confirmed the earlier ones.<a class="footnote-ref" id="fnref:205" href="#fn:205">205</a> However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on its orbital dynamics, since its orbit is primarily controlled by Neptune with only occasional perturbations by Pluto.<a class="footnote-ref" id="fnref:206" href="#fn:206">206</a><a class="footnote-ref" id="fnref:207" href="#fn:207">207</a><a class="footnote-ref" id="fnref:208" href="#fn:208">208</a>

<h2 id="origin">Origin </h2>
Further information: <a href="/facts/Kuiper_belt/IvLzYp5P">Kuiper belt</a> and <a href="/facts/Nice_model/Mv0ADIy8">Nice model</a>

Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune<a class="footnote-ref" id="fnref:209" href="#fn:209">209</a> knocked out of orbit by Neptune's largest moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.<a class="footnote-ref" id="fnref:210" href="#fn:210">210</a>
Pluto's true place in the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a> began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is thought to be the source of many <a href="/facts/Short-period_comet/w34utQCd">short-period comets</a>. Pluto is the largest member of the <a href="/facts/Kuiper_belt/IvLzYp5P">Kuiper belt</a>,<a class="footnote-ref" id="fnref:211" href="#fn:211">211</a> a stable belt of objects located between 30 and 50 AU from the Sun. As of 2011, surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto-sized objects are expected to be beyond 100 AU from the Sun.<a class="footnote-ref" id="fnref:212" href="#fn:212">212</a> Like other Kuiper-belt objects (KBOs), Pluto shares features with <a href="/facts/Comet/w34utQCd">comets</a>; for example, the <a href="/facts/Solar_wind/BvuB3dT8">solar wind</a> is gradually blowing Pluto's surface into space.<a class="footnote-ref" id="fnref:213" href="#fn:213">213</a> It has been claimed that if Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.<a class="footnote-ref" id="fnref:214" href="#fn:214">214</a> This claim has been disputed with the argument that Pluto's escape velocity is too high for this to happen.<a class="footnote-ref" id="fnref:215" href="#fn:215">215</a> It has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper-belt objects.<a class="footnote-ref" id="fnref:216" href="#fn:216">216</a><a class="footnote-ref" id="fnref:217" href="#fn:217">217</a>
Though Pluto is the largest Kuiper belt object discovered,<a class="footnote-ref" id="fnref:218" href="#fn:218">218</a> Neptune's moon <a href="/facts/Triton_(moon)/mxxRUygc">Triton</a>, which is larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object.<a class="footnote-ref" id="fnref:219" href="#fn:219">219</a> <a href="/facts/Eris_(dwarf_planet)/vuaR65F7">Eris</a> (see above) is about the same size as Pluto (though more massive) but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the <a href="/facts/Scattered_disc/cYWDkWUL">scattered disc</a>.<a class="footnote-ref" id="fnref:220" href="#fn:220">220</a>
Like other members of the Kuiper belt, Pluto is thought to be a residual <a href="/facts/Planetesimal/PK60dofs">planetesimal</a>; a component of the original <a href="/facts/Protoplanetary_disc/FtfSbP61">protoplanetary disc</a> around the <a href="/facts/Sun/CadPCVHh">Sun</a> that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its position to a <a href="/facts/Planetary_migration/rH6cEwNR">sudden migration</a> undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself (Triton), locking others into resonances, and knocking others into chaotic orbits. The objects in the <a href="/facts/Scattered_disc/cYWDkWUL">scattered disc</a>, a dynamically unstable region overlapping the Kuiper belt, are thought to have been placed in their positions by interactions with Neptune's migrating resonances.<a class="footnote-ref" id="fnref:221" href="#fn:221">221</a> A computer model created in 2004 by <a href="/facts/Alessandro_Morbidelli_(astronomer)/Sdz6zMpx">Alessandro Morbidelli</a> of the <a href="/facts/Observatoire_de_la_C%25C3%25B4te_d%2527Azur/EtfjaURW">Observatoire de la Côte d'Azur</a> in <a href="/facts/Nice/83vtP8Lk">Nice</a> suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the <a href="/facts/Late_Heavy_Bombardment/PBUlnPJX">Late Heavy Bombardment</a> 600 million years after the Solar System's formation and the origin of the <a href="/facts/Jupiter_trojan/KN9dgpUr">Jupiter trojans</a>.<a class="footnote-ref" id="fnref:222" href="#fn:222">222</a> It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration <a href="/facts/Perturbation_(astronomy)/og5hPpjQ">perturbed</a> it into a resonant capture.<a class="footnote-ref" id="fnref:223" href="#fn:223">223</a> The <a href="/facts/Nice_model/Mv0ADIy8">Nice model</a> requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk, which included Triton and Eris.<a class="footnote-ref" id="fnref:224" href="#fn:224">224</a>

<h2 id="observation-and-exploration">Observation and exploration</h2>
<h3>Observation</h3>

Pluto's distance from Earth makes its in-depth study and <a href="/facts/Space_exploration/2eKXYVdq">exploration</a> difficult. Pluto's visual <a href="/facts/Apparent_magnitude/7pCTF1Me">apparent magnitude</a> averages 15.1, brightening to 13.65 at perihelion.<a class="footnote-ref" id="fnref:225" href="#fn:225">225</a> To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.<a class="footnote-ref" id="fnref:226" href="#fn:226">226</a> It looks star-like and without a visible disk even in large telescopes,<a class="footnote-ref" id="fnref:227" href="#fn:227">227</a> because its <a href="/facts/Angular_diameter/p2xv4vcQ">angular diameter</a> is maximum 0.11".<a class="footnote-ref" id="fnref:228" href="#fn:228">228</a>
The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.<a class="footnote-ref" id="fnref:229" href="#fn:229">229</a><a class="footnote-ref" id="fnref:230" href="#fn:230">230</a>
Better maps were produced from images taken by the <a href="/facts/Hubble_Space_Telescope/NQeIf0vw">Hubble Space Telescope</a> (HST), which offered higher <a href="/facts/Angular_resolution/wJarPBWF">resolution</a>, and showed considerably more detail,<a class="footnote-ref" id="fnref:231" href="#fn:231">231</a> resolving variations several hundred kilometers across, including polar regions and large bright spots.<a class="footnote-ref" id="fnref:232" href="#fn:232">232</a> These maps were produced by complex computer processing, which finds the best-fit projected maps for the few pixels of the Hubble images.<a class="footnote-ref" id="fnref:233" href="#fn:233">233</a> These remained the most detailed maps of Pluto until the flyby of New Horizons in July 2015, because the two cameras on the HST used for these maps were no longer in service.<a class="footnote-ref" id="fnref:234" href="#fn:234">234</a>

<h3>Exploration</h3>
Main articles: <a href="/facts/Exploration_of_Pluto/I5ytz92T">Exploration of Pluto</a> and <a href="/facts/New_Horizons/G5XTU5DK">New Horizons</a>

The New Horizons spacecraft, which <a href="/facts/Planetary_flyby/g65t37fh">flew by</a> Pluto in July 2015, is the first and so far only attempt to explore Pluto directly. Launched in 2006, it captured its first (distant) images of Pluto in late September 2006 during a test of the Long Range Reconnaissance Imager.<a class="footnote-ref" id="fnref:235" href="#fn:235">235</a> The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a <a href="/facts/Gravity_assist/4sqIByot">gravity assist</a> from <a href="/facts/Jupiter/95m7XjMT">Jupiter</a>.
New Horizons made its closest approach to Pluto on July 14, 2015, after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. Observations were conducted using a <a href="/facts/Remote_sensing/pj2crmGs">remote sensing</a> package that included <a href="/facts/Digital_imaging/nTNga50f">imaging</a> instruments and a radio science investigation tool, as well as <a href="/facts/Spectroscopic/mj71FuNK">spectroscopic</a> and other experiments. The scientific goals of New Horizons were to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition, and analyze Pluto's neutral atmosphere and its escape rate. On October 25, 2016, at 05:48 pm ET, the last bit of data (of a total of 50 billion bits of data; or 6.25 gigabytes) was received from New Horizons from its close encounter with Pluto.<a class="footnote-ref" id="fnref:236" href="#fn:236">236</a><a class="footnote-ref" id="fnref:237" href="#fn:237">237</a><a class="footnote-ref" id="fnref:238" href="#fn:238">238</a><a class="footnote-ref" id="fnref:239" href="#fn:239">239</a>
Since the New Horizons flyby, scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives.<a class="footnote-ref" id="fnref:240" href="#fn:240">240</a><a class="footnote-ref" id="fnref:241" href="#fn:241">241</a><a class="footnote-ref" id="fnref:242" href="#fn:242">242</a> They include mapping the surface at 9.1 m (30 ft) per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, investigations of a possible subsurface ocean, and topographic mapping of Pluto's regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using laser pulses to generate a complete topographic map of Pluto. New Horizons principal investigator Alan Stern has advocated for a <a href="/facts/Cassini%25E2%2580%2593Huygens/PWknrDeY">Cassini</a>-style orbiter that would launch around 2030 (the 100th anniversary of Pluto's discovery) and use Charon's gravity to adjust its orbit as needed to fulfill science objectives after arriving at the Pluto system.<a class="footnote-ref" id="fnref:243" href="#fn:243">243</a> The orbiter could then use Charon's gravity to leave the Pluto system and study more KBOs after all Pluto science objectives are completed. A conceptual study funded by the NASA Innovative Advanced Concepts (<a href="/facts/NASA_Institute_for_Advanced_Concepts/4hYhDWSK">NIAC</a>) program describes a fusion-enabled Pluto orbiter and lander based on the <a href="/facts/Princeton_field-reversed_configuration_experiment/Q8d3QVNS">Princeton field-reversed configuration reactor</a>.<a class="footnote-ref" id="fnref:244" href="#fn:244">244</a><a class="footnote-ref" id="fnref:245" href="#fn:245">245</a>
New Horizons imaged all of Pluto's northern hemisphere, and the equatorial regions down to about 30° South. Higher southern latitudes have only been observed, at very low resolution, from Earth.<a class="footnote-ref" id="fnref:246" href="#fn:246">246</a> Images from the Hubble Space Telescope in 1996 cover 85% of Pluto and show large albedo features down to about 75° South.<a class="footnote-ref" id="fnref:247" href="#fn:247">247</a><a class="footnote-ref" id="fnref:248" href="#fn:248">248</a> This is enough to show the extent of the temperate-zone maculae. Later images had slightly better resolution, due to minor improvements in Hubble instrumentation.<a class="footnote-ref" id="fnref:249" href="#fn:249">249</a> The equatorial region of the sub-Charon hemisphere of Pluto has only been imaged at low resolution, as New Horizons made its closest approach to the anti-Charon hemisphere.<a class="footnote-ref" id="fnref:250" href="#fn:250">250</a>
Some albedo variations in the higher southern latitudes could be detected by New Horizons using <a href="/facts/Planetshine/efcw3Fdc">Charon-shine</a> (light reflected off Charon). The south polar region seems to be darker than the north polar region, but there is a high-albedo region in the southern hemisphere that may be a regional nitrogen or methane ice deposit.<a class="footnote-ref" id="fnref:251" href="#fn:251">251</a>

<ul><li><a href="/facts/How_I_Killed_Pluto_and_Why_It_Had_It_Coming/06Nk66B8">How I Killed Pluto and Why It Had It Coming</a></li>
<li><a href="/facts/List_of_geological_features_on_Pluto/HeXMRNEK">List of geological features on Pluto</a></li>
<li><a href="/facts/Planets_in_astrology/2B9Qv38e">Pluto in astrology</a></li>
<li><a href="/facts/Pluto_in_fiction/XhIb7IX0">Pluto in fiction</a></li>
<li><a href="/facts/List_of_gravitationally_rounded_objects_of_the_Solar_System/VUqhybx1">Statistics of planets in the Solar System</a></li></ul>

<h2 id="notes">Notes</h2>

<h2 id="further-reading">Further reading</h2>

<ul><li>Codex Regius (2016). <a href="https://books.google.com/books?id=dTs2DQAAQBAJ">Pluto & Charon: the new horizons spacecraft at the farthest worldly shores</a>. Wiesbaden: Codex Regius. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-1-5349-6074-9.</li>
<li>Stern, Alan; Tholen, David J., eds. (1997). <a href="https://books.google.com/books?id=VcY7iYJwJZoC">Pluto and Charon</a>. Space science series. Tucson: <a href="/facts/University_of_Arizona_Press/gkGd3099">University of Arizona Press</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-8165-1840-1.</li>
<li><a href="/facts/Alan_Stern/kCKGyeIt">Stern, Alan</a>; <a href="/facts/David_Grinspoon/akEJJ6ze">Grinspoon, David</a> (2018). <a href="/facts/Chasing_New_Horizons/myJxIBDx">Chasing new horizons: inside the epic first mission to Pluto</a>. New York: <a href="/facts/Picador_(imprint)/QSGCtNmy">Picador</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-1-250-09896-2.</li>
<li><a href="/facts/Alan_Stern/kCKGyeIt">Stern, Alan</a>; Moore, J.; Grundy, William M.; Young, Leslie A.; <a href="/facts/Richard_P._Binzel/mcqEdFdg">Binzel, Richard P.</a>, eds. (2021). <a href="https://books.google.com/books?id=bi01EAAAQBAJ">The Pluto system after New Horizons</a>. The University of Arizona Space science series. Tucson: <a href="/facts/University_of_Arizona_Press/gkGd3099">University of Arizona Press</a>. p. 688. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-8165-4094-5.</li></ul>

<h2 id="external-links">External links</h2>

<ul><li><a href="https://www.nasa.gov/mission_pages/newhorizons/main/index.html">New Horizons homepage</a> <a href="https://web.archive.org/web/20150726155211/http://www.nasa.gov/mission_pages/newhorizons/main/index.html">Archived</a> July 26, 2015, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li>
<li><a href="https://web.archive.org/web/20120729075223/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto">Pluto Profile</a> at <a href="https://solarsystem.nasa.gov/">NASA's Solar System Exploration site</a></li>
<li><a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html">NASA Pluto factsheet</a> <a href="https://archive.today/20151119095810/http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html">Archived</a> November 19, 2015, at <a href="/facts/Archive.today/B9vA7Vmt">archive.today</a></li>
<li><a href="http://www.lowell.edu/">Website of the observatory that discovered Pluto</a> <a href="https://web.archive.org/web/20110302233420/http://www.lowell.edu/">Archived</a> March 2, 2011, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li>
<li><a href="https://web.archive.org/web/20121004232224/http://www.astrobio.net/pressrelease/5055/sharpest-ever-views-of-pluto-and-charon">Earth telescope image of Pluto system</a></li>
<li><a href="http://www.ifa.hawaii.edu/info/press-releases/PlutoPictures/Pluto-Tholen-10-07.html">Keck infrared with AO of Pluto system</a> <a href="https://web.archive.org/web/20201109033030/http://www.ifa.hawaii.edu/info/press-releases/PlutoPictures/Pluto-Tholen-10-07.html">Archived</a> November 9, 2020, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li>
<li><a href="https://www.nasa.gov/image-feature/goddard/views-of-pluto-through-the-years">Video – Pluto – viewed through the years (GIF)</a> <a href="https://web.archive.org/web/20150726170002/http://www.nasa.gov/image-feature/goddard/views-of-pluto-through-the-years">Archived</a> July 26, 2015, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> (NASA; animation; July 15, 2015).</li>
<li><a href="https://photojournal.jpl.nasa.gov/archive/PIA19873_FLYTHROUGH_ANIMATION_V5.mp4">Video – Pluto – "FlyThrough" (00:22; MP4)</a> <a href="https://web.archive.org/web/20210929080856/http://photojournal.jpl.nasa.gov/archive/PIA19873_FLYTHROUGH_ANIMATION_V5.mp4">Archived</a> September 29, 2021, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> <a href="https://www.youtube.com/watch?v=ds_OlZnV9qk">(YouTube)</a> <a href="https://web.archive.org/web/20201202025129/https://www.youtube.com/watch?v=ds_OlZnV9qk">Archived</a> December 2, 2020, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> (NASA; animation; August 31, 2015).</li>
<li><a href="http://www.scientificamerican.com/video/a-day-on-pluto-reconstructed-from-new-horizons-images/">"A Day on Pluto Video made from July 2015 New Horizon Images"</a> <a href="https://web.archive.org/web/20160223013923/http://www.scientificamerican.com/video/a-day-on-pluto-reconstructed-from-new-horizons-images/">Archived</a> February 23, 2016, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> <a href="/facts/Scientific_American/jkl8bhUN">Scientific American</a></li>
<li>NASA CGI <a href="http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20170714-2">video</a> <a href="https://web.archive.org/web/20170801114952/http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20170714-2">Archived</a> August 1, 2017, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> of Pluto flyover (July 14, 2017)</li>
<li><a href="https://www.flickr.com/photos/136797589@N04/35260746413/in/photostream/">CGI video</a> <a href="https://web.archive.org/web/20201003081511/https://www.flickr.com/photos/136797589@N04/35260746413/in/photostream/">Archived</a> October 3, 2020, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> simulation of rotating Pluto by Seán Doran (see <a href="https://www.flickr.com/photos/136797589@N04/albums/72157686474817595">album</a> <a href="https://web.archive.org/web/20200727235247/https://www.flickr.com/photos/136797589@N04/albums/72157686474817595">Archived</a> July 27, 2020, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a> for more)</li>
<li><a href="https://www.google.com/maps/space/pluto/@26.4828801,-19.6602925,11338964m/data=!3m1!1e3">Google Pluto 3D</a> <a href="https://web.archive.org/web/20200806100353/https://www.google.com/maps/space/pluto/@26.4828801,-19.6602925,11338964m/data=!3m1!1e3">Archived</a> August 6, 2020, at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a>, interactive map of the dwarf planet</li>
<li><a href="https://web.archive.org/web/20200611221454/https://gravitysimulator.org/solar-system/the-plutonian-system/">"Interactive 3D gravity simulation of the Plutonian system"</a>. Archived from <a href="https://gravitysimulator.org/solar-system/the-plutonian-system">the original</a> on June 11, 2020.</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Clyde Tombaugh & Patrick Moore (2008) Out of the Darkness: The Planet Pluto <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. p. 43. ISBN 978-0-684-83252-4. Archived from the original on February 26, 2024. Retrieved April 15, 2022. <a href="978-0-684-83252-4" target="_blank">978-0-684-83252-4</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Tombaugh, Clyde W. (1946). "The Search for the Ninth Planet, Pluto". Astronomical Society of the Pacific Leaflets. 5 (209): 73–80. Bibcode:1946ASPL....5...73T. <a href="/wiki/Clyde_Tombaugh" target="_blank">/wiki/Clyde_Tombaugh</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Hoyt, William G. (1976). "W. H. Pickering's Planetary Predictions and the Discovery of Pluto". Isis. 67 (4): 551–564. doi:10.1086/351668. JSTOR 230561. PMID 794024. S2CID 26512655. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Hoyt, William G. (1976). "W. H. Pickering's Planetary Predictions and the Discovery of Pluto". Isis. 67 (4): 551–564. doi:10.1086/351668. JSTOR 230561. PMID 794024. S2CID 26512655. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Littman, Mark (1990). Planets Beyond: Discovering the Outer Solar System. Wiley. p. 70. ISBN 978-0-471-51053-6. <a href="978-0-471-51053-6" target="_blank">978-0-471-51053-6</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Buchwald, Greg; Dimario, Michael; Wild, Walter (2000). "Pluto is Discovered Back in Time". Amateur–Professional Partnerships in Astronomy. 220. San Francisco: 335. Bibcode:2000ASPC..220..355B. ISBN 978-1-58381-052-1. <a href="978-1-58381-052-1" target="_blank">978-1-58381-052-1</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Croswell 1997, p. 50. - Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. p. 43. ISBN 978-0-684-83252-4. Archived from the original on February 26, 2024. Retrieved April 15, 2022. <a href="https://books.google.com/books?id=60sPD6yjbVAC" target="_blank">https://books.google.com/books?id=60sPD6yjbVAC</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Croswell 1997, p. 50. - Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. p. 43. ISBN 978-0-684-83252-4. Archived from the original on February 26, 2024. Retrieved April 15, 2022. <a href="https://books.google.com/books?id=60sPD6yjbVAC" target="_blank">https://books.google.com/books?id=60sPD6yjbVAC</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Croswell 1997, p. 52. - Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. p. 43. ISBN 978-0-684-83252-4. Archived from the original on February 26, 2024. Retrieved April 15, 2022. <a href="https://books.google.com/books?id=60sPD6yjbVAC" target="_blank">https://books.google.com/books?id=60sPD6yjbVAC</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Hoyt, William G. (1976). "W. H. Pickering's Planetary Predictions and the Discovery of Pluto". Isis. 67 (4): 551–564. doi:10.1086/351668. JSTOR 230561. PMID 794024. S2CID 26512655. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Williams, David R. (July 24, 2015). "Pluto Fact Sheet". NASA. Archived from the original on November 19, 2015. Retrieved August 6, 2015. <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html" target="_blank">https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Rao, Joe (March 11, 2005). "Finding Pluto: Tough Task, Even 75 Years Later". Space.com. Archived from the original on August 23, 2010. Retrieved September 8, 2006. <a href="http://www.space.com/spacewatch/050311_pluto_guide.html" target="_blank">http://www.space.com/spacewatch/050311_pluto_guide.html</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Kevin Schindler & William Grundy (2018) Pluto and Lowell Observatory, pp. 73–79. <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Clyde Tombaugh & Patrick Moore (2008) Out of the Darkness: The Planet Pluto <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Croswell 1997, pp. 54–55. - Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. p. 43. ISBN 978-0-684-83252-4. Archived from the original on February 26, 2024. Retrieved April 15, 2022. <a href="https://books.google.com/books?id=60sPD6yjbVAC" target="_blank">https://books.google.com/books?id=60sPD6yjbVAC</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">Rincon, Paul (January 13, 2006). "The girl who named a planet". BBC News. Archived from the original on October 4, 2018. Retrieved April 12, 2007. <a href="http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm" target="_blank">http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">"Pluto Research at Lowell". Lowell Observatory. Archived from the original on April 18, 2016. Retrieved March 22, 2017. <a href="https://web.archive.org/web/20160418140312/http://lowell.edu/in-depth/pluto/pluto-research-at-lowell/" target="_blank">https://web.archive.org/web/20160418140312/http://lowell.edu/in-depth/pluto/pluto-research-at-lowell/</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">A French astronomer had suggested the name Pluto for Planet X in 1919, but there is no indication that the Lowell staff knew of this.[28] <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Clyde Tombaugh & Patrick Moore (2008) Out of the Darkness: The Planet Pluto <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Rincon, Paul (January 13, 2006). "The girl who named a planet". BBC News. Archived from the original on October 4, 2018. Retrieved April 12, 2007. <a href="http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm" target="_blank">http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Kevin Schindler & William Grundy (2018) Pluto and Lowell Observatory, pp. 73–79. <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">Rincon, Paul (January 13, 2006). "The girl who named a planet". BBC News. Archived from the original on October 4, 2018. Retrieved April 12, 2007. <a href="http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm" target="_blank">http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">Scott & Powell (2018) The Universe as It Really Is <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
<li id="fn:25">Kevin Schindler & William Grundy (2018) Pluto and Lowell Observatory, pp. 73–79. <a href="#fnref:25" class="footnote-back-ref">↩</a></li>
<li id="fn:26">Coincidentally, as popular science author Martin Gardner and others have noted of the name "Pluto", "the last two letters are the first two letters of Tombaugh's name" Martin Gardner, Puzzling Questions about the Solar System (Dover Publications, 1997) p. 55 <a href="/wiki/Martin_Gardner" target="_blank">/wiki/Martin_Gardner</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></li>
<li id="fn:27">"NASA's Solar System Exploration: Multimedia: Gallery: Pluto's Symbol". NASA. Archived from the original on October 1, 2006. Retrieved November 29, 2011. <a href="https://web.archive.org/web/20061001015053/http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263" target="_blank">https://web.archive.org/web/20061001015053/http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></li>
<li id="fn:28">For example, ⟨♇⟩ (in Unicode: U+2647 ♇ PLUTO) occurs in a table of the planets identified by their symbols in a 2004 article written before the 2006 IAU definition,[32] but not in a graph of planets, dwarf planets and moons from 2016, where only the eight IAU planets are identified by their symbols.[33]
(Planetary symbols in general are uncommon in astronomy, and are discouraged by the IAU.)[34] <a href="/wiki/Unicode" target="_blank">/wiki/Unicode</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></li>
<li id="fn:29">Dane Rudhyar (1936) The Astrology of Personality, credits it to Paul Clancy Publications, founded in 1933. <a href="#fnref:29" class="footnote-back-ref">↩</a></li>
<li id="fn:30">The bident symbol (U+2BD3 ⯓ PLUTO FORM TWO) has seen some astronomical use as well since the IAU decision on dwarf planets, for example in a public-education poster on dwarf planets published by the NASA/JPL Dawn mission in 2015, in which each of the five dwarf planets announced by the IAU receives a symbol.[36] There are in addition several other symbols for Pluto found in astrological sources,[37] including three accepted by Unicode: , U+2BD4 ⯔ PLUTO FORM THREE, used principally in southern Europe; /, U+2BD6 ⯖ PLUTO FORM FIVE (found in various orientations, showing Pluto's orbit cutting across that of Neptune), used principally in northern Europe; and , U+2BD5 ⯕ PLUTO FORM FOUR, used in Uranian astrology.[38] <a href="#fnref:30" class="footnote-back-ref">↩</a></li>
<li id="fn:31">Heinrichs, Allison M. (2006). "Dwarfed by comparison". Pittsburgh Tribune-Review. Archived from the original on November 14, 2007. Retrieved March 26, 2007. <a href="https://web.archive.org/web/20071114081539/http://www.pittsburghlive.com/x/pittsburghtrib/news/cityregion/s_467650.html" target="_blank">https://web.archive.org/web/20071114081539/http://www.pittsburghlive.com/x/pittsburghtrib/news/cityregion/s_467650.html</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></li>
<li id="fn:32">Clark, David L.; Hobart, David E. (2000). "Reflections on the Legacy of a Legend" (PDF). Archived (PDF) from the original on June 3, 2016. Retrieved November 29, 2011. <a href="https://fas.org/sgp/othergov/doe/lanl/pubs/00818011.pdf" target="_blank">https://fas.org/sgp/othergov/doe/lanl/pubs/00818011.pdf</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></li>
<li id="fn:33">The equivalence is less close in languages whose phonology differs widely from Greek's, such as Somali Buluuto and Navajo Tłóotoo. <a href="/wiki/Phonology" target="_blank">/wiki/Phonology</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></li>
<li id="fn:34">"Planetary Linguistics". Archived from the original on December 17, 2007. Retrieved June 12, 2007. <a href="https://web.archive.org/web/20071217070734/http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html" target="_blank">https://web.archive.org/web/20071217070734/http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></li>
<li id="fn:35">"Planetary Linguistics". Archived from the original on December 17, 2007. Retrieved June 12, 2007. <a href="https://web.archive.org/web/20071217070734/http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html" target="_blank">https://web.archive.org/web/20071217070734/http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></li>
<li id="fn:36">"Planetary Linguistics". Archived from the original on December 17, 2007. Retrieved June 12, 2007. <a href="https://web.archive.org/web/20071217070734/http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html" target="_blank">https://web.archive.org/web/20071217070734/http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></li>
<li id="fn:37">Renshaw, Steve; Ihara, Saori (2000). "A Tribute to Houei Nojiri". Archived from the original on December 6, 2012. Retrieved November 29, 2011. <a href="https://archive.today/20121206025620/http://www2.gol.com/users/stever/nojiri.htm" target="_blank">https://archive.today/20121206025620/http://www2.gol.com/users/stever/nojiri.htm</a> <a href="#fnref:37" class="footnote-back-ref">↩</a></li>
<li id="fn:38">Bathrobe. "Uranus, Neptune, and Pluto in Chinese, Japanese, and Vietnamese". cjvlang.com. Archived from the original on July 20, 2011. Retrieved November 29, 2011. <a href="https://web.archive.org/web/20110720140817/http://www.cjvlang.com/Dow/UrNepPl.html" target="_blank">https://web.archive.org/web/20110720140817/http://www.cjvlang.com/Dow/UrNepPl.html</a> <a href="#fnref:38" class="footnote-back-ref">↩</a></li>
<li id="fn:39">Tombaugh, Clyde W. (1946). "The Search for the Ninth Planet, Pluto". Astronomical Society of the Pacific Leaflets. 5 (209): 73–80. Bibcode:1946ASPL....5...73T. <a href="/wiki/Clyde_Tombaugh" target="_blank">/wiki/Clyde_Tombaugh</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></li>
<li id="fn:40">Stern, Alan; Tholen, David James (1997). Pluto and Charon. University of Arizona Press. pp. 206–208. ISBN 978-0-8165-1840-1. <a href="978-0-8165-1840-1" target="_blank">978-0-8165-1840-1</a> <a href="#fnref:40" class="footnote-back-ref">↩</a></li>
<li id="fn:41">Tombaugh, Clyde W. (1946). "The Search for the Ninth Planet, Pluto". Astronomical Society of the Pacific Leaflets. 5 (209): 73–80. Bibcode:1946ASPL....5...73T. <a href="/wiki/Clyde_Tombaugh" target="_blank">/wiki/Clyde_Tombaugh</a> <a href="#fnref:41" class="footnote-back-ref">↩</a></li>
<li id="fn:42">Crommelin, Andrew Claude de la Cherois (1931). "The Discovery of Pluto". Monthly Notices of the Royal Astronomical Society. 91 (4): 380–385. Bibcode:1931MNRAS..91..380.. doi:10.1093/mnras/91.4.380. <a href="https://doi.org/10.1093%2Fmnras%2F91.4.380" target="_blank">https://doi.org/10.1093%2Fmnras%2F91.4.380</a> <a href="#fnref:42" class="footnote-back-ref">↩</a></li>
<li id="fn:43">Nicholson, Seth B.; Mayall, Nicholas U. (December 1930). "The Probable Value of the Mass of Pluto". Publications of the Astronomical Society of the Pacific. 42 (250): 350. Bibcode:1930PASP...42..350N. doi:10.1086/124071. <a href="/wiki/Seth_Barnes_Nicholson" target="_blank">/wiki/Seth_Barnes_Nicholson</a> <a href="#fnref:43" class="footnote-back-ref">↩</a></li>
<li id="fn:44">Nicholson, Seth B.; Mayall, Nicholas U. (January 1931). "Positions, Orbit, and Mass of Pluto". Astrophysical Journal. 73: 1. Bibcode:1931ApJ....73....1N. doi:10.1086/143288. <a href="/wiki/Seth_Barnes_Nicholson" target="_blank">/wiki/Seth_Barnes_Nicholson</a> <a href="#fnref:44" class="footnote-back-ref">↩</a></li>
<li id="fn:45">Kuiper, Gerard P. (1950). "The Diameter of Pluto". Publications of the Astronomical Society of the Pacific. 62 (366): 133–137. Bibcode:1950PASP...62..133K. doi:10.1086/126255. <a href="https://doi.org/10.1086%2F126255" target="_blank">https://doi.org/10.1086%2F126255</a> <a href="#fnref:45" class="footnote-back-ref">↩</a></li>
<li id="fn:46">Croswell 1997, p. 57. - Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. p. 43. ISBN 978-0-684-83252-4. Archived from the original on February 26, 2024. Retrieved April 15, 2022. <a href="https://books.google.com/books?id=60sPD6yjbVAC" target="_blank">https://books.google.com/books?id=60sPD6yjbVAC</a> <a href="#fnref:46" class="footnote-back-ref">↩</a></li>
<li id="fn:47">Christy, James W.; Harrington, Robert Sutton (1978). "The Satellite of Pluto". Astronomical Journal. 83 (8): 1005–1008. Bibcode:1978AJ.....83.1005C. doi:10.1086/112284. S2CID 120501620. <a href="/wiki/Robert_Sutton_Harrington" target="_blank">/wiki/Robert_Sutton_Harrington</a> <a href="#fnref:47" class="footnote-back-ref">↩</a></li>
<li id="fn:48">Buie, Marc W.; Grundy, William M.; Young, Eliot F.; et al. (2006). "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2". Astronomical Journal. 132 (1): 290–298. arXiv:astro-ph/0512491. Bibcode:2006AJ....132..290B. doi:10.1086/504422. S2CID 119386667. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:48" class="footnote-back-ref">↩</a></li>
<li id="fn:49">Nicholson, Seth B.; Mayall, Nicholas U. (December 1930). "The Probable Value of the Mass of Pluto". Publications of the Astronomical Society of the Pacific. 42 (250): 350. Bibcode:1930PASP...42..350N. doi:10.1086/124071. <a href="/wiki/Seth_Barnes_Nicholson" target="_blank">/wiki/Seth_Barnes_Nicholson</a> <a href="#fnref:49" class="footnote-back-ref">↩</a></li>
<li id="fn:50">Kuiper, Gerard P. (1950). "The Diameter of Pluto". Publications of the Astronomical Society of the Pacific. 62 (366): 133–137. Bibcode:1950PASP...62..133K. doi:10.1086/126255. <a href="https://doi.org/10.1086%2F126255" target="_blank">https://doi.org/10.1086%2F126255</a> <a href="#fnref:50" class="footnote-back-ref">↩</a></li>
<li id="fn:51">Croswell 1997, p. 57. - Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York: The Free Press. p. 43. ISBN 978-0-684-83252-4. Archived from the original on February 26, 2024. Retrieved April 15, 2022. <a href="https://books.google.com/books?id=60sPD6yjbVAC" target="_blank">https://books.google.com/books?id=60sPD6yjbVAC</a> <a href="#fnref:51" class="footnote-back-ref">↩</a></li>
<li id="fn:52">Williams, David R. (July 24, 2015). "Pluto Fact Sheet". NASA. Archived from the original on November 19, 2015. Retrieved August 6, 2015. <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html" target="_blank">https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html</a> <a href="#fnref:52" class="footnote-back-ref">↩</a></li>
<li id="fn:53">Seidelmann, P. Kenneth; Harrington, Robert Sutton (1988). "Planet X – The current status". Celestial Mechanics and Dynamical Astronomy. 43 (1–4): 55–68. Bibcode:1988CeMec..43...55S. doi:10.1007/BF01234554. S2CID 189831334. <a href="/wiki/Robert_Sutton_Harrington" target="_blank">/wiki/Robert_Sutton_Harrington</a> <a href="#fnref:53" class="footnote-back-ref">↩</a></li>
<li id="fn:54">Standish, E. Myles (1993). "Planet X – No dynamical evidence in the optical observations". Astronomical Journal. 105 (5): 200–2006. Bibcode:1993AJ....105.2000S. doi:10.1086/116575. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:54" class="footnote-back-ref">↩</a></li>
<li id="fn:55">Standage, Tom (2000). The Neptune File. Penguin. p. 168. ISBN 978-0-8027-1363-6. <a href="978-0-8027-1363-6" target="_blank">978-0-8027-1363-6</a> <a href="#fnref:55" class="footnote-back-ref">↩</a></li>
<li id="fn:56">Ernest W. Brown, On the predictions of trans-Neptunian planets from the perturbations of Uranus Archived January 18, 2022, at the Wayback Machine, PNAS May 15, 1930, 16 (5) 364–371. <a href="https://www.pnas.org/content/16/5/364" target="_blank">https://www.pnas.org/content/16/5/364</a> <a href="#fnref:56" class="footnote-back-ref">↩</a></li>
<li id="fn:57">Tyson, Neil deGrasse (February 2, 2001). "Astronomer Responds to Pluto-Not-a-Planet Claim". Space.com. Archived from the original on May 12, 2020. Retrieved November 30, 2011. <a href="/wiki/Neil_deGrasse_Tyson" target="_blank">/wiki/Neil_deGrasse_Tyson</a> <a href="#fnref:57" class="footnote-back-ref">↩</a></li>
<li id="fn:58">Metzger, Philip T.; Sykes, Mark V.; Stern, Alan; Runyon, Kirby (2019). "The Reclassification of Asteroids from Planets to Non-Planets". Icarus. 319: 21–32. arXiv:1805.04115. Bibcode:2019Icar..319...21M. doi:10.1016/j.icarus.2018.08.026. S2CID 119206487. <a href="/wiki/Philip_T._Metzger" target="_blank">/wiki/Philip_T._Metzger</a> <a href="#fnref:58" class="footnote-back-ref">↩</a></li>
<li id="fn:59">Metzger, Philip T.; Grundy, W. M.; Sykes, Mark V.; Stern, Alan; Bell III, James F.; Detelich, Charlene E.; Runyon, Kirby; Summers, Michael (2022). "Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science". Icarus. 374: 114768. arXiv:2110.15285. Bibcode:2022Icar..37414768M. doi:10.1016/j.icarus.2021.114768. S2CID 240071005. Archived from the original on September 11, 2022. Retrieved August 8, 2022. <a href="/wiki/Philip_T._Metzger" target="_blank">/wiki/Philip_T._Metzger</a> <a href="#fnref:59" class="footnote-back-ref">↩</a></li>
<li id="fn:60">"Is Pluto a giant comet?". Central Bureau for Astronomical Telegrams. 1999. Retrieved June 27, 2025. Marsden had first suggested in 1980 that "we should "[drop] the appellation 'ninth planet' and [classify] Pluto [...] as an unusual minor planet." <a href="http://www.icq.eps.harvard.edu/ICQPluto.html" target="_blank">http://www.icq.eps.harvard.edu/ICQPluto.html</a> <a href="#fnref:60" class="footnote-back-ref">↩</a></li>
<li id="fn:61">Moomaw, Bruce (June 28, 1999). "Pluto: A Planet By Default". spacedaily.com. Archived from the original on August 16, 2000. Retrieved June 27, 2025. <a href="https://www.spacedaily.com/news/pluto-99a.html" target="_blank">https://www.spacedaily.com/news/pluto-99a.html</a> <a href="#fnref:61" class="footnote-back-ref">↩</a></li>
<li id="fn:62">"The Status of Pluto: A clarification". International Astronomical Union, Press release. February 3, 1999. Archived from the original on April 22, 1999. <a href="https://web.archive.org/web/19990422054633/http://www.iau.org/PlutoPR.html" target="_blank">https://web.archive.org/web/19990422054633/http://www.iau.org/PlutoPR.html</a> <a href="#fnref:62" class="footnote-back-ref">↩</a></li>
<li id="fn:63">Moomaw, Bruce (June 28, 1999). "Pluto: A Planet By Default". spacedaily.com. Archived from the original on August 16, 2000. Retrieved June 27, 2025. <a href="https://www.spacedaily.com/news/pluto-99a.html" target="_blank">https://www.spacedaily.com/news/pluto-99a.html</a> <a href="#fnref:63" class="footnote-back-ref">↩</a></li>
<li id="fn:64">"NASA-Funded Scientists Discover Tenth Planet". NASA press releases. July 29, 2005. Archived from the original on May 12, 2020. Retrieved February 22, 2007. <a href="https://www.nasa.gov/home/hqnews/2005/jul/HQ_05209_10th_Planet.html" target="_blank">https://www.nasa.gov/home/hqnews/2005/jul/HQ_05209_10th_Planet.html</a> <a href="#fnref:64" class="footnote-back-ref">↩</a></li>
<li id="fn:65">Soter, Steven (November 2, 2006). "What Is a Planet?". The Astronomical Journal. 132 (6): 2513–2519. arXiv:astro-ph/0608359. Bibcode:2006AJ....132.2513S. doi:10.1086/508861. S2CID 14676169. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:65" class="footnote-back-ref">↩</a></li>
<li id="fn:66">Federation, International Astronautical. "IAF : Gonzalo Tancredi". www.iafastro.org. Retrieved March 1, 2025. <a href="https://www.iafastro.org/biographie/gonzalo-tancredi.html" target="_blank">https://www.iafastro.org/biographie/gonzalo-tancredi.html</a> <a href="#fnref:66" class="footnote-back-ref">↩</a></li>
<li id="fn:67">"Cómo fue el día en que dos uruguayos lograron que Plutón dejara de ser considerado un planeta". BBC News Mundo (in Spanish). Retrieved March 1, 2025. <a href="https://www.bbc.com/mundo/noticias-37164992" target="_blank">https://www.bbc.com/mundo/noticias-37164992</a> <a href="#fnref:67" class="footnote-back-ref">↩</a></li>
<li id="fn:68">"IAU 2006 General Assembly: Resolutions 5 and 6" (PDF). IAU. August 24, 2006. Archived (PDF) from the original on June 20, 2009. Retrieved June 15, 2008. <a href="http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf" target="_blank">http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf</a> <a href="#fnref:68" class="footnote-back-ref">↩</a></li>
<li id="fn:69">"IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union (News Release – IAU0603). August 24, 2006. Archived from the original on April 29, 2014. Retrieved June 15, 2008. <a href="http://www.iau.org/news/pressreleases/detail/iau0603/" target="_blank">http://www.iau.org/news/pressreleases/detail/iau0603/</a> <a href="#fnref:69" class="footnote-back-ref">↩</a></li>
<li id="fn:70">Margot, Jean-Luc (2015). "A Quantitative Criterion for Defining Planets". The Astronomical Journal. 150 (6): 185. arXiv:1507.06300. Bibcode:2015AJ....150..185M. doi:10.1088/0004-6256/150/6/185. S2CID 51684830. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:70" class="footnote-back-ref">↩</a></li>
<li id="fn:71">Soter, Steven (2007). "What is a Planet?". The Astronomical Journal. 132 (6). Department of Astrophysics, American Museum of Natural History: 2513–2519. arXiv:astro-ph/0608359. Bibcode:2006AJ....132.2513S. doi:10.1086/508861. S2CID 14676169. Archived from the original on November 6, 2013. Retrieved April 9, 2012. <a href="http://www.scientificamerican.com/article.cfm?id=what-is-a-planet&page=2" target="_blank">http://www.scientificamerican.com/article.cfm?id=what-is-a-planet&page=2</a> <a href="#fnref:71" class="footnote-back-ref">↩</a></li>
<li id="fn:72">"IAU 2006 General Assembly: Result of the IAU Resolution votes". International Astronomical Union (News Release – IAU0603). August 24, 2006. Archived from the original on April 29, 2014. Retrieved June 15, 2008. <a href="http://www.iau.org/news/pressreleases/detail/iau0603/" target="_blank">http://www.iau.org/news/pressreleases/detail/iau0603/</a> <a href="#fnref:72" class="footnote-back-ref">↩</a></li>
<li id="fn:73">Green, Daniel W. E. (September 13, 2006). "(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia)" (PDF). IAU Circular (8747): 1. Bibcode:2006IAUC.8747....1G. Archived from the original on February 5, 2007. Retrieved December 1, 2011. <a href="https://web.archive.org/web/20070205035336/http://www.cfa.harvard.edu/iau/special/08747.pdf" target="_blank">https://web.archive.org/web/20070205035336/http://www.cfa.harvard.edu/iau/special/08747.pdf</a> <a href="#fnref:73" class="footnote-back-ref">↩</a></li>
<li id="fn:74">"JPL Small-Body Database Browser". California Institute of Technology. Archived from the original on July 21, 2011. Retrieved July 15, 2015. <a href="https://ssd.jpl.nasa.gov/sbdb.cgi#top" target="_blank">https://ssd.jpl.nasa.gov/sbdb.cgi#top</a> <a href="#fnref:74" class="footnote-back-ref">↩</a></li>
<li id="fn:75">Britt, Robert Roy (August 24, 2006). "Pluto Demoted: No Longer a Planet in Highly Controversial Definition". Space.com. Archived from the original on December 27, 2010. Retrieved September 8, 2006. <a href="https://web.archive.org/web/20101227092545/http://www.space.com/scienceastronomy/060824_planet_definition.html" target="_blank">https://web.archive.org/web/20101227092545/http://www.space.com/scienceastronomy/060824_planet_definition.html</a> <a href="#fnref:75" class="footnote-back-ref">↩</a></li>
<li id="fn:76">Ruibal, Sal (January 6, 1999). "Astronomers question if Pluto is real planet". USA Today. <a href="/wiki/USA_Today" target="_blank">/wiki/USA_Today</a> <a href="#fnref:76" class="footnote-back-ref">↩</a></li>
<li id="fn:77">Britt, Robert Roy (November 21, 2006). "Why Planets Will Never Be Defined". Space.com. Archived from the original on May 24, 2009. Retrieved December 1, 2006. <a href="http://www.space.com/scienceastronomy/061121_exoplanet_definition.html" target="_blank">http://www.space.com/scienceastronomy/061121_exoplanet_definition.html</a> <a href="#fnref:77" class="footnote-back-ref">↩</a></li>
<li id="fn:78">Britt, Robert Roy (August 24, 2006). "Scientists decide Pluto's no longer a planet". NBC News. Archived from the original on February 11, 2013. Retrieved September 8, 2006. <a href="https://www.nbcnews.com/id/wbna14489259" target="_blank">https://www.nbcnews.com/id/wbna14489259</a> <a href="#fnref:78" class="footnote-back-ref">↩</a></li>
<li id="fn:79">Shiga, David (August 25, 2006). "New planet definition sparks furore". NewScientist.com. Archived from the original on October 3, 2010. Retrieved September 8, 2006. <a href="https://www.newscientist.com/article/dn9846-new-planet-definition-sparks-furore.html" target="_blank">https://www.newscientist.com/article/dn9846-new-planet-definition-sparks-furore.html</a> <a href="#fnref:79" class="footnote-back-ref">↩</a></li>
<li id="fn:80">Kluger, Jeffrey (February 18, 2025). "What Makes Pluto So Intriguing". Time. Stern thinks the dwarf planet distinction is nonsensical—an arbitrary parsing of cosmic definitions. "Small planets are planets too," he says. "Just because the sun is a small star we don't call it a dwarf star. We're not afraid of large numbers of planets; we're not afraid of schoolchildren having to learn all their names. After all, kids don't have to memorize every element in the periodic table." <a href="https://time.com/7221576/what-makes-pluto-intriguing/" target="_blank">https://time.com/7221576/what-makes-pluto-intriguing/</a> <a href="#fnref:80" class="footnote-back-ref">↩</a></li>
<li id="fn:81">Shiga, David (August 25, 2006). "New planet definition sparks furore". NewScientist.com. Archived from the original on October 3, 2010. Retrieved September 8, 2006. <a href="https://www.newscientist.com/article/dn9846-new-planet-definition-sparks-furore.html" target="_blank">https://www.newscientist.com/article/dn9846-new-planet-definition-sparks-furore.html</a> <a href="#fnref:81" class="footnote-back-ref">↩</a></li>
<li id="fn:82">Buie, Marc W. (September 2006). "My response to 2006 IAU Resolutions 5a and 6a". Southwest Research Institute. Archived from the original on June 3, 2007. Retrieved December 1, 2011. <a href="https://web.archive.org/web/20070603104622/http://www.lowell.edu/users/buie/pluto/iauresponse.html" target="_blank">https://web.archive.org/web/20070603104622/http://www.lowell.edu/users/buie/pluto/iauresponse.html</a> <a href="#fnref:82" class="footnote-back-ref">↩</a></li>
<li id="fn:83">Overbye, Dennis (August 24, 2006). "Pluto Is Demoted to 'Dwarf Planet'". The New York Times. Archived from the original on June 22, 2022. Retrieved December 1, 2011. <a href="https://www.nytimes.com/2006/08/24/science/space/25pluto.html" target="_blank">https://www.nytimes.com/2006/08/24/science/space/25pluto.html</a> <a href="#fnref:83" class="footnote-back-ref">↩</a></li>
<li id="fn:84">DeVore, Edna (September 7, 2006). "Planetary Politics: Protecting Pluto". Space.com. Archived from the original on August 4, 2011. Retrieved December 1, 2011. <a href="http://www.space.com/2855-planetary-politics-protecting-pluto.html" target="_blank">http://www.space.com/2855-planetary-politics-protecting-pluto.html</a> <a href="#fnref:84" class="footnote-back-ref">↩</a></li>
<li id="fn:85">Holden, Constance (March 23, 2007). "Rehabilitating Pluto". Science. 315 (5819): 1643. doi:10.1126/science.315.5819.1643c. S2CID 220102037. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:85" class="footnote-back-ref">↩</a></li>
<li id="fn:86">Gutierrez, Joni Marie (2007). "A joint memorial. Declaring Pluto a planet and declaring March 13, 2007, 'Pluto planet day' at the legislature". Legislature of New Mexico. Archived from the original on May 11, 2020. Retrieved September 5, 2009. <a href="http://www.nmlegis.gov/Sessions/07%20Regular/memorials/house/HJM054.html" target="_blank">http://www.nmlegis.gov/Sessions/07%20Regular/memorials/house/HJM054.html</a> <a href="#fnref:86" class="footnote-back-ref">↩</a></li>
<li id="fn:87">"Illinois General Assembly: Bill Status of SR0046, 96th General Assembly". ilga.gov. Illinois General Assembly. Archived from the original on May 14, 2011. Retrieved March 16, 2011. <a href="http://www.ilga.gov/legislation/BillStatus.asp?DocNum=46&GAID=10&DocTypeID=SR&LegId=40752&SessionID=76&GA=96" target="_blank">http://www.ilga.gov/legislation/BillStatus.asp?DocNum=46&GAID=10&DocTypeID=SR&LegId=40752&SessionID=76&GA=96</a> <a href="#fnref:87" class="footnote-back-ref">↩</a></li>
<li id="fn:88">"Pluto's still the same Pluto". Independent Newspapers. Associated Press. October 21, 2006. Archived from the original on December 1, 2017. Retrieved November 29, 2011. Mickey Mouse has a cute dog. <a href="https://www.iol.co.za/business-report/technology/plutos-still-the-same-pluto-299586" target="_blank">https://www.iol.co.za/business-report/technology/plutos-still-the-same-pluto-299586</a> <a href="#fnref:88" class="footnote-back-ref">↩</a></li>
<li id="fn:89">"'Plutoed' chosen as '06 Word of the Year". Associated Press. January 8, 2007. Archived from the original on March 1, 2013. Retrieved January 10, 2007. <a href="https://www.nbcnews.com/id/wbna16529756" target="_blank">https://www.nbcnews.com/id/wbna16529756</a> <a href="#fnref:89" class="footnote-back-ref">↩</a></li>
<li id="fn:90">Sanchez, Cameron. "Pluto is a planet again — at least in Arizona". npr.org. NPR. Retrieved April 12, 2024. <a href="https://www.npr.org/2024/04/06/1243230463/pluto-was-discovered-at-an-arizona-observatory-it-might-be-named-the-state-plane" target="_blank">https://www.npr.org/2024/04/06/1243230463/pluto-was-discovered-at-an-arizona-observatory-it-might-be-named-the-state-plane</a> <a href="#fnref:90" class="footnote-back-ref">↩</a></li>
<li id="fn:91">Minkel, J. R. (April 10, 2008). "Is Rekindling the Pluto Planet Debate a Good Idea?". Scientific American. Archived from the original on August 11, 2011. Retrieved December 1, 2011. <a href="http://www.scientificamerican.com/article.cfm?id=rekindling-the-pluto-planet-debate" target="_blank">http://www.scientificamerican.com/article.cfm?id=rekindling-the-pluto-planet-debate</a> <a href="#fnref:91" class="footnote-back-ref">↩</a></li>
<li id="fn:92">"The Great Planet Debate: Science as Process. A Scientific Conference and Educator Workshop". gpd.jhuapl.edu. Johns Hopkins University Applied Physics Laboratory. June 27, 2008. Archived from the original on August 17, 2011. Retrieved December 1, 2011. <a href="http://gpd.jhuapl.edu/" target="_blank">http://gpd.jhuapl.edu/</a> <a href="#fnref:92" class="footnote-back-ref">↩</a></li>
<li id="fn:93">"Scientists Debate Planet Definition and Agree to Disagree", Planetary Science Institute press release of September 19, 2008, PSI.edu Archived July 15, 2011, at the Wayback Machine <a href="http://www.psi.edu/press/archive/20080919planetdebate/" target="_blank">http://www.psi.edu/press/archive/20080919planetdebate/</a> <a href="#fnref:93" class="footnote-back-ref">↩</a></li>
<li id="fn:94">"Plutoid chosen as name for Solar System objects like Pluto". Paris: International Astronomical Union (News Release – IAU0804). June 11, 2008. Archived from the original on August 10, 2011. Retrieved December 1, 2011. <a href="http://www.iau.org/news/pressreleases/detail/iau0804/" target="_blank">http://www.iau.org/news/pressreleases/detail/iau0804/</a> <a href="#fnref:94" class="footnote-back-ref">↩</a></li>
<li id="fn:95">"Plutoids Join the Solar Family", Discover Magazine, January 2009, p. 76 <a href="#fnref:95" class="footnote-back-ref">↩</a></li>
<li id="fn:96">Science News, July 5, 2008, p. 7 <a href="#fnref:96" class="footnote-back-ref">↩</a></li>
<li id="fn:97">"Horizon Online Ephemeris System for Pluto Barycenter". JPL Horizons On-Line Ephemeris System @ Solar System Dynamics Group. Archived from the original on May 10, 2011. Retrieved January 16, 2011. (Observer Location @sun with the observer at the center of the Sun) <a href="https://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=9" target="_blank">https://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=9</a> <a href="#fnref:97" class="footnote-back-ref">↩</a></li>
<li id="fn:98">The discovery of Charon in 1978 allowed astronomers to accurately calculate the mass of the Plutonian system. But it did not indicate the two bodies' individual masses, which could only be estimated after other moons of Pluto were discovered in late 2005. As a result, because Pluto came to perihelion in 1989, most Pluto perihelion date estimates are based on the Pluto–Charon barycenter. Charon came to perihelion 4 September 1989. The Pluto–Charon barycenter came to perihelion 5 September 1989. Pluto came to perihelion 8 September 1989. <a href="/wiki/Barycenter" target="_blank">/wiki/Barycenter</a> <a href="#fnref:98" class="footnote-back-ref">↩</a></li>
<li id="fn:99">"Pluto to become most distant planet". JPL/NASA. January 28, 1999. Archived from the original on September 2, 2010. Retrieved January 16, 2011. <a href="https://web.archive.org/web/20100902073240/http://www.jpl.nasa.gov/releases/99/pluto990209.html" target="_blank">https://web.archive.org/web/20100902073240/http://www.jpl.nasa.gov/releases/99/pluto990209.html</a> <a href="#fnref:99" class="footnote-back-ref">↩</a></li>
<li id="fn:100">Sussman, Gerald Jay; Wisdom, Jack (1988). "Numerical evidence that the motion of Pluto is chaotic". Science. 241 (4864): 433–437. Bibcode:1988Sci...241..433S. doi:10.1126/science.241.4864.433. hdl:1721.1/6038. PMID 17792606. S2CID 1398095. Archived from the original on September 24, 2017. Retrieved May 16, 2018. <a href="https://web.archive.org/web/20170924164847/http://www.dtic.mil/get-tr-doc/pdf?AD=ADA195920" target="_blank">https://web.archive.org/web/20170924164847/http://www.dtic.mil/get-tr-doc/pdf?AD=ADA195920</a> <a href="#fnref:100" class="footnote-back-ref">↩</a></li>
<li id="fn:101">Wisdom, Jack; Holman, Matthew (1991). "Symplectic maps for the n-body problem". Astronomical Journal. 102: 1528–1538. Bibcode:1991AJ....102.1528W. doi:10.1086/115978. Archived from the original on July 10, 2021. Retrieved October 18, 2021. <a href="http://cdsads.u-strasbg.fr/pdf/1991AJ....102.1528W" target="_blank">http://cdsads.u-strasbg.fr/pdf/1991AJ....102.1528W</a> <a href="#fnref:101" class="footnote-back-ref">↩</a></li>
<li id="fn:102">Williams, James G.; Benson, G. S. (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal. 76: 167. Bibcode:1971AJ.....76..167W. doi:10.1086/111100. S2CID 120122522. <a href="https://doi.org/10.1086%2F111100" target="_blank">https://doi.org/10.1086%2F111100</a> <a href="#fnref:102" class="footnote-back-ref">↩</a></li>
<li id="fn:103">Wan, Xiao-Sheng; Huang, Tian-Yi; Innanen, Kim A. (2001). "The 1:1 Superresonance in Pluto's Motion". The Astronomical Journal. 121 (2): 1155–1162. Bibcode:2001AJ....121.1155W. doi:10.1086/318733. <a href="https://doi.org/10.1086%2F318733" target="_blank">https://doi.org/10.1086%2F318733</a> <a href="#fnref:103" class="footnote-back-ref">↩</a></li>
<li id="fn:104">Hunter, Maxwell W. (2004). "Unmanned scientific exploration throughout the Solar System". Space Science Reviews. 6 (5): 501. Bibcode:1967SSRv....6..601H. doi:10.1007/BF00168793. S2CID 125982610. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:104" class="footnote-back-ref">↩</a></li>
<li id="fn:105">Malhotra, Renu (1997). "Pluto's Orbit". Archived from the original on July 31, 2019. Retrieved March 26, 2007. <a href="http://www.nineplanets.org/plutodyn.html" target="_blank">http://www.nineplanets.org/plutodyn.html</a> <a href="#fnref:105" class="footnote-back-ref">↩</a></li>
<li id="fn:106">Because of the eccentricity of Pluto's orbit, some have theorized that it was once a satellite of Neptune.[100] <a href="/wiki/Satellite_of_Neptune" target="_blank">/wiki/Satellite_of_Neptune</a> <a href="#fnref:106" class="footnote-back-ref">↩</a></li>
<li id="fn:107">Williams, James G.; Benson, G. S. (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal. 76: 167. Bibcode:1971AJ.....76..167W. doi:10.1086/111100. S2CID 120122522. <a href="https://doi.org/10.1086%2F111100" target="_blank">https://doi.org/10.1086%2F111100</a> <a href="#fnref:107" class="footnote-back-ref">↩</a></li>
<li id="fn:108">The ecliptic longitude of Pluto Archived February 13, 2024, at the Wayback Machine and of Neptune Archived February 13, 2024, at the Wayback Machine are available from the JPL Horizons On-Line Ephemeris System. <a href="https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27" target="_blank">https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27</a> <a href="#fnref:108" class="footnote-back-ref">↩</a></li>
<li id="fn:109">Malhotra, Renu (1997). "Pluto's Orbit". Archived from the original on July 31, 2019. Retrieved March 26, 2007. <a href="http://www.nineplanets.org/plutodyn.html" target="_blank">http://www.nineplanets.org/plutodyn.html</a> <a href="#fnref:109" class="footnote-back-ref">↩</a></li>
<li id="fn:110">Williams, James G.; Benson, G. S. (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal. 76: 167. Bibcode:1971AJ.....76..167W. doi:10.1086/111100. S2CID 120122522. <a href="https://doi.org/10.1086%2F111100" target="_blank">https://doi.org/10.1086%2F111100</a> <a href="#fnref:110" class="footnote-back-ref">↩</a></li>
<li id="fn:111">Alfvén, Hannes; Arrhenius, Gustaf (1976). "SP-345 Evolution of the Solar System". Archived from the original on May 13, 2007. Retrieved March 28, 2007. <a href="https://history.nasa.gov/SP-345/ch8.htm" target="_blank">https://history.nasa.gov/SP-345/ch8.htm</a> <a href="#fnref:111" class="footnote-back-ref">↩</a></li>
<li id="fn:112">Malhotra, Renu (1997). "Pluto's Orbit". Archived from the original on July 31, 2019. Retrieved March 26, 2007. <a href="http://www.nineplanets.org/plutodyn.html" target="_blank">http://www.nineplanets.org/plutodyn.html</a> <a href="#fnref:112" class="footnote-back-ref">↩</a></li>
<li id="fn:113">The ecliptic longitude of Pluto Archived February 13, 2024, at the Wayback Machine and of Neptune Archived February 13, 2024, at the Wayback Machine are available from the JPL Horizons On-Line Ephemeris System. <a href="https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27" target="_blank">https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27</a> <a href="#fnref:113" class="footnote-back-ref">↩</a></li>
<li id="fn:114">Malhotra, Renu (1997). "Pluto's Orbit". Archived from the original on July 31, 2019. Retrieved March 26, 2007. <a href="http://www.nineplanets.org/plutodyn.html" target="_blank">http://www.nineplanets.org/plutodyn.html</a> <a href="#fnref:114" class="footnote-back-ref">↩</a></li>
<li id="fn:115">Alfvén, Hannes; Arrhenius, Gustaf (1976). "SP-345 Evolution of the Solar System". Archived from the original on May 13, 2007. Retrieved March 28, 2007. <a href="https://history.nasa.gov/SP-345/ch8.htm" target="_blank">https://history.nasa.gov/SP-345/ch8.htm</a> <a href="#fnref:115" class="footnote-back-ref">↩</a></li>
<li id="fn:116">Cohen, C. J.; Hubbard, E. C. (1965). "Libration of the close approaches of Pluto to Neptune". Astronomical Journal. 70: 10. Bibcode:1965AJ.....70...10C. doi:10.1086/109674. <a href="https://doi.org/10.1086%2F109674" target="_blank">https://doi.org/10.1086%2F109674</a> <a href="#fnref:116" class="footnote-back-ref">↩</a></li>
<li id="fn:117">Wan, Xiao-Sheng; Huang, Tian-Yi; Innanen, Kim A. (2001). "The 1:1 Superresonance in Pluto's Motion". The Astronomical Journal. 121 (2): 1155–1162. Bibcode:2001AJ....121.1155W. doi:10.1086/318733. <a href="https://doi.org/10.1086%2F318733" target="_blank">https://doi.org/10.1086%2F318733</a> <a href="#fnref:117" class="footnote-back-ref">↩</a></li>
<li id="fn:118">Williams, James G.; Benson, G. S. (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal. 76: 167. Bibcode:1971AJ.....76..167W. doi:10.1086/111100. S2CID 120122522. <a href="https://doi.org/10.1086%2F111100" target="_blank">https://doi.org/10.1086%2F111100</a> <a href="#fnref:118" class="footnote-back-ref">↩</a></li>
<li id="fn:119">Williams, James G.; Benson, G. S. (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal. 76: 167. Bibcode:1971AJ.....76..167W. doi:10.1086/111100. S2CID 120122522. <a href="https://doi.org/10.1086%2F111100" target="_blank">https://doi.org/10.1086%2F111100</a> <a href="#fnref:119" class="footnote-back-ref">↩</a></li>
<li id="fn:120">Wan, Xiao-Sheng; Huang, Tian-Yi; Innanen, Kim A. (2001). "The 1:1 Superresonance in Pluto's Motion". The Astronomical Journal. 121 (2): 1155–1162. Bibcode:2001AJ....121.1155W. doi:10.1086/318733. <a href="https://doi.org/10.1086%2F318733" target="_blank">https://doi.org/10.1086%2F318733</a> <a href="#fnref:120" class="footnote-back-ref">↩</a></li>
<li id="fn:121">Alfvén, Hannes; Arrhenius, Gustaf (1976). "SP-345 Evolution of the Solar System". Archived from the original on May 13, 2007. Retrieved March 28, 2007. <a href="https://history.nasa.gov/SP-345/ch8.htm" target="_blank">https://history.nasa.gov/SP-345/ch8.htm</a> <a href="#fnref:121" class="footnote-back-ref">↩</a></li>
<li id="fn:122">Wan, Xiao-Sheng; Huang, Tian-Yi; Innanen, Kim A. (2001). "The 1:1 Superresonance in Pluto's Motion". The Astronomical Journal. 121 (2): 1155–1162. Bibcode:2001AJ....121.1155W. doi:10.1086/318733. <a href="https://doi.org/10.1086%2F318733" target="_blank">https://doi.org/10.1086%2F318733</a> <a href="#fnref:122" class="footnote-back-ref">↩</a></li>
<li id="fn:123">Williams, David R. (July 24, 2015). "Pluto Fact Sheet". NASA. Archived from the original on November 19, 2015. Retrieved August 6, 2015. <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html" target="_blank">https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html</a> <a href="#fnref:123" class="footnote-back-ref">↩</a></li>
<li id="fn:124">Faure, Gunter; Mensing, Teresa M. (2007). "Pluto and Charon: The Odd Couple". Introduction to Planetary Science. Springer. pp. 401–408. doi:10.1007/978-1-4020-5544-7. ISBN 978-1-4020-5544-7. <a href="978-1-4020-5544-7" target="_blank">978-1-4020-5544-7</a> <a href="#fnref:124" class="footnote-back-ref">↩</a></li>
<li id="fn:125">Schombert, Jim; University of Oregon Astronomy 121 Lecture notes Archived July 23, 2011, at the Wayback Machine, Pluto Orientation diagram Archived March 25, 2009, at the Wayback Machine <a href="http://abyss.uoregon.edu/~js/ast121/lectures/lec21.html" target="_blank">http://abyss.uoregon.edu/~js/ast121/lectures/lec21.html</a> <a href="#fnref:125" class="footnote-back-ref">↩</a></li>
<li id="fn:126">Kirschvink, Joseph L.; Ripperdan, Robert L.; Evans, David A. (July 25, 1997). "Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander". Science. 277 (5325): 541–545. doi:10.1126/science.277.5325.541. ISSN 0036-8075. S2CID 177135895. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:126" class="footnote-back-ref">↩</a></li>
<li id="fn:127">Keane, James T.; Matsuyama, Isamu; Kamata, Shunichi; Steckloff, Jordan K. (2016). "Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia". Nature. 540 (7631): 90–93. Bibcode:2016Natur.540...90K. doi:10.1038/nature20120. PMID 27851731. S2CID 4468636. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:127" class="footnote-back-ref">↩</a></li>
<li id="fn:128">Owen, Tobias C.; Roush, Ted L.; Cruikshank, Dale P.; et al. (1993). "Surface Ices and the Atmospheric Composition of Pluto". Science. 261 (5122): 745–748. Bibcode:1993Sci...261..745O. doi:10.1126/science.261.5122.745. JSTOR 2882241. PMID 17757212. S2CID 6039266. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:128" class="footnote-back-ref">↩</a></li>
<li id="fn:129">Grundy, W.M.; Olkin, C.B.; Young, L.A.; Buie, M.W.; Young, E.F. (2013). "Near-infrared spectral monitoring of Pluto's ices: Spatial distribution and secular evolution" (PDF). Icarus. 223 (2): 710–721. arXiv:1301.6284. Bibcode:2013Icar..223..710G. doi:10.1016/j.icarus.2013.01.019. S2CID 26293543. Archived from the original (PDF) on November 8, 2015. <a href="https://web.archive.org/web/20151108210124/http://www2.lowell.edu/~grundy/abstracts/preprints/2013.Pluto_SpeX.pdf" target="_blank">https://web.archive.org/web/20151108210124/http://www2.lowell.edu/~grundy/abstracts/preprints/2013.Pluto_SpeX.pdf</a> <a href="#fnref:129" class="footnote-back-ref">↩</a></li>
<li id="fn:130">Drake, Nadia (November 9, 2015). "Floating Mountains on Pluto – You Can't Make This Stuff Up". National Geographic. Archived from the original on November 13, 2015. Retrieved December 23, 2016. <a href="/wiki/Nadia_Drake" target="_blank">/wiki/Nadia_Drake</a> <a href="#fnref:130" class="footnote-back-ref">↩</a></li>
<li id="fn:131">Buie, Marc W.; Grundy, William M.; Young, Eliot F.; et al. (2010). "Pluto and Charon with the Hubble Space Telescope: I. Monitoring global change and improved surface properties from light curves". Astronomical Journal. 139 (3): 1117–1127. Bibcode:2010AJ....139.1117B. CiteSeerX 10.1.1.625.7795. doi:10.1088/0004-6256/139/3/1117. S2CID 1725219. Archived from the original on July 20, 2015. Retrieved February 10, 2010. <a href="http://www.boulder.swri.edu/~buie/biblio/pub072.html" target="_blank">http://www.boulder.swri.edu/~buie/biblio/pub072.html</a> <a href="#fnref:131" class="footnote-back-ref">↩</a></li>
<li id="fn:132">Buie, Marc W. "Pluto map information". Archived from the original on June 29, 2011. Retrieved February 10, 2010. <a href="https://web.archive.org/web/20110629005310/http://www.boulder.swri.edu/~buie/pluto/hrcmap.html" target="_blank">https://web.archive.org/web/20110629005310/http://www.boulder.swri.edu/~buie/pluto/hrcmap.html</a> <a href="#fnref:132" class="footnote-back-ref">↩</a></li>
<li id="fn:133">Villard, Ray; Buie, Marc W. (February 4, 2010). "New Hubble Maps of Pluto Show Surface Changes". News Release Number: STScI-2010-06. Archived from the original on September 1, 2016. Retrieved February 10, 2010. <a href="http://hubblesite.org/newscenter/archive/releases/2010/06/full/" target="_blank">http://hubblesite.org/newscenter/archive/releases/2010/06/full/</a> <a href="#fnref:133" class="footnote-back-ref">↩</a></li>
<li id="fn:134">Buie, Marc W.; Grundy, William M.; Young, Eliot F.; et al. (2010). "Pluto and Charon with the Hubble Space Telescope: II. Resolving changes on Pluto's surface and a map for Charon". Astronomical Journal. 139 (3): 1128–1143. Bibcode:2010AJ....139.1128B. CiteSeerX 10.1.1.625.7795. doi:10.1088/0004-6256/139/3/1128. S2CID 9343680. Archived from the original on July 7, 2015. Retrieved February 10, 2010. <a href="http://www.boulder.swri.edu/~buie/biblio/pub073.html" target="_blank">http://www.boulder.swri.edu/~buie/biblio/pub073.html</a> <a href="#fnref:134" class="footnote-back-ref">↩</a></li>
<li id="fn:135">Stern, S.A.; Grundy, W.; McKinnon, W.B.; Weaver, H.A.; Young, L.A. (2017). "The Pluto System After New Horizons". Annual Review of Astronomy and Astrophysics. 2018: 357–392. arXiv:1712.05669. Bibcode:2018ARA&A..56..357S. doi:10.1146/annurev-astro-081817-051935. ISSN 0066-4146. S2CID 119072504. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:135" class="footnote-back-ref">↩</a></li>
<li id="fn:136">Lakdawalla, Emily (October 26, 2016). "DPS/EPSC update on New Horizons at the Pluto system and beyond". The Planetary Society. Archived from the original on October 8, 2018. Retrieved October 26, 2016. <a href="/wiki/Emily_Lakdawalla" target="_blank">/wiki/Emily_Lakdawalla</a> <a href="#fnref:136" class="footnote-back-ref">↩</a></li>
<li id="fn:137">McKinnon, W. B.; Nimmo, F.; Wong, T.; Schenk, P. M.; White, O. L.; et al. (June 1, 2016). "Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigour". Nature. 534 (7605): 82–85. arXiv:1903.05571. Bibcode:2016Natur.534...82M. doi:10.1038/nature18289. PMID 27251279. S2CID 30903520. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:137" class="footnote-back-ref">↩</a></li>
<li id="fn:138">Trowbridge, A. J.; Melosh, H. J.; Steckloff, J. K.; Freed, A. M. (June 1, 2016). "Vigorous convection as the explanation for Pluto's polygonal terrain". Nature. 534 (7605): 79–81. Bibcode:2016Natur.534...79T. doi:10.1038/nature18016. PMID 27251278. S2CID 6743360. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:138" class="footnote-back-ref">↩</a></li>
<li id="fn:139">Lakdawalla, Emily (December 21, 2015). "Pluto updates from AGU and DPS: Pretty pictures from a confusing world". The Planetary Society. Archived from the original on December 24, 2015. Retrieved January 24, 2016. <a href="/wiki/Emily_Lakdawalla" target="_blank">/wiki/Emily_Lakdawalla</a> <a href="#fnref:139" class="footnote-back-ref">↩</a></li>
<li id="fn:140">Umurhan, O. (January 8, 2016). "Probing the Mysterious Glacial Flow on Pluto's Frozen 'Heart'". blogs.nasa.gov. NASA. Archived from the original on April 19, 2016. Retrieved January 24, 2016. <a href="https://blogs.nasa.gov/pluto/2016/01/08/probing-the-mysterious-glacial-flow-on-plutos-frozen-heart/" target="_blank">https://blogs.nasa.gov/pluto/2016/01/08/probing-the-mysterious-glacial-flow-on-plutos-frozen-heart/</a> <a href="#fnref:140" class="footnote-back-ref">↩</a></li>
<li id="fn:141">Marchis, F.; Trilling, D. E. (January 20, 2016). "The Surface Age of Sputnik Planum, Pluto, Must Be Less than 10 Million Years". PLOS ONE. 11 (1): e0147386. arXiv:1601.02833. Bibcode:2016PLoSO..1147386T. doi:10.1371/journal.pone.0147386. PMC 4720356. PMID 26790001. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4720356" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4720356</a> <a href="#fnref:141" class="footnote-back-ref">↩</a></li>
<li id="fn:142">Buhler, P. B.; Ingersoll, A. P. (March 23, 2017). "Sublimation pit distribution indicates convection cell surface velocity of ~10 centimeters per year in Sputnik Planitia, Pluto" (PDF). 48th Lunar and Planetary Science Conference. Archived (PDF) from the original on August 13, 2017. Retrieved May 11, 2017. <a href="https://www.hou.usra.edu/meetings/lpsc2017/pdf/1746.pdf" target="_blank">https://www.hou.usra.edu/meetings/lpsc2017/pdf/1746.pdf</a> <a href="#fnref:142" class="footnote-back-ref">↩</a></li>
<li id="fn:143">Stern, S.A.; et al. (2015). "The Pluto system: Initial results from its exploration by New Horizons". Science. 350 (6258): 249–352. arXiv:1510.07704. Bibcode:2015Sci...350.1815S. doi:10.1126/science.aad1815. PMID 26472913. S2CID 1220226. <a href="/wiki/Science_(journal)" target="_blank">/wiki/Science_(journal)</a> <a href="#fnref:143" class="footnote-back-ref">↩</a></li>
<li id="fn:144">Telfer, Matt W; Parteli, Eric J. R; Radebaugh, Jani; Beyer, Ross A; Bertrand, Tanguy; Forget, François; Nimmo, Francis; Grundy, Will M; Moore, Jeffrey M; Stern, S. Alan; Spencer, John; Lauer, Tod R; Earle, Alissa M; Binzel, Richard P; Weaver, Hal A; Olkin, Cathy B; Young, Leslie A; Ennico, Kimberly; Runyon, Kirby (2018). "Dunes on Pluto" (PDF). Science. 360 (6392): 992–997. Bibcode:2018Sci...360..992T. doi:10.1126/science.aao2975. PMID 29853681. S2CID 44159592. Archived (PDF) from the original on October 23, 2020. Retrieved April 12, 2020. <a href="https://pearl.plymouth.ac.uk/bitstream/handle/10026.1/11613/UoP_Deposit_Agreement%20v1.1%2020160217.pdf?sequence=2&isAllowed=y" target="_blank">https://pearl.plymouth.ac.uk/bitstream/handle/10026.1/11613/UoP_Deposit_Agreement%20v1.1%2020160217.pdf?sequence=2&isAllowed=y</a> <a href="#fnref:144" class="footnote-back-ref">↩</a></li>
<li id="fn:145">Brozović, Marina; Jacobson, Robert A. (May 8, 2024). "Post-new-horizons Orbits and Masses for the Satellites of Pluto". The Astronomical Journal. 167 (256): 256. Bibcode:2024AJ....167..256B. doi:10.3847/1538-3881/ad39f0. <a href="https://doi.org/10.3847%2F1538-3881%2Fad39f0" target="_blank">https://doi.org/10.3847%2F1538-3881%2Fad39f0</a> <a href="#fnref:145" class="footnote-back-ref">↩</a></li>
<li id="fn:146">Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus. 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005. Archived (PDF) from the original on August 31, 2015. Retrieved October 25, 2018. <a href="https://www.researchgate.net/publication/225019299" target="_blank">https://www.researchgate.net/publication/225019299</a> <a href="#fnref:146" class="footnote-back-ref">↩</a></li>
<li id="fn:147">Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus. 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005. Archived (PDF) from the original on August 31, 2015. Retrieved October 25, 2018. <a href="https://www.researchgate.net/publication/225019299" target="_blank">https://www.researchgate.net/publication/225019299</a> <a href="#fnref:147" class="footnote-back-ref">↩</a></li>
<li id="fn:148">"The Inside Story". pluto.jhuapl.edu – NASA New Horizons mission site. Johns Hopkins University Applied Physics Laboratory. 2007. Archived from the original on May 16, 2008. Retrieved February 15, 2014. <a href="https://web.archive.org/web/20080516222133/http://pluto.jhuapl.edu/science/everything_pluto/9_insideStory.html" target="_blank">https://web.archive.org/web/20080516222133/http://pluto.jhuapl.edu/science/everything_pluto/9_insideStory.html</a> <a href="#fnref:148" class="footnote-back-ref">↩</a></li>
<li id="fn:149">Overlooked Ocean Worlds Fill the Outer Solar System Archived December 26, 2018, at the Wayback Machine. John Wenz, Scientific American. October 4, 2017. <a href="https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/" target="_blank">https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/</a> <a href="#fnref:149" class="footnote-back-ref">↩</a></li>
<li id="fn:150">Samantha Cole. "An Incredibly Deep Ocean Could Be Hiding Beneath Pluto's Icy Heart". Popular Science. Archived from the original on September 27, 2016. Retrieved September 24, 2016. <a href="http://www.popsci.com/an-incredibly-deep-ocean-could-be-hiding-beneath-plutos-icy-heart" target="_blank">http://www.popsci.com/an-incredibly-deep-ocean-could-be-hiding-beneath-plutos-icy-heart</a> <a href="#fnref:150" class="footnote-back-ref">↩</a></li>
<li id="fn:151">Rabie, Passant (June 22, 2020). "New Evidence Suggests Something Strange and Surprising about Pluto — The findings will make scientists rethink the habitability of Kuiper Belt objects". Inverse. Archived from the original on June 23, 2020. Retrieved June 23, 2020. <a href="https://www.inverse.com/science/pluto-hot-star" target="_blank">https://www.inverse.com/science/pluto-hot-star</a> <a href="#fnref:151" class="footnote-back-ref">↩</a></li>
<li id="fn:152">Bierson, Carver; et al. (June 22, 2020). "Evidence for a hot start and early ocean formation on Pluto". Nature Geoscience. 769 (7): 468–472. Bibcode:2020NatGe..13..468B. doi:10.1038/s41561-020-0595-0. S2CID 219976751. Archived from the original on June 22, 2020. Retrieved June 23, 2020. <a href="https://www.nature.com/articles/s41561-020-0595-0" target="_blank">https://www.nature.com/articles/s41561-020-0595-0</a> <a href="#fnref:152" class="footnote-back-ref">↩</a></li>
<li id="fn:153">Singer, Kelsi N. (March 29, 2022). "Large-scale cryovolcanic resurfacing on Pluto". Nature Communications. 13 (1): 1542. arXiv:2207.06557. Bibcode:2022NatCo..13.1542S. doi:10.1038/s41467-022-29056-3. PMC 8964750. PMID 35351895. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8964750" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8964750</a> <a href="#fnref:153" class="footnote-back-ref">↩</a></li>
<li id="fn:154">Nimmo, Francis; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus. 287: 12–29. arXiv:1603.00821. Bibcode:2017Icar..287...12N. doi:10.1016/j.icarus.2016.06.027. S2CID 44935431. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:154" class="footnote-back-ref">↩</a></li>
<li id="fn:155">Davies, John (2001). "Beyond Pluto (extract)" (PDF). Royal Observatory, Edinburgh. Archived from the original (PDF) on July 15, 2011. Retrieved March 26, 2007. <a href="https://web.archive.org/web/20110715233608/http://assets.cambridge.org/052180/0196/excerpt/0521800196_excerpt.pdf" target="_blank">https://web.archive.org/web/20110715233608/http://assets.cambridge.org/052180/0196/excerpt/0521800196_excerpt.pdf</a> <a href="#fnref:155" class="footnote-back-ref">↩</a></li>
<li id="fn:156">Williams, David R. (July 24, 2015). "Pluto Fact Sheet". NASA. Archived from the original on November 19, 2015. Retrieved August 6, 2015. <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html" target="_blank">https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html</a> <a href="#fnref:156" class="footnote-back-ref">↩</a></li>
<li id="fn:157">Nimmo, Francis; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus. 287: 12–29. arXiv:1603.00821. Bibcode:2017Icar..287...12N. doi:10.1016/j.icarus.2016.06.027. S2CID 44935431. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:157" class="footnote-back-ref">↩</a></li>
<li id="fn:158">"How Big Is Pluto? New Horizons Settles Decades-Long Debate". NASA. July 13, 2015. Archived from the original on July 1, 2017. Retrieved July 13, 2015. <a href="https://web.archive.org/web/20170701005734/https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate/" target="_blank">https://web.archive.org/web/20170701005734/https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate/</a> <a href="#fnref:158" class="footnote-back-ref">↩</a></li>
<li id="fn:159">"Pluto and Charon | Astronomy". courses.lumenlearning.com. Archived from the original on March 24, 2022. Retrieved April 6, 2022. For a long time, it was thought that the mass of Pluto was similar to that of Earth, so that it was classed as a fifth terrestrial planet, somehow misplaced in the far outer reaches of the solar system. There were other anomalies, however, as Pluto's orbit was more eccentric and inclined to the plane of our solar system than that of any other planet. Only after the discovery of its moon Charon in 1978 could the mass of Pluto be measured, and it turned out to be far less than the mass of Earth. <a href="https://courses.lumenlearning.com/astronomy/chapter/pluto-and-charon/" target="_blank">https://courses.lumenlearning.com/astronomy/chapter/pluto-and-charon/</a> <a href="#fnref:159" class="footnote-back-ref">↩</a></li>
<li id="fn:160">Close, Laird M.; Merline, William J.; Tholen, David J.; et al. (2000). "Adaptive optics imaging of Pluto-Charon and the discovery of a moon around the Asteroid 45 Eugenia: The potential of adaptive optics in planetary astronomy". In Wizinowich, Peter L. (ed.). Adaptive Optical Systems Technology. Vol. 4007. pp. 787–795. Bibcode:2000SPIE.4007..787C. doi:10.1117/12.390379. S2CID 122678656. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:160" class="footnote-back-ref">↩</a></li>
<li id="fn:161">Young, Eliot F.; Young, Leslie A.; Buie, Marc W. (2007). "Pluto's Radius". American Astronomical Society, DPS Meeting No. 39, #62.05; Bulletin of the American Astronomical Society. 39: 541. Bibcode:2007DPS....39.6205Y. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:161" class="footnote-back-ref">↩</a></li>
<li id="fn:162">Brown, Michael E. (November 22, 2010). "How big is Pluto, anyway?". Mike Brown's Planets. Archived from the original on July 21, 2011. Retrieved June 9, 2015. (Franck Marchis on 8 November 2010)[dead link] <a href="http://www.mikebrownsplanets.com/2010/11/how-big-is-pluto-anyway.html" target="_blank">http://www.mikebrownsplanets.com/2010/11/how-big-is-pluto-anyway.html</a> <a href="#fnref:162" class="footnote-back-ref">↩</a></li>
<li id="fn:163">Lellouch, Emmanuel; de Bergh, Catherine; Sicardy, Bruno; et al. (January 15, 2015). "Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere". Icarus. 246: 268–278. arXiv:1403.3208. Bibcode:2015Icar..246..268L. doi:10.1016/j.icarus.2014.03.027. S2CID 119194193. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:163" class="footnote-back-ref">↩</a></li>
<li id="fn:164">"How Big Is Pluto? New Horizons Settles Decades-Long Debate". NASA. July 13, 2015. Archived from the original on July 1, 2017. Retrieved July 13, 2015. <a href="https://web.archive.org/web/20170701005734/https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate/" target="_blank">https://web.archive.org/web/20170701005734/https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate/</a> <a href="#fnref:164" class="footnote-back-ref">↩</a></li>
<li id="fn:165">Lakdawalla, Emily (July 13, 2015). "Pluto minus one day: Very first New Horizons Pluto encounter science results". The Planetary Society. Archived from the original on March 2, 2020. Retrieved July 13, 2015. <a href="http://www.planetary.org/blogs/emily-lakdawalla/2015/07131311-pluto-first-science.html" target="_blank">http://www.planetary.org/blogs/emily-lakdawalla/2015/07131311-pluto-first-science.html</a> <a href="#fnref:165" class="footnote-back-ref">↩</a></li>
<li id="fn:166">NASA's New Horizons Team Reveals New Scientific Findings on Pluto. NASA. July 24, 2015. Event occurs at 52:30. Archived from the original on October 28, 2021. Retrieved July 30, 2015. We had an uncertainty that ranged over maybe 70 kilometers, we've collapsed that to plus and minus two, and it's centered around 1186 <a href="https://ghostarchive.org/varchive/youtube/20211028/dWr29KIs2Ns" target="_blank">https://ghostarchive.org/varchive/youtube/20211028/dWr29KIs2Ns</a> <a href="#fnref:166" class="footnote-back-ref">↩</a></li>
<li id="fn:167">Stern, S.A.; et al. (2015). "The Pluto system: Initial results from its exploration by New Horizons". Science. 350 (6258): 249–352. arXiv:1510.07704. Bibcode:2015Sci...350.1815S. doi:10.1126/science.aad1815. PMID 26472913. S2CID 1220226. <a href="/wiki/Science_(journal)" target="_blank">/wiki/Science_(journal)</a> <a href="#fnref:167" class="footnote-back-ref">↩</a></li>
<li id="fn:168">Nimmo, Francis; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus. 287: 12–29. arXiv:1603.00821. Bibcode:2017Icar..287...12N. doi:10.1016/j.icarus.2016.06.027. S2CID 44935431. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:168" class="footnote-back-ref">↩</a></li>
<li id="fn:169">"Conditions on Pluto: Incredibly Hazy With Flowing Ice". New York Times. July 24, 2015. Archived from the original on July 28, 2015. Retrieved July 24, 2015. <a href="https://www.nytimes.com/aponline/2015/07/24/science/ap-us-sci-pluto.html" target="_blank">https://www.nytimes.com/aponline/2015/07/24/science/ap-us-sci-pluto.html</a> <a href="#fnref:169" class="footnote-back-ref">↩</a></li>
<li id="fn:170">Croswell, Ken (1992). "Nitrogen in Pluto's Atmosphere". KenCroswell.com. New Scientist. Archived from the original on May 11, 2020. Retrieved April 27, 2007. <a href="http://www.kencroswell.com/NitrogenInPlutosAtmosphere.html" target="_blank">http://www.kencroswell.com/NitrogenInPlutosAtmosphere.html</a> <a href="#fnref:170" class="footnote-back-ref">↩</a></li>
<li id="fn:171">Stern, S.A.; et al. (2015). "The Pluto system: Initial results from its exploration by New Horizons". Science. 350 (6258): 249–352. arXiv:1510.07704. Bibcode:2015Sci...350.1815S. doi:10.1126/science.aad1815. PMID 26472913. S2CID 1220226. <a href="/wiki/Science_(journal)" target="_blank">/wiki/Science_(journal)</a> <a href="#fnref:171" class="footnote-back-ref">↩</a></li>
<li id="fn:172">Olkin, C.B.; Young, L.A.; Borncamp, D.; et al. (January 2015). "Evidence that Pluto's atmosphere does not collapse from occultations including the 2013 May 04 event". Icarus. 246: 220–225. Bibcode:2015Icar..246..220O. doi:10.1016/j.icarus.2014.03.026. hdl:10261/167246. Archived from the original on September 29, 2021. Retrieved September 8, 2017. <a href="https://www.researchgate.net/publication/262937542" target="_blank">https://www.researchgate.net/publication/262937542</a> <a href="#fnref:172" class="footnote-back-ref">↩</a></li>
<li id="fn:173">Kelly Beatty (2016). "Pluto's Atmosphere Confounds Researchers". Sky & Telescope. Archived from the original on April 7, 2016. Retrieved April 2, 2016. <a href="http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/" target="_blank">http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/</a> <a href="#fnref:173" class="footnote-back-ref">↩</a></li>
<li id="fn:174">Kelly Beatty (2016). "Pluto's Atmosphere Confounds Researchers". Sky & Telescope. Archived from the original on April 7, 2016. Retrieved April 2, 2016. <a href="http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/" target="_blank">http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/</a> <a href="#fnref:174" class="footnote-back-ref">↩</a></li>
<li id="fn:175">Kelly Beatty (2016). "Pluto's Atmosphere Confounds Researchers". Sky & Telescope. Archived from the original on April 7, 2016. Retrieved April 2, 2016. <a href="http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/" target="_blank">http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/</a> <a href="#fnref:175" class="footnote-back-ref">↩</a></li>
<li id="fn:176">Than, Ker (2006). "Astronomers: Pluto colder than expected". Space.com. Archived from the original on October 19, 2012. Retrieved November 30, 2011 – via CNN. <a href="http://www.cnn.com/2006/TECH/space/01/03/pluto.temp/index.html" target="_blank">http://www.cnn.com/2006/TECH/space/01/03/pluto.temp/index.html</a> <a href="#fnref:176" class="footnote-back-ref">↩</a></li>
<li id="fn:177">Gladstone, G. R.; Stern, S. A.; Ennico, K.; et al. (March 2016). "The atmosphere of Pluto as observed by New Horizons" (PDF). Science. 351 (6279): aad8866. arXiv:1604.05356. Bibcode:2016Sci...351.8866G. doi:10.1126/science.aad8866. PMID 26989258. S2CID 32043359. Archived from the original (PDF) on May 21, 2016. Retrieved June 12, 2016. (Supplementary Material) <a href="https://web.archive.org/web/20160521090831/https://www.astro.umd.edu/~dphamil/research/reprints/GlaSteEnn16.pdf" target="_blank">https://web.archive.org/web/20160521090831/https://www.astro.umd.edu/~dphamil/research/reprints/GlaSteEnn16.pdf</a> <a href="#fnref:177" class="footnote-back-ref">↩</a></li>
<li id="fn:178">"What is happening to Pluto's Atmosphere". May 22, 2020. Archived from the original on October 24, 2021. Retrieved October 7, 2021. <a href="https://astronomy.com/news/2020/05/plutos-strange-atmosphere-just-collapsed" target="_blank">https://astronomy.com/news/2020/05/plutos-strange-atmosphere-just-collapsed</a> <a href="#fnref:178" class="footnote-back-ref">↩</a></li>
<li id="fn:179">"SwRI Scientists Confirm Decrease In Pluto's Atmospheric Density". Southwest Research Institute. October 4, 2021. Archived from the original on October 15, 2021. Retrieved October 7, 2021. <a href="https://www.swri.org/press-release/scientists-confirm-decrease-plutos-atmospheric-density" target="_blank">https://www.swri.org/press-release/scientists-confirm-decrease-plutos-atmospheric-density</a> <a href="#fnref:179" class="footnote-back-ref">↩</a></li>
<li id="fn:180">Lellouch, Emmanuel; Sicardy, Bruno; de Bergh, Catherine; et al. (2009). "Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations". Astronomy and Astrophysics. 495 (3): L17 – L21. arXiv:0901.4882. Bibcode:2009A&A...495L..17L. doi:10.1051/0004-6361/200911633. S2CID 17779043. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:180" class="footnote-back-ref">↩</a></li>
<li id="fn:181">Kelly Beatty (2016). "Pluto's Atmosphere Confounds Researchers". Sky & Telescope. Archived from the original on April 7, 2016. Retrieved April 2, 2016. <a href="http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/" target="_blank">http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/</a> <a href="#fnref:181" class="footnote-back-ref">↩</a></li>
<li id="fn:182">Stern, S.A.; et al. (2015). "The Pluto system: Initial results from its exploration by New Horizons". Science. 350 (6258): 249–352. arXiv:1510.07704. Bibcode:2015Sci...350.1815S. doi:10.1126/science.aad1815. PMID 26472913. S2CID 1220226. <a href="/wiki/Science_(journal)" target="_blank">/wiki/Science_(journal)</a> <a href="#fnref:182" class="footnote-back-ref">↩</a></li>
<li id="fn:183">Kelly Beatty (2016). "Pluto's Atmosphere Confounds Researchers". Sky & Telescope. Archived from the original on April 7, 2016. Retrieved April 2, 2016. <a href="http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/" target="_blank">http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/</a> <a href="#fnref:183" class="footnote-back-ref">↩</a></li>
<li id="fn:184">Gugliotta, Guy (November 1, 2005). "Possible New Moons for Pluto". The Washington Post. Archived from the original on October 20, 2012. Retrieved October 10, 2006. <a href="https://www.washingtonpost.com/wp-dyn/content/article/2005/10/31/AR2005103101426.html" target="_blank">https://www.washingtonpost.com/wp-dyn/content/article/2005/10/31/AR2005103101426.html</a> <a href="#fnref:184" class="footnote-back-ref">↩</a></li>
<li id="fn:185">"NASA's Hubble Discovers Another Moon Around Pluto". NASA. July 20, 2011. Archived from the original on May 12, 2020. Retrieved July 20, 2011. <a href="https://www.nasa.gov/mission_pages/hubble/science/pluto-moon.html" target="_blank">https://www.nasa.gov/mission_pages/hubble/science/pluto-moon.html</a> <a href="#fnref:185" class="footnote-back-ref">↩</a></li>
<li id="fn:186">Wall, Mike (July 11, 2012). "Pluto Has a Fifth Moon, Hubble Telescope Reveals". Space.com. Archived from the original on May 14, 2020. Retrieved July 11, 2012. <a href="http://www.space.com/16531-pluto-fifth-moon-hubble-discovery.html" target="_blank">http://www.space.com/16531-pluto-fifth-moon-hubble-discovery.html</a> <a href="#fnref:186" class="footnote-back-ref">↩</a></li>
<li id="fn:187">Buie, M.; Tholen, D.; Grundy, W. (2012). "The Orbit of Charon is Circular" (PDF). The Astronomical Journal. 144 (1): 15. Bibcode:2012AJ....144...15B. doi:10.1088/0004-6256/144/1/15. S2CID 15009477. Archived from the original (PDF) on April 12, 2020. <a href="https://web.archive.org/web/20200412141438/http://pdfs.semanticscholar.org/bfb8/1eb1887c28df5f5348a491cff7d4870e8c77.pdf" target="_blank">https://web.archive.org/web/20200412141438/http://pdfs.semanticscholar.org/bfb8/1eb1887c28df5f5348a491cff7d4870e8c77.pdf</a> <a href="#fnref:187" class="footnote-back-ref">↩</a></li>
<li id="fn:188">Showalter, M.R.; Hamilton, D.P. (June 3, 2015). "Resonant interactions and chaotic rotation of Pluto's small moons". Nature. 522 (7554): 45–49. Bibcode:2015Natur.522...45S. doi:10.1038/nature14469. PMID 26040889. S2CID 205243819. <a href="/wiki/Mark_R._Showalter" target="_blank">/wiki/Mark_R._Showalter</a> <a href="#fnref:188" class="footnote-back-ref">↩</a></li>
<li id="fn:189">Stern, S. Alan; Weaver, Harold A. Jr.; Steffl, Andrew J.; et al. (2005). "Characteristics and Origin of the Quadruple System at Pluto". arXiv:astro-ph/0512599. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:189" class="footnote-back-ref">↩</a></li>
<li id="fn:190">Showalter, M.R.; Hamilton, D.P. (June 3, 2015). "Resonant interactions and chaotic rotation of Pluto's small moons". Nature. 522 (7554): 45–49. Bibcode:2015Natur.522...45S. doi:10.1038/nature14469. PMID 26040889. S2CID 205243819. <a href="/wiki/Mark_R._Showalter" target="_blank">/wiki/Mark_R._Showalter</a> <a href="#fnref:190" class="footnote-back-ref">↩</a></li>
<li id="fn:191">Witze, Alexandra (2015). "Pluto's moons move in synchrony". Nature. doi:10.1038/nature.2015.17681. S2CID 134519717. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:191" class="footnote-back-ref">↩</a></li>
<li id="fn:192">Showalter, M.R.; Hamilton, D.P. (June 3, 2015). "Resonant interactions and chaotic rotation of Pluto's small moons". Nature. 522 (7554): 45–49. Bibcode:2015Natur.522...45S. doi:10.1038/nature14469. PMID 26040889. S2CID 205243819. <a href="/wiki/Mark_R._Showalter" target="_blank">/wiki/Mark_R._Showalter</a> <a href="#fnref:192" class="footnote-back-ref">↩</a></li>
<li id="fn:193">Showalter, M.R.; Hamilton, D.P. (June 3, 2015). "Resonant interactions and chaotic rotation of Pluto's small moons". Nature. 522 (7554): 45–49. Bibcode:2015Natur.522...45S. doi:10.1038/nature14469. PMID 26040889. S2CID 205243819. <a href="/wiki/Mark_R._Showalter" target="_blank">/wiki/Mark_R._Showalter</a> <a href="#fnref:193" class="footnote-back-ref">↩</a></li>
<li id="fn:194">Matson, J. (July 11, 2012). "New Moon for Pluto: Hubble Telescope Spots a 5th Plutonian Satellite". Scientific American web site. Archived from the original on August 31, 2016. Retrieved July 12, 2012. <a href="http://www.scientificamerican.com/article.cfm?id=pluto-moon-p5" target="_blank">http://www.scientificamerican.com/article.cfm?id=pluto-moon-p5</a> <a href="#fnref:194" class="footnote-back-ref">↩</a></li>
<li id="fn:195">Richardson, Derek C.; Walsh, Kevin J. (2005). "Binary Minor Planets". Annual Review of Earth and Planetary Sciences. 34 (1): 47–81. Bibcode:2006AREPS..34...47R. doi:10.1146/annurev.earth.32.101802.120208. S2CID 1692921. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:195" class="footnote-back-ref">↩</a></li>
<li id="fn:196">Sicardy, Bruno; Bellucci, Aurélie; Gendron, Éric; et al. (2006). "Charon's size and an upper limit on its atmosphere from a stellar occultation". Nature. 439 (7072): 52–54. Bibcode:2006Natur.439...52S. doi:10.1038/nature04351. hdl:11336/39754. PMID 16397493. S2CID 4411478. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:196" class="footnote-back-ref">↩</a></li>
<li id="fn:197">Szakáts, R.; Kiss, Cs.; Ortiz, J.L.; Morales, N.; Pál, A.; Müller, T.G.; et al. (2023). "Tidally locked rotation of the dwarf planet (136199) Eris discovered from long-term ground based and space photometry". Astronomy & Astrophysics. L3: 669. arXiv:2211.07987. Bibcode:2023A&A...669L...3S. doi:10.1051/0004-6361/202245234. S2CID 253522934. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:197" class="footnote-back-ref">↩</a></li>
<li id="fn:198">Young, Leslie A. (1997). "The Once and Future Pluto". Southwest Research Institute, Boulder, Colorado. Archived from the original on March 30, 2004. Retrieved March 26, 2007. <a href="http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt" target="_blank">http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt</a> <a href="#fnref:198" class="footnote-back-ref">↩</a></li>
<li id="fn:199">Faure, Gunter; Mensing, Teresa M. (2007). "Pluto and Charon: The Odd Couple". Introduction to Planetary Science. Springer. pp. 401–408. doi:10.1007/978-1-4020-5544-7. ISBN 978-1-4020-5544-7. <a href="978-1-4020-5544-7" target="_blank">978-1-4020-5544-7</a> <a href="#fnref:199" class="footnote-back-ref">↩</a></li>
<li id="fn:200">"NASA's Hubble Finds Pluto's Moons Tumbling in Absolute Chaos". June 3, 2015. Archived from the original on April 6, 2020. Retrieved June 3, 2015. <a href="https://www.nasa.gov/press-release/nasa-s-hubble-finds-pluto-s-moons-tumbling-in-absolute-chaos" target="_blank">https://www.nasa.gov/press-release/nasa-s-hubble-finds-pluto-s-moons-tumbling-in-absolute-chaos</a> <a href="#fnref:200" class="footnote-back-ref">↩</a></li>
<li id="fn:201">de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2012). "Plutino 15810 (1994 JR1), an accidental quasi-satellite of Pluto". Monthly Notices of the Royal Astronomical Society Letters. 427 (1): L85. arXiv:1209.3116. Bibcode:2012MNRAS.427L..85D. doi:10.1111/j.1745-3933.2012.01350.x. S2CID 118570875. <a href="https://doi.org/10.1111%2Fj.1745-3933.2012.01350.x" target="_blank">https://doi.org/10.1111%2Fj.1745-3933.2012.01350.x</a> <a href="#fnref:201" class="footnote-back-ref">↩</a></li>
<li id="fn:202">de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2012). "Plutino 15810 (1994 JR1), an accidental quasi-satellite of Pluto". Monthly Notices of the Royal Astronomical Society Letters. 427 (1): L85. arXiv:1209.3116. Bibcode:2012MNRAS.427L..85D. doi:10.1111/j.1745-3933.2012.01350.x. S2CID 118570875. <a href="https://doi.org/10.1111%2Fj.1745-3933.2012.01350.x" target="_blank">https://doi.org/10.1111%2Fj.1745-3933.2012.01350.x</a> <a href="#fnref:202" class="footnote-back-ref">↩</a></li>
<li id="fn:203">"Pluto's fake moon". Sky & Telescope. September 24, 2012. Archived from the original on February 20, 2020. Retrieved September 24, 2012. <a href="http://www.skyandtelescope.com/astronomy-news/plutos-fake-moon/" target="_blank">http://www.skyandtelescope.com/astronomy-news/plutos-fake-moon/</a> <a href="#fnref:203" class="footnote-back-ref">↩</a></li>
<li id="fn:204">"New Horizons Collects First Science on a Post-Pluto Object". NASA. May 13, 2016. Archived from the original on June 7, 2016. Retrieved June 5, 2016. <a href="https://web.archive.org/web/20160607150433/http://www.nasa.gov/feature/new-horizons-collects-first-science-on-a-post-pluto-object/" target="_blank">https://web.archive.org/web/20160607150433/http://www.nasa.gov/feature/new-horizons-collects-first-science-on-a-post-pluto-object/</a> <a href="#fnref:204" class="footnote-back-ref">↩</a></li>
<li id="fn:205">de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2016). "The analemma criterion: accidental quasi-satellites are indeed true quasi-satellites". Monthly Notices of the Royal Astronomical Society. 462 (3): 3344–3349. arXiv:1607.06686. Bibcode:2016MNRAS.462.3344D. doi:10.1093/mnras/stw1833. S2CID 119284843. <a href="https://doi.org/10.1093%2Fmnras%2Fstw1833" target="_blank">https://doi.org/10.1093%2Fmnras%2Fstw1833</a> <a href="#fnref:205" class="footnote-back-ref">↩</a></li>
<li id="fn:206">Porter, Simon B.; et al. (2016). "The First High-phase Observations of a KBO: New Horizons Imaging of (15810) 1994 JR1 from the Kuiper Belt". The Astrophysical Journal Letters. 828 (2): L15. arXiv:1605.05376. Bibcode:2016ApJ...828L..15P. doi:10.3847/2041-8205/828/2/L15. S2CID 54507506. <a href="https://doi.org/10.3847%2F2041-8205%2F828%2F2%2FL15" target="_blank">https://doi.org/10.3847%2F2041-8205%2F828%2F2%2FL15</a> <a href="#fnref:206" class="footnote-back-ref">↩</a></li>
<li id="fn:207">"New Horizons Collects First Science on a Post-Pluto Object". NASA. May 13, 2016. Archived from the original on June 7, 2016. Retrieved June 5, 2016. <a href="https://web.archive.org/web/20160607150433/http://www.nasa.gov/feature/new-horizons-collects-first-science-on-a-post-pluto-object/" target="_blank">https://web.archive.org/web/20160607150433/http://www.nasa.gov/feature/new-horizons-collects-first-science-on-a-post-pluto-object/</a> <a href="#fnref:207" class="footnote-back-ref">↩</a></li>
<li id="fn:208">de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2016). "The analemma criterion: accidental quasi-satellites are indeed true quasi-satellites". Monthly Notices of the Royal Astronomical Society. 462 (3): 3344–3349. arXiv:1607.06686. Bibcode:2016MNRAS.462.3344D. doi:10.1093/mnras/stw1833. S2CID 119284843. <a href="https://doi.org/10.1093%2Fmnras%2Fstw1833" target="_blank">https://doi.org/10.1093%2Fmnras%2Fstw1833</a> <a href="#fnref:208" class="footnote-back-ref">↩</a></li>
<li id="fn:209">Kuiper, Gerard (1961). Planets and Satellites. Chicago: University of Chicago Press. p. 576. <a href="#fnref:209" class="footnote-back-ref">↩</a></li>
<li id="fn:210">Stern, S. Alan; Tholen, David J. (1997). Pluto and Charon. University of Arizona Press. p. 623. ISBN 978-0-8165-1840-1. Archived from the original on February 26, 2024. Retrieved October 23, 2015. <a href="978-0-8165-1840-1" target="_blank">978-0-8165-1840-1</a> <a href="#fnref:210" class="footnote-back-ref">↩</a></li>
<li id="fn:211">The dwarf planet Eris is roughly the same size as Pluto, about 2330 km; Eris is 28% more massive than Pluto. Eris is a scattered-disc object, often considered a distinct population from Kuiper-belt objects like Pluto; Pluto is the largest body in the Kuiper belt proper, which excludes the scattered-disc objects. <a href="/wiki/Eris_(dwarf_planet)" target="_blank">/wiki/Eris_(dwarf_planet)</a> <a href="#fnref:211" class="footnote-back-ref">↩</a></li>
<li id="fn:212">Sheppard, Scott S.; Trujillo, Chadwick A.; Udalski, Andrzej; et al. (2011). "A Southern Sky and Galactic Plane Survey for Bright Kuiper Belt Objects". Astronomical Journal. 142 (4): 98. arXiv:1107.5309. Bibcode:2011AJ....142...98S. doi:10.1088/0004-6256/142/4/98. S2CID 53552519. <a href="/wiki/Scott_S._Sheppard" target="_blank">/wiki/Scott_S._Sheppard</a> <a href="#fnref:212" class="footnote-back-ref">↩</a></li>
<li id="fn:213">"Colossal Cousin to a Comet?". pluto.jhuapl.edu – NASA New Horizons mission site. Johns Hopkins University Applied Physics Laboratory. Archived from the original on November 13, 2014. Retrieved February 15, 2014. <a href="https://web.archive.org/web/20141113225441/http://pluto.jhuapl.edu/science/everything_pluto/8_cousin.php" target="_blank">https://web.archive.org/web/20141113225441/http://pluto.jhuapl.edu/science/everything_pluto/8_cousin.php</a> <a href="#fnref:213" class="footnote-back-ref">↩</a></li>
<li id="fn:214">Tyson, Neil deGrasse (1999). "Pluto Is Not a Planet". The Planetary Society. Archived from the original on September 27, 2011. Retrieved November 30, 2011. <a href="/wiki/Neil_deGrasse_Tyson" target="_blank">/wiki/Neil_deGrasse_Tyson</a> <a href="#fnref:214" class="footnote-back-ref">↩</a></li>
<li id="fn:215">Philip Metzger (April 13, 2015). "Nine Reasons Why Pluto Is a Planet". Philip Metzger. Archived from the original on April 15, 2015. <a href="/wiki/Philip_T._Metzger" target="_blank">/wiki/Philip_T._Metzger</a> <a href="#fnref:215" class="footnote-back-ref">↩</a></li>
<li id="fn:216">Wall, Mike (May 24, 2018). "Pluto May Have Formed from 1 Billion Comets". Space.com. Archived from the original on May 24, 2018. Retrieved May 24, 2018. <a href="https://www.space.com/40687-pluto-formation-1-billion-comets.html" target="_blank">https://www.space.com/40687-pluto-formation-1-billion-comets.html</a> <a href="#fnref:216" class="footnote-back-ref">↩</a></li>
<li id="fn:217">Glein, Christopher R.; Waite, J. Hunter Jr. (May 24, 2018). "Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto". Icarus. 313 (2018): 79–92. arXiv:1805.09285. Bibcode:2018Icar..313...79G. doi:10.1016/j.icarus.2018.05.007. S2CID 102343522. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:217" class="footnote-back-ref">↩</a></li>
<li id="fn:218">Brown, Michael E. (November 22, 2010). "How big is Pluto, anyway?". Mike Brown's Planets. Archived from the original on July 21, 2011. Retrieved June 9, 2015. (Franck Marchis on 8 November 2010)[dead link] <a href="http://www.mikebrownsplanets.com/2010/11/how-big-is-pluto-anyway.html" target="_blank">http://www.mikebrownsplanets.com/2010/11/how-big-is-pluto-anyway.html</a> <a href="#fnref:218" class="footnote-back-ref">↩</a></li>
<li id="fn:219">"Neptune's Moon Triton". The Planetary Society. Archived from the original on December 10, 2011. Retrieved November 30, 2011. <a href="https://web.archive.org/web/20111210143839/http://www.planetary.org/explore/topics/neptune/triton.html" target="_blank">https://web.archive.org/web/20111210143839/http://www.planetary.org/explore/topics/neptune/triton.html</a> <a href="#fnref:219" class="footnote-back-ref">↩</a></li>
<li id="fn:220">Gomes R. S.; Gallardo T.; Fernández J. A.; Brunini A. (2005). "On the origin of the High-Perihelion Scattered Disk: the role of the Kozai mechanism and mean motion resonances". Celestial Mechanics and Dynamical Astronomy. 91 (1–2): 109–129. Bibcode:2005CeMDA..91..109G. doi:10.1007/s10569-004-4623-y. hdl:11336/38379. S2CID 18066500. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:220" class="footnote-back-ref">↩</a></li>
<li id="fn:221">Hahn, Joseph M. (2005). "Neptune's Migration into a Stirred-up Kuiper Belt: A Detailed Comparison of Simulations to Observations" (PDF). The Astronomical Journal. 130 (5): 2392–2414. arXiv:astro-ph/0507319. Bibcode:2005AJ....130.2392H. doi:10.1086/452638. S2CID 14153557. Archived (PDF) from the original on July 23, 2011. Retrieved March 5, 2008. <a href="http://gemelli.colorado.edu/~hahnjm/pubs/migrate.pdf" target="_blank">http://gemelli.colorado.edu/~hahnjm/pubs/migrate.pdf</a> <a href="#fnref:221" class="footnote-back-ref">↩</a></li>
<li id="fn:222">Levison, Harold F.; Morbidelli, Alessandro; Van Laerhoven, Christa; et al. (2007). "Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune". Icarus. 196 (1): 258–273. arXiv:0712.0553. Bibcode:2008Icar..196..258L. doi:10.1016/j.icarus.2007.11.035. S2CID 7035885. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:222" class="footnote-back-ref">↩</a></li>
<li id="fn:223">Malhotra, Renu (1995). "The Origin of Pluto's Orbit: Implications for the Solar System Beyond Neptune". Astronomical Journal. 110: 420. arXiv:astro-ph/9504036. Bibcode:1995AJ....110..420M. doi:10.1086/117532. S2CID 10622344. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:223" class="footnote-back-ref">↩</a></li>
<li id="fn:224">Levison, Harold F.; Morbidelli, Alessandro; Van Laerhoven, Christa; et al. (2007). "Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune". Icarus. 196 (1): 258–273. arXiv:0712.0553. Bibcode:2008Icar..196..258L. doi:10.1016/j.icarus.2007.11.035. S2CID 7035885. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:224" class="footnote-back-ref">↩</a></li>
<li id="fn:225">Williams, David R. (July 24, 2015). "Pluto Fact Sheet". NASA. Archived from the original on November 19, 2015. Retrieved August 6, 2015. <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html" target="_blank">https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html</a> <a href="#fnref:225" class="footnote-back-ref">↩</a></li>
<li id="fn:226">"This month Pluto's apparent magnitude is m=14.1. Could we see it with an 11" reflector of focal length 3400 mm?". Singapore Science Centre. 2002. Archived from the original on November 11, 2005. Retrieved November 29, 2011. <a href="https://web.archive.org/web/20051111151435/http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66" target="_blank">https://web.archive.org/web/20051111151435/http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66</a> <a href="#fnref:226" class="footnote-back-ref">↩</a></li>
<li id="fn:227">"How to Scope Out Pluto in the Night Sky Friday". Space.com. July 3, 2014. Archived from the original on April 6, 2022. Retrieved April 6, 2022. <a href="https://www.space.com/26426-pluto-telescope-skywatching-friday.html" target="_blank">https://www.space.com/26426-pluto-telescope-skywatching-friday.html</a> <a href="#fnref:227" class="footnote-back-ref">↩</a></li>
<li id="fn:228">Williams, David R. (July 24, 2015). "Pluto Fact Sheet". NASA. Archived from the original on November 19, 2015. Retrieved August 6, 2015. <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html" target="_blank">https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html</a> <a href="#fnref:228" class="footnote-back-ref">↩</a></li>
<li id="fn:229">Young, Eliot F.; Binzel, Richard P.; Crane, Keenan (2001). "A Two-Color Map of Pluto's Sub-Charon Hemisphere". The Astronomical Journal. 121 (1): 552–561. Bibcode:2001AJ....121..552Y. doi:10.1086/318008. <a href="https://doi.org/10.1086%2F318008" target="_blank">https://doi.org/10.1086%2F318008</a> <a href="#fnref:229" class="footnote-back-ref">↩</a></li>
<li id="fn:230">Buie, Marc W.; Tholen, David J.; Horne, Keith (1992). "Albedo maps of Pluto and Charon: Initial mutual event results". Icarus. 97 (2): 221–227. Bibcode:1992Icar...97..211B. doi:10.1016/0019-1035(92)90129-U. Archived from the original on June 22, 2011. Retrieved February 10, 2010. <a href="http://www.boulder.swri.edu/~buie/biblio/pub015.html" target="_blank">http://www.boulder.swri.edu/~buie/biblio/pub015.html</a> <a href="#fnref:230" class="footnote-back-ref">↩</a></li>
<li id="fn:231">Buie, Marc W. "Pluto map information". Archived from the original on June 29, 2011. Retrieved February 10, 2010. <a href="https://web.archive.org/web/20110629005310/http://www.boulder.swri.edu/~buie/pluto/hrcmap.html" target="_blank">https://web.archive.org/web/20110629005310/http://www.boulder.swri.edu/~buie/pluto/hrcmap.html</a> <a href="#fnref:231" class="footnote-back-ref">↩</a></li>
<li id="fn:232">Buie, Marc W.; Grundy, William M.; Young, Eliot F.; et al. (2010). "Pluto and Charon with the Hubble Space Telescope: II. Resolving changes on Pluto's surface and a map for Charon". Astronomical Journal. 139 (3): 1128–1143. Bibcode:2010AJ....139.1128B. CiteSeerX 10.1.1.625.7795. doi:10.1088/0004-6256/139/3/1128. S2CID 9343680. Archived from the original on July 7, 2015. Retrieved February 10, 2010. <a href="http://www.boulder.swri.edu/~buie/biblio/pub073.html" target="_blank">http://www.boulder.swri.edu/~buie/biblio/pub073.html</a> <a href="#fnref:232" class="footnote-back-ref">↩</a></li>
<li id="fn:233">Buie, Marc W. "How the Pluto maps were made". Archived from the original on February 9, 2010. Retrieved February 10, 2010. <a href="https://web.archive.org/web/20100209202908/http://www.boulder.swri.edu/~buie/pluto/mapstory.html" target="_blank">https://web.archive.org/web/20100209202908/http://www.boulder.swri.edu/~buie/pluto/mapstory.html</a> <a href="#fnref:233" class="footnote-back-ref">↩</a></li>
<li id="fn:234">Buie, Marc W. "How the Pluto maps were made". Archived from the original on February 9, 2010. Retrieved February 10, 2010. <a href="https://web.archive.org/web/20100209202908/http://www.boulder.swri.edu/~buie/pluto/mapstory.html" target="_blank">https://web.archive.org/web/20100209202908/http://www.boulder.swri.edu/~buie/pluto/mapstory.html</a> <a href="#fnref:234" class="footnote-back-ref">↩</a></li>
<li id="fn:235">"New Horizons, Not Quite to Jupiter, Makes First Pluto Sighting". pluto.jhuapl.edu – NASA New Horizons mission site. Johns Hopkins University Applied Physics Laboratory. November 28, 2006. Archived from the original on November 13, 2014. Retrieved November 29, 2011. <a href="https://web.archive.org/web/20141113224846/http://pluto.jhuapl.edu/news_center/news/112806.php" target="_blank">https://web.archive.org/web/20141113224846/http://pluto.jhuapl.edu/news_center/news/112806.php</a> <a href="#fnref:235" class="footnote-back-ref">↩</a></li>
<li id="fn:236">Chang, Kenneth (October 28, 2016). "No More Data From Pluto". New York Times. Archived from the original on March 29, 2019. Retrieved October 28, 2016. <a href="https://www.nytimes.com/2016/10/29/science/pluto-nasa-new-horizons.html" target="_blank">https://www.nytimes.com/2016/10/29/science/pluto-nasa-new-horizons.html</a> <a href="#fnref:236" class="footnote-back-ref">↩</a></li>
<li id="fn:237">"Pluto Exploration Complete: New Horizons Returns Last Bits of 2015 Flyby Data to Earth". Johns Hopkins Applied Research Laboratory. October 27, 2016. Archived from the original on October 28, 2016. Retrieved October 28, 2016. <a href="http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20161027" target="_blank">http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20161027</a> <a href="#fnref:237" class="footnote-back-ref">↩</a></li>
<li id="fn:238">Brown, Dwayne; Buckley, Michael; Stothoff, Maria (January 15, 2015). "Release 15-011 – NASA's New Horizons Spacecraft Begins First Stages of Pluto Encounter". NASA. Archived from the original on April 7, 2020. Retrieved January 15, 2015. <a href="https://www.nasa.gov/press/2015/january/nasa-s-new-horizons-spacecraft-begins-first-stages-of-pluto-encounter" target="_blank">https://www.nasa.gov/press/2015/january/nasa-s-new-horizons-spacecraft-begins-first-stages-of-pluto-encounter</a> <a href="#fnref:238" class="footnote-back-ref">↩</a></li>
<li id="fn:239">"New Horizons". pluto.jhuapl.edu. Archived from the original on October 8, 2016. Retrieved May 15, 2016. <a href="http://pluto.jhuapl.edu/Mission/Spacecraft/Data-Collection.php" target="_blank">http://pluto.jhuapl.edu/Mission/Spacecraft/Data-Collection.php</a> <a href="#fnref:239" class="footnote-back-ref">↩</a></li>
<li id="fn:240">"Why a group of scientists think we need another mission to Pluto". The Verge. Archived from the original on July 8, 2018. Retrieved July 14, 2018. <a href="https://www.theverge.com/2017/4/26/15424770/nasa-spacecraft-new-horizons-flyby-pluto-moons-orbiter-mission" target="_blank">https://www.theverge.com/2017/4/26/15424770/nasa-spacecraft-new-horizons-flyby-pluto-moons-orbiter-mission</a> <a href="#fnref:240" class="footnote-back-ref">↩</a></li>
<li id="fn:241">"Why NASA should visit Pluto again". MIT Technology Review. Archived from the original on January 18, 2022. Retrieved January 18, 2022. <a href="https://www.technologyreview.com/2021/07/19/1029680/nasa-pluto-mission-persephone/" target="_blank">https://www.technologyreview.com/2021/07/19/1029680/nasa-pluto-mission-persephone/</a> <a href="#fnref:241" class="footnote-back-ref">↩</a></li>
<li id="fn:242">"New videos simulate Pluto and Charon flyby; return mission to Pluto proposed". August 2021. Archived from the original on September 4, 2021. Retrieved September 4, 2021. <a href="https://web.archive.org/web/20210904132010/https://www.spaceflightinsider.com/missions/solar-system/videos-simulate-pluto-charon-flyby-follow-up-mission-proposed/" target="_blank">https://web.archive.org/web/20210904132010/https://www.spaceflightinsider.com/missions/solar-system/videos-simulate-pluto-charon-flyby-follow-up-mission-proposed/</a> <a href="#fnref:242" class="footnote-back-ref">↩</a></li>
<li id="fn:243">"Going Back to Pluto? Scientists to Push for Orbiter Mission". Space.com. Archived from the original on July 14, 2018. Retrieved July 14, 2018. <a href="https://www.space.com/36697-pluto-orbiter-mission-after-new-horizons.html" target="_blank">https://www.space.com/36697-pluto-orbiter-mission-after-new-horizons.html</a> <a href="#fnref:243" class="footnote-back-ref">↩</a></li>
<li id="fn:244">Hall, Loura (April 5, 2017). "Fusion-Enabled Pluto Orbiter and Lander". NASA. Archived from the original on April 21, 2017. Retrieved July 14, 2018. <a href="https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_Lander" target="_blank">https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_Lander</a> <a href="#fnref:244" class="footnote-back-ref">↩</a></li>
<li id="fn:245">Fusion-Enabled Pluto Orbiter and Lander – Phase I Final Report Archived April 29, 2019, at the Wayback Machine. (PDF) Stephanie Thomas, Princeton Satellite Systems. 2017. <a href="https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003126.pdf" target="_blank">https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003126.pdf</a> <a href="#fnref:245" class="footnote-back-ref">↩</a></li>
<li id="fn:246">Nadia Drake (July 14, 2016). "5 Amazing Things We've Learned a Year After Visiting Pluto". National Geographic. Archived from the original on March 7, 2021. Retrieved August 19, 2021. <a href="https://web.archive.org/web/20210307022014/https://www.nationalgeographic.com/science/article/pluto-planets-new-horizons-one-year-anniversary-nasa-space-science" target="_blank">https://web.archive.org/web/20210307022014/https://www.nationalgeographic.com/science/article/pluto-planets-new-horizons-one-year-anniversary-nasa-space-science</a> <a href="#fnref:246" class="footnote-back-ref">↩</a></li>
<li id="fn:247">"HUBBLE REVEALS SURFACE OF PLUTO FOR FIRST TIME". HubbleSite.org. Space Telescope Science Institute. March 7, 1996. Archived from the original on August 19, 2021. Retrieved October 18, 2021. <a href="https://hubblesite.org/contents/news-releases/1996/news-1996-09.html" target="_blank">https://hubblesite.org/contents/news-releases/1996/news-1996-09.html</a> <a href="#fnref:247" class="footnote-back-ref">↩</a></li>
<li id="fn:248">"MAP OF PLUTO'S SURFACE". HubbleSite.org. Space Telescope Science Institute. March 7, 1996. Archived from the original on August 19, 2021. Retrieved October 18, 2021. <a href="https://hubblesite.org/contents/media/images/1996/09/401-Image.html?news=true" target="_blank">https://hubblesite.org/contents/media/images/1996/09/401-Image.html?news=true</a> <a href="#fnref:248" class="footnote-back-ref">↩</a></li>
<li id="fn:249">A.S.Ganesh (March 7, 2021). "Seeing Pluto like never before". The Hindu. Archived from the original on August 19, 2021. Retrieved August 19, 2021. <a href="https://www.thehindu.com/children/seeing-pluto-like-never-before/article33941881.ece" target="_blank">https://www.thehindu.com/children/seeing-pluto-like-never-before/article33941881.ece</a> <a href="#fnref:249" class="footnote-back-ref">↩</a></li>
<li id="fn:250">Rothery, David A (October 2015). "Pluto and Charon from New Horizons". Astronomy & Geophysics. 56 (5): 5.19 – 5.22. doi:10.1093/astrogeo/atv168. <a href="https://doi.org/10.1093%2Fastrogeo%2Fatv168" target="_blank">https://doi.org/10.1093%2Fastrogeo%2Fatv168</a> <a href="#fnref:250" class="footnote-back-ref">↩</a></li>
<li id="fn:251">Lauer, Todd R.; Spencer, John R.; Bertrand, Tanguy; Beyer, Ross A.; Runyon, Kirby D.; White, Oliver L.; Young, Leslie A.; Ennico, Kimberly; MacKinnon, William B.; Moore, Jeffrey M.; Olkin, Catherine B.; Stern, S. Alan; Weaver, Harold A. (October 20, 2021). "The Dark Side of Pluto". The Planetary Science Journal. 2 (214): 214. arXiv:2110.11976. Bibcode:2021PSJ.....2..214L. doi:10.3847/PSJ/ac2743. S2CID 239047659. <a href="https://doi.org/10.3847%2FPSJ%2Fac2743" target="_blank">https://doi.org/10.3847%2FPSJ%2Fac2743</a> <a href="#fnref:251" class="footnote-back-ref">↩</a></li>
</ol>

Pluto open-in-new

Pluto