WASP-121b

<h2 id="nomenclature">Nomenclature</h2>
In August 2022, this planet and its host star were included among 20 systems to be named by the third <a href="/facts/NameExoWorlds/NpRMt4m0">NameExoWorlds</a> project.<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> The approved names, proposed by a team from <a href="/facts/Bahrain/ndXdWPhr">Bahrain</a>, were announced in June 2023. WASP-121b is named Tylos after the <a href="/facts/Tylos/Ha7hut6j">ancient Greek name</a> for Bahrain, and its host star is named <a href="/facts/WASP-121/5M8bjPWq">Dilmun</a> after the <a href="/facts/Dilmun/7MaD1Xyy">ancient civilization</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a>

<h2 id="characteristics">Characteristics</h2>

WASP-121b is an <a href="/facts/Ultra-hot_Jupiter/9wSFCuxk">ultra-hot Jupiter</a> <a href="/facts/Exoplanet/zjfmqfUf">exoplanet</a> with a mass about 1.16 times that of <a href="/facts/Jupiter/95m7XjMT">Jupiter</a> and a radius about 1.75 times that of Jupiter. The exoplanet orbits <a href="/facts/WASP-121/5M8bjPWq">WASP-121</a>, its host star, every 1.27 days.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a>
In 2019 a work by Hellard et al. discussed the possibility of measuring the <a href="/facts/Love_number/gDV4lBFw">Love number</a> of transiting hot Jupiters using HST (<a href="/facts/Hubble_Space_Telescope/NQeIf0vw">Hubble Space Telescope</a>)/<a href="/facts/Space_Telescope_Imaging_Spectrograph/obppRqIS">STIS</a>. A tentative measurement of 
 
 
 
 
 h
 
 2
 
 
 =
 1.4
 ±
 0.8
 
 
 {\displaystyle h_{2}=1.4\pm 0.8}
 
 for WASP-121b was published in the same work.<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a><a class="footnote-ref" id="fnref:14" href="#fn:14">14</a>
The planetary orbit is inclined to the equatorial plane of the star by 8.1°.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>

<h3>Atmospheric composition</h3>
A spectral survey in 2015 attributed 2,500 °C (4,530 °F), hot<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a> <a href="/facts/Stratosphere/taBra67V">stratosphere</a> absorption bands to <a href="/facts/Water/0lkXnJPK">water molecules</a>, <a href="/facts/Titanium(II)_oxide/5ek4Vasd">titanium(II) oxide</a> (TiO) and <a href="/facts/Vanadium(II)_oxide/5b9whSBm">vanadium(II) oxide</a> (VO).<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a> Neutral iron was also detected in the stratosphere of WASP-121b in 2020,<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a><a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> along with neutral <a href="/facts/Chromium/6sVk6Uu7">chromium</a> and <a href="/facts/Vanadium/UvKkKTSm">vanadium</a>.<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a> A number of other studies, however, failed to detect TiO and VO.<a class="footnote-ref" id="fnref:21" href="#fn:21">21</a><a class="footnote-ref" id="fnref:22" href="#fn:22">22</a><a class="footnote-ref" id="fnref:23" href="#fn:23">23</a><a class="footnote-ref" id="fnref:24" href="#fn:24">24</a>
Reanalysis of collected spectral data was published in June 2020. Neutral <a href="/facts/Magnesium/XyIhd2FG">magnesium</a>, <a href="/facts/Calcium/hf252CC0">calcium</a>, vanadium, chromium, iron (Fe), and <a href="/facts/Nickel/KwMCHYRP">nickel</a> (Ni), along with ionized <a href="/facts/Sodium/eYuSPu2V">sodium atoms</a>, were detected. However the low quality of available data precluded a positive identification of any molecular species, including water. The atmosphere appears to be significantly out of <a href="/facts/Chemical_equilibrium/y8ToYXHp">chemical equilibrium</a> and possibly escaping.<a class="footnote-ref" id="fnref:25" href="#fn:25">25</a> The strong atmospheric flows beyond the <a href="/facts/Roche_lobe/4YglV2Kr">Roche lobe</a>, indicating ongoing atmosphere loss, were confirmed in late 2020.<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a>
In 2021, the planetary atmosphere was revealed to be slightly more blue and less absorbing, which may be an indication of planetary weather patterns.<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a> By mid-2021, the presence of ions of <a href="/facts/Iron/DkICCKIC">iron</a>, chromium, vanadium and calcium in the planetary atmosphere was confirmed.<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a> In 2022, ionized barium was also detected.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a> By 2022, an absence of titanium in the planetary atmosphere was confirmed and attributed to the nightside condensation of highly refractory <a href="/facts/Titanium_dioxide/T0KWmKcz">titanium dioxide</a>.<a class="footnote-ref" id="fnref:30" href="#fn:30">30</a> Observations by HST from 2016-2019, published in 2024, confirmed variability in the atmosphere of WASP-121b.<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a><a class="footnote-ref" id="fnref:32" href="#fn:32">32</a>
A 2025 study revealed the first 3D structure of its atmosphere, showing it to be formed of at least three layers. The upper layer consists of hydrogen gas, the middle layer contains sodium and the lower layer iron. A super-rotational sodium-containing jet stream moves material around the equator while the layer below moves the gas from the hot side of the planet to the cooler side.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a> Titanium is detected at a lower latitude below the equatorial jet stream.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a> Another study in 2025 constraining the abundance of volatile elements (<a href="/facts/Carbon/ukrgQexo">carbon</a> and <a href="/facts/Oxygen/acyn6o8r">oxygen</a>) and refractory elements (iron and nickel) shows that WASP-121b likely have formed faraway from its host star, in an ice-rich environment, before migrating inward.<a class="footnote-ref" id="fnref:35" href="#fn:35">35</a>

<h2 id="possible-exomoon">Possible exomoon</h2>
The sodium detected via <a href="/facts/Absorption_spectroscopy/GH7RC14D">absorption spectroscopy</a> around WASP-121b<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a> is consistent with an extrasolar <a href="/facts/Gas_torus/Hv95vhu8">gas torus</a>, possibly fueled by an <a href="/facts/Io_(moon)/p3Uc8FmK">Io</a>-like <a href="/facts/Exomoon/v8ncCW28">exomoon</a>.<a class="footnote-ref" id="fnref:37" href="#fn:37">37</a>

<h2 id="see-also">See also</h2>

Wikimedia Commons has media related to WASP-121 b.

<ul><li><a href="/facts/List_of_exoplanet_firsts/8WzIH0Jm">List of exoplanet firsts</a></li>
<li><a href="/facts/List_of_exoplanets_discovered_in_2015/b25UssPQ">List of exoplanets discovered in 2015</a>, including WASP-121b</li>
<li><a href="/facts/SuperWASP/fp58Y205">SuperWASP</a></li>
<li><a href="/facts/WASP-33b/1X2Lx5xX">WASP-33b</a>, another ultra-hot Jupiter</li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="http://wasp-planets.net/wasp-planets/">SuperWASP Wide Angle Search for Planets: The Planets</a>, <a href="/facts/SuperWASP/fp58Y205">SuperWASP</a>.</li>
<li>Pultarova, Tereza (3 August 2017). <a href="https://www.space.com/37705-exoplanet-stratosphere-detected-wasp-121b.html">"Hubble Telescope Detects Stratosphere on Huge Alien Planet"</a>. <a href="/facts/Space.com/1H06DeGz">space.com</a>. Retrieved 3 August 2017.</li></ul>

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

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 Gaia DR3 record for this source at VizieR. <a href="https://doi.org/10.1051%2F0004-6361%2F202243940" target="_blank">https://doi.org/10.1051%2F0004-6361%2F202243940</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
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<li id="fn:15">Borsa, F.; et al. (2021), "Atmospheric Rossiter–Mc Laughlin effect and transmission spectroscopy of WASP-121b with ESPRESSO", Astronomy & Astrophysics, 645: A24, arXiv:2011.01245, Bibcode:2021A&A...645A..24B, doi:10.1051/0004-6361/202039344, S2CID 226237425 <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
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<li id="fn:18">Gibson, Neale P.; Merritt, Stephanie; Nugroho, Stevanus K.; Cubillos, Patricio E.; de Mooij, Ernst J. W.; Mikal-Evans, Thomas; Fossati, Luca; Lothringer, Joshua; Nikolov, Nikolay; Sing, David K.; Spake, Jessica J.; Watson, Chris A.; Wilson, Jamie (2020). "Detection of Fe I in the atmosphere of the ultra-hot Jupiter WASP-121b, and a new likelihood-based approach for Doppler-resolved spectroscopy". Monthly Notices of the Royal Astronomical Society. 493 (2): 2215. arXiv:2001.06430. Bibcode:2020MNRAS.493.2215G. doi:10.1093/mnras/staa228. S2CID 210714233. <a href="https://doi.org/10.1093%2Fmnras%2Fstaa228" target="_blank">https://doi.org/10.1093%2Fmnras%2Fstaa228</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">Cabot, Samuel H. C.; Madhusudhan, Nikku; Welbanks, Luis; Piette, Anjali; Gandhi, Siddharth (2020). "Detection of neutral atomic species in the ultra-hot jupiter WASP-121b". Monthly Notices of the Royal Astronomical Society. 494 (1): 363–377. arXiv:2001.07196. Bibcode:2020MNRAS.494..363C. doi:10.1093/mnras/staa748. S2CID 210838889. <a href="https://doi.org/10.1093%2Fmnras%2Fstaa748" target="_blank">https://doi.org/10.1093%2Fmnras%2Fstaa748</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Ben-Yami, Maya; Madhusudhan, Nikku; Cabot, Samuel H. C.; Constantinou, Savvas; Piette, Anjali; Gandhi, Siddharth; Welbanks, Luis (2020). "Neutral Cr and V in the Atmosphere of Ultra-hot Jupiter WASP-121 B". The Astrophysical Journal. 897 (1): L5. arXiv:2006.05995. Bibcode:2020ApJ...897L...5B. doi:10.3847/2041-8213/ab94aa. S2CID 219573825. <a href="https://doi.org/10.3847%2F2041-8213%2Fab94aa" target="_blank">https://doi.org/10.3847%2F2041-8213%2Fab94aa</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Evans, Thomas M.; Sing, David K.; et al. (2 August 2017). "An ultrahot gas-giant exoplanet with a stratosphere". Nature. 548 (7665): 58–61. arXiv:1708.01076. Bibcode:2017Natur.548...58E. doi:10.1038/nature23266. ISSN 1476-4687. PMID 28770846. S2CID 205258293. <a href="/wiki/Nature_(journal)" target="_blank">/wiki/Nature_(journal)</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Mikal-Evans, Thomas (27 June 2019). "An emission spectrum for WASP-121b measured across the 0.8–1.1 μm wavelength range using the Hubble Space Telescope". Monthly Notices of the Royal Astronomical Society. 488 (2): 2222–2234. arXiv:1906.06326. Bibcode:2019MNRAS.488.2222M. doi:10.1093/mnras/stz1753. hdl:10150/634587. <a href="https://doi.org/10.1093%2Fmnras%2Fstz1753" target="_blank">https://doi.org/10.1093%2Fmnras%2Fstz1753</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">Merritt, S. R.; Gibson, N. P.; Nugroho, S. K.; Mooij, E. J. W. de; Hooton, M. J.; Matthews, S. M.; McKemmish, L. K.; Mikal-Evans, T.; Nikolov, N.; Sing, D. K.; Spake, J. J. (1 April 2020). "Non-detection of TiO and VO in the atmosphere of WASP-121b using high-resolution spectroscopy". Astronomy & Astrophysics. 636: A117. arXiv:2002.02795. Bibcode:2020A&A...636A.117M. doi:10.1051/0004-6361/201937409. ISSN 0004-6361. <a href="https://www.aanda.org/articles/aa/abs/2020/04/aa37409-19/aa37409-19.html" target="_blank">https://www.aanda.org/articles/aa/abs/2020/04/aa37409-19/aa37409-19.html</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">Mikal-Evans, Thomas; Sing, David K.; Kataria, Tiffany; Wakeford, Hannah R.; Mayne, Nathan J.; Lewis, Nikole K.; Barstow, Joanna K.; Spake, Jessica J. (2020). "Confirmation of water emission in the dayside spectrum of the ultrahot Jupiter WASP-121b". Monthly Notices of the Royal Astronomical Society. 496 (2): 1638–1644. arXiv:2005.09631. Bibcode:2020MNRAS.496.1638M. doi:10.1093/mnras/staa1628. S2CID 218684532. <a href="https://doi.org/10.1093%2Fmnras%2Fstaa1628" target="_blank">https://doi.org/10.1093%2Fmnras%2Fstaa1628</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
<li id="fn:25">Hoeijmakers, H.J.; Seidel, J.V.; Pino, L.; Kitzmann, D.; Sindel, J.P.; Ehrenreich, D.; Oza, A.V.; Bourrier, V.; Allart, R.; Gebek, A.; Lovis, C.; Yurchenko, S.N.; Astudillo-Defru, N.; Bayliss, D.; Cegla, H.; Lavie, B.; Lendl, M.; Melo, C.; Murgas, F.; Nascimbeni, V.; Pepe, F.; Segransan, D.; Udry, S.; Wyttenbach, A.; Heng, K. (18 September 2020). "Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) - IV. A spectral inventory of atoms and molecules in the high-resolution transmission spectrum of WASP-121 b". Astronomy & Astrophysics. 641: A123. arXiv:2006.11308. Bibcode:2020A&A...641A.123H. doi:10.1051/0004-6361/202038365. S2CID 219966241. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></li>
<li id="fn:26">Borsa, F.; et al. (2021), "Atmospheric Rossiter–Mc Laughlin effect and transmission spectroscopy of WASP-121b with ESPRESSO", Astronomy & Astrophysics, 645: A24, arXiv:2011.01245, Bibcode:2021A&A...645A..24B, doi:10.1051/0004-6361/202039344, S2CID 226237425 <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></li>
<li id="fn:27">Wilson, Jamie; Gibson, Neale P.; Lothringer, Joshua D.; Sing, David K.; Mikal-Evans, Thomas; De Mooij, Ernst J W.; Nikolov, Nikolay; Watson, Chris A. (2021), "Gemini/GMOS optical transmission spectroscopy of WASP-121b: Signs of variability in an ultra-hot Jupiter?", Monthly Notices of the Royal Astronomical Society, 503 (4): 4787–4801, arXiv:2103.05698, doi:10.1093/mnras/stab797 <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></li>
<li id="fn:28">Merritt, Stephanie R.; Gibson, Neale P.; Nugroho, Stevanus K.; De Mooij, Ernst J W.; Hooton, Matthew J.; Lothringer, Joshua D.; Matthews, Shannon M.; Mikal-Evans, Thomas; Nikolov, Nikolay; Sing, David K.; Watson, Chris A. (2021), "An inventory of atomic species in the atmosphere of WASP-121b using UVES high-resolution spectroscopy", Monthly Notices of the Royal Astronomical Society, 506 (3): 3853–3871, arXiv:2106.15394, doi:10.1093/mnras/stab1878 <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></li>
<li id="fn:29">Azevedo Silva, T.; et al. (2022), "Detection of barium in the atmospheres of the ultra-hot gas giants WASP-76b and WASP-121b", Astronomy & Astrophysics, 666: L10, arXiv:2210.06892, doi:10.1051/0004-6361/202244489, S2CID 252873126 <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></li>
<li id="fn:30">Hoeijmakers, H. J.; Kitzmann, D.; Morris, B. M.; Prinoth, B.; Borsato, N.; Pino, L.; Lee, E. K. H.; Akın, C.; Heng, K. (2022), "The Mantis Network III: A titanium cold-trap on the ultra-hot Jupiter WASP-121 b.", Astronomy and Astrophysics, 685, arXiv:2210.12847, Bibcode:2024A&A...685A.139H, doi:10.1051/0004-6361/202244968 <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></li>
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</ol>

WASP-121b open-in-new

WASP-121b