WASP-18b

WASP-18b is an <a href="/facts/Exoplanet/zjfmqfUf">exoplanet</a> that is notable for having an orbital period of less than one day. It has a mass equal to 10 <a href="/facts/Jupiter_mass/EtLUmeEN">Jupiter masses</a>, just below the boundary line between planets and <a href="/facts/Brown_dwarf/fa7kr9lH">brown dwarfs</a> (about 13 Jupiter masses). Due to <a href="/facts/Tidal_acceleration/PwDTMyjb">tidal deceleration</a>, it is expected to spiral toward and eventually merge with its host star, <a href="/facts/WASP-18/sdgwFGrb">WASP-18</a>, in less than a million years. The planet is approximately 3.1 million <a href="/facts/Kilometre/hdQp79QE">km</a> (1.9 million <a href="/facts/Mile/e22bY569">mi</a>; 0.021 <a href="/facts/Astronomical_unit/gjsbu5xU">AU</a>) from its star, which is about 400 <a href="/facts/Light-year/IGc5NdQb">light-years</a> (120 <a href="/facts/Parsec/SwSsk57J">parsecs</a>) from Earth. A team led by Coel Hellier, a professor of astrophysics at <a href="/facts/Keele_University/3JumIoNJ">Keele University</a> in England, discovered the exoplanet in 2009.
Scientists at Keele and at the <a href="/facts/University_of_Maryland/Wmc5R1kM">University of Maryland</a> are working to understand whether the discovery of this planet so shortly before its expected demise (with less than 0.1% of its lifetime remaining) was fortuitous, or whether tidal dissipation by WASP-18 is actually much less efficient than astrophysicists typically assume. Observations made over the next decade should yield a measurement of the rate at which WASP-18b's orbit is decaying.
The closest example of a similar situation in the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a> is <a href="/facts/Mars/AQGtVvz0">Mars</a>' moon <a href="/facts/Phobos_(moon)/qqS9JvuG">Phobos</a>. Phobos orbits Mars at a distance of only about 9,000 km (5,600 mi), 40 times closer than the Moon is to the Earth and is expected to be destroyed in about eleven million years.
The planet's dayside temperature, as measured in 2020, is 3,029 ± 50 <a href="/facts/Kelvin/E3trzC4C">K</a> (2,755.8 ± 50.0 <a href="/facts/Celsius/okAEu1CA">°C</a>; 4,992.5 ± 90.0 <a href="/facts/Fahrenheit/2TVmyJod">°F</a>). A 2023 study found an average dayside temperature of 2,781+25−13 K (2,508 °C; 4,546 °F).
A study in 2012, utilizing the <a href="/facts/Rossiter%25E2%2580%2593McLaughlin_effect/QEMgxao5">Rossiter–McLaughlin effect</a>, determined that the planetary orbit is well aligned with the equatorial plane of the star, with a misalignment equal to 13±7°.
A 2017 study detected <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a> in the planet's atmosphere, without signs of <a href="/facts/Water_vapor/DX5IUFG0">water vapor</a>. However, in 2023, the <a href="/facts/James_Webb_Space_Telescope/w4GAWnWX">James Webb Space Telescope</a> detected water vapor in the planet's atmosphere.

WASP-18b open-in-new

WASP-18b