WASP-18b

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Extrasolar planet that has an orbital period of less than one day

title: "WASP-18b" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["exoplanets-discovered-by-wasp", "exoplanets-discovered-in-2009", "giant-planets", "hot-jupiters", "transiting-exoplanets", "phoenix-(constellation)"] description: "Extrasolar planet that has an orbital period of less than one day" topic_path: "general/exoplanets-discovered-by-wasp" source: "https://en.wikipedia.org/wiki/WASP-18b" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Extrasolar planet that has an orbital period of less than one day ::

::data[format=table title="Infobox planet"]

Field	Value
name	WASP-18b
image	Exoplanet Comparison WASP-18 b.png
caption	Size comparison of WASP-18b with Jupiter.
discovery_ref
discoverer	Hellier et al. (SuperWASP)
discovered	August 27, 2009
discovery_method	Transit (including secondary eclipses)
orbit_ref
apsis	astron
semimajor
(0.02041 AU km)
eccentricity
period	days
(0.941452379 day hours)
inclination
arg_peri
semi-amplitude
star	WASP-18
mean_radius
mass
density
single_temperature	3061 ±
albedo	Geometric:
::

| name = WASP-18b | image = Exoplanet Comparison WASP-18 b.png | caption = Size comparison of WASP-18b with Jupiter. | discovery_ref = | discoverer = Hellier et al. (SuperWASP) | discovered = August 27, 2009 | discovery_method = Transit (including secondary eclipses) | orbit_ref = | apsis = astron | aphelion = | perihelion = | semimajor = (0.02041 AU km) | eccentricity = | period = days (0.941452379 day hours) | inclination = | arg_peri = | semi-amplitude = | star = WASP-18 | physical_ref = | mean_radius = | mass = | density = | single_temperature = 3061 ± | albedo = Geometric: WASP-18b is an exoplanet that is notable for having an orbital period of less than one day. It has a mass equal to 10 Jupiter masses, just below the boundary line between planets and brown dwarfs (about 13 Jupiter masses). Due to tidal deceleration, it is expected to spiral toward and eventually merge with its host star, WASP-18, in less than a million years. The planet is approximately 3.1 e6km from its star, which is about 400 ly from Earth. A team led by Coel Hellier, a professor of astrophysics at Keele University in England, discovered the exoplanet in 2009.

Scientists at Keele and at the University of Maryland 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 Solar System is Mars' moon Phobos. Phobos orbits Mars at a distance of only about 9000 km, 40 times closer than the Moon is to the Earth and is expected to be destroyed in about eleven million years.{{cite arXiv | last = Sharma | first = Bijay Kumar | title = Theoretical Formulation of the Phobos, moon of Mars, rate of altitudinal loss | date = 2008-05-10 | eprint = 0805.1454 | class = astro-ph

A study in 2012, utilizing the Rossiter–McLaughlin effect, determined that the planetary orbit is well aligned with the equatorial plane of the star, with a misalignment equal to 13°.

Temperature

The planet's dayside temperature, as measured in 2020, is 3029 ±. A 2023 study found an average dayside temperature of (2781 K). It has been theorized that highly irradiated ultra-hot Jupiters like WASP-18b have large variations in atmospheric temperature and chemistry as a function of longitude, latitude and altitude. WASP-18b was observed with the Near Infrared Imager and Slitless Spectrograph instruments on JWST creating a resolved atmosphere in multiple dimensions. Mapping confirmed theoretical models revealing a weaker longitudinal temperature gradient. It also indicated the importance of hydrogen dissociation and/or nightside clouds role in shaping thermal emission on the global scale.

Atmosphere

A 2017 study detected carbon monoxide in the planet's atmosphere, without signs of water vapor. However, in 2023, the James Webb Space Telescope detected water vapor in the planet's atmosphere.

There are two thermally distinct identified regions on the planets atmosphere. The first is the “hotspot” surrounding the substellar point. near the dayside limbs. The hotspot region shows a strongly inverted thermal structure due to the presence of optical absorbers and a water abundance marginally lower than average. The second region is a “ring” near the dayside limb. This region shows colder temperatures and poorly constrained chemical abundances.

::figure[src="https://upload.wikimedia.org/wikipedia/commons/8/8c/PIA22087-Exoplanet-Wasp18b-COinStratosphere-20171129.jpg" caption="Exoplanet WASP-18b − high [[carbon monoxide]] levels detected in [[stratosphere]] (artist concept)}}"] ::

References

| last = Thompson | first = Andrea | title = Newfound Planet Might Be Near Death | work = Space.Com | publisher = Imaginova | date = 2009-08-26 | url = http://www.space.com/scienceastronomy/090826-strange-planet.html | access-date = 2009-08-28}}

References

Johnson, John Jr.; [http://www.latimes.com/news/nationworld/nation/la-sci-planet27-2009aug27,0,5882308.story Astrophysicists puzzle over planet that's too close to its sun], ''Los Angeles Times'' (August 27, 2009).
(2023-06-01). "Webb telescope discovers traces of water in atmosphere of exoplanet with mass of 10 Jupiters".
(2009). "An orbital period of 0.94days for the hot-Jupiter planet WASP-18b".
(2012). "Obliquities of Hot Jupiter Host Stars: Evidence for Tidal Interactions and Primordial Misalignments". The Astrophysical Journal.
(2015). "Secondary eclipse observations for seven hot-Jupiters from the Anglo-Australian Telescope". Monthly Notices of the Royal Astronomical Society.
(2025). "Dark skies of the slightly eccentric WASP-18 b from its optical-to-infrared dayside emission". Astronomy & Astrophysics.
(December 2017). "Evidence for a Dayside Thermal Inversion and High Metallicity for the Hot Jupiter WASP-18b". [[The Astrophysical Journal Letters]].
(29 November 2017). "Exoplanet Has Smothering Stratosphere Without Water". [[NASA]].
(2018). "Pre-discovery transits of the exoplanets WASP-18b and WASP-33b from Hipparcos". Monthly Notices of the Royal Astronomical Society: Letters.
(April 2020). "TraMoS. V. Updated ephemeris and multi-epoch monitoring of the hot Jupiters WASP-18Ab, WASP-19b, and WASP-77Ab". [[Astronomy & Astrophysics]].
(2020). "Systematic Phase Curve Study of Known Transiting Systems from Year One of the TESS Mission". The Astronomical Journal.
(August 2023). "A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b". [[Nature (journal).

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