 Size comparison of WASP-18b with Jupiter. | |
| Discovery[1] | |
|---|---|
| Discovered by | Hellier et al. (SuperWASP) |
| Discovery date | August 27, 2009 |
| Transit (including secondary eclipses) | |
| Orbital characteristics[2] | |
| 0.02041+0.00015 −0.00014 au (3,053,000 km) | |
| Eccentricity | 0.0051+0.0070 −0.0037[3] |
| 0.941452379(16) days (22.6 hours) | |
| Inclination | 84.08°±0.17° |
| −85°+72° −96°[3] | |
| Semi-amplitude | 1,814+23 −24 m/s[3] |
| Star | WASP-18 |
| Physical characteristics | |
| 1.1926±0.0077 RJ[2] | |
| Mass | 10.20±0.35 MJ[3] |
Meandensity | 6.6+1.2 −1.1 g/cm3[3] |
| Albedo | Geometric:0.027±0.011[2] |
| Temperature | 3,061 ± 29 K (2,787.8 ± 29.0 °C; 5,050.1 ± 52.2 °F)[2] |
WASP-18b is anexoplanet that is notable for having an orbital period of less than one day. It has a mass equal to 10Jupiter masses,[1] just below the boundary line between planets andbrown dwarfs (about 13 Jupiter masses). Due totidal deceleration, it is expected to spiral toward and eventually merge with its host star,WASP-18, in less than a million years.[1] The planet is approximately 3.1 million km (1.9 million mi; 0.021 AU) from its star, which is about 400light-years (120parsecs) from Earth. A team led by Coel Hellier, a professor of astrophysics atKeele University in England, discovered the exoplanet in 2009.[1]
Scientists at Keele and at theUniversity 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.[1][4] Observations made over the next decade should yield a measurement of the rate at which WASP-18b's orbit is decaying.[5] The closest example of a similar situation in theSolar System isMars' moonPhobos. 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[6] and is expected to be destroyed in about eleven million years.[7]
A study in 2012, utilizing theRossiter–McLaughlin effect, determined that the planetary orbit is well aligned with the equatorial plane of the star, with a misalignment equal to 13±7°.[8]
The planet's dayside temperature, as measured in 2020, is 3,029 ± 50 K (2,755.8 ± 50.0 °C; 4,992.5 ± 90.0 °F).[9] A 2023 study found an average dayside temperature of2,781+25
−13 K (2,508 °C; 4,546 °F).[10] 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 onJWST 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.[7]
A 2017 study detectedcarbon monoxide in the planet's atmosphere, without signs ofwater vapor.[11][12] However, in 2023, theJames Webb Space Telescope detected water vapor in the planet's atmosphere.[10][13]
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.[7]
Media related toWASP-18b at Wikimedia Commons
