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Exoplanet List of exoplanets
Exoplanet Comparison WASP-18 b.png
Size comparison of WASP-18b with Jupiter.
Parent star
Star WASP-18
Constellation Phoenix[1]
Right ascension (α) 01h 37m 24.95s[1]
Declination (δ) −45° 40′ 40.8″[1]
Apparent magnitude (mV) 9.29[1]
Distance 325 ly
(100 ± 10[2] pc)
Spectral type F6[2]
Mass (m) 1.25[3] M
Orbital elements
Semi-major axis (a) 0.02026 ± 0.00068[2] AU
Periastron (q) 0.02007 AU
Apastron (Q) 0.02045 AU
Eccentricity (e) 0.0092 ± 0.0028[2]
Orbital period (P) 0.94145299[1] d
    (22.59487 h)
Inclination (i) 86 ± 2.5[2]°
Argument of
(ω) 96 ± 10[2]°
Time of transit (Tt) 2454221.48163 ± 0.00038[2] JD
Physical characteristics
Mass (m) 10.3 ± 0.69[2] MJ
Radius (r) 1.106+0.072
[2] RJ
Discovery information
Discovery date August 27, 2009[4]
Discoverer(s) Hellier et al. (SuperWASP)[4]
Discovery method Transit[4] (including secondary eclipses)
Other detection methods Radial velocity
Discovery status Published[4]
Other designations
HD 10069 b, HIP 7562 b
Database references
Extrasolar Planets
Exoplanet Archive data
Open Exoplanet Catalogue data

WASP-18b is an extrasolar planet that is notable for having an orbital period of less than one day. It has a mass equal to 10 Jupiter masses,[4] just below the boundary line between planets and brown dwarfs, about 13 Jupiter masses. Due to tidal deceleration, it is expected to spiral towards and eventually merge with its host star, WASP-18, in less than a million years.[4] The planet is approximately 3.1 million kilometres (1.9 million miles) from its star, which is about 325 light-years from Earth. It was discovered by a team led by Coel Hellier, a professor of astrophysics at Keele University in England.[4]

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.[4][5] Observations made over the next decade should yield a measurement of the rate at which WASP-18b's orbit is decaying.[6]

The closest example of a similar situation in the Solar System is Mars' moon Phobos. 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,[7] and is expected to be destroyed in about eleven million years.[8]

See also[edit]


  1. ^ a b c d e "WASP-18b". Exoplanet Transit Database. Retrieved 2009-08-29. 
  2. ^ a b c d e f g h i "Notes for planet WASP-18b". The Extrasolar Planets Encyclopaedia. Retrieved 2009-09-15. 
  3. ^ PlanetQuest: WASP-18 b
  4. ^ a b c d e f g h Hellier, Coel; et al. (2009). "An orbital period of 0.94days for the hot-Jupiter planet WASP-18b" (PDF). Nature. 460 (7259): 1098–1100. Bibcode:2009Natur.460.1098H. doi:10.1038/nature08245. PMID 19713926. 
  5. ^ Hamilton, D. P. (2009-08-27). "Extrasolar planets: Secrets that only tides will tell". Nature. Nature Publishing Group. 460 (7259): 1086–1087. Bibcode:2009Natur.460.1086H. doi:10.1038/4601086a. PMID 19713920. Retrieved 2009-08-28. 
  6. ^ Thompson, Andrea (2009-08-26). "Newfound Planet Might Be Near Death". Space.Com. Imaginova. Retrieved 2009-08-28. 
  7. ^ John Johnson Jr., Astrophysicists puzzle over planet that's too close to its sun, Los Angeles Times (August 27, 2009).
  8. ^ Sharma, B. K. (2008-05-10). "Theoretical Formulation of the Phobos, moon of Mars, rate of altitudinal loss". arXiv:0805.1454free to read [astro-ph]. 

External links[edit]

Media related to WASP-18b at Wikimedia Commons

Coordinates: Sky map 01h 37m 25s, −45° 40′ 41″