Wasp-17b: An Ultra-Low Density Planet in a Probable Retrograde Orbit

• Published in: The Astrophysical journal Vol. 709; no. 1; pp. 159 - 167
• Main Authors: Anderson, D. R, Hellier, C, Gillon, M, Triaud, A. H. M. J, Smalley, B, Hebb, L, Cameron, A. Collier, Maxted, P. F. L, Queloz, D, West, R. G, Bentley, S. J, Enoch, B, Horne, K, Lister, T. A, Mayor, M, Parley, N. R, Pepe, F, Pollacco, D, Ségransan, D, Udry, S, Wilson, D. M
• Format: Journal Article Web Resource
• Language: English
• Published: Bristol IOP Publishing 20.01.2010; IOP; University of Chicago Press
• Subjects: Astronomy; Astrophysics - Earth and Planetary Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Aérospatiale, astronomie & astrophysique; Density; DIMENSIONS; Earth, ocean, space; ECLIPSE; Exact sciences and technology; GRAVITATION; HEIGHT; JUPITER PLANET; MASS; ORBITS; Physical, chemical, mathematical & earth Sciences; Physique, chimie, mathématiques & sciences de la terre; planetary systems; PLANETS; RADIAL VELOCITY; Retrograde orbits; SATURN PLANET; SCALE HEIGHT; Space science, astronomy & astrophysics; SPECTROSCOPY; STARS; stars: individual: WASP-17; VELOCITY; Orbits; Planets; planetary systems; stars: individual (WASP-17); Planetary system
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/709/1/159

We report the discovery of the transiting giant planet WASP-17b, the least-dense planet currently known. It is 1.6 Saturn masses but 1.5-2 Jupiter radii, giving a density of 6-14 per cent that of Jupiter. WASP-17b is in a 3.7-day orbit around a sub-solar metallicity, V = 11.6, F6 star. Preliminary detection of the Rossiter-McLaughlin effect suggests that WASP-17b is in a retrograde orbit (lambda ~ -150 deg), indicative of a violent history involving planet-planet or planet-star scattering. WASP-17b's bloated radius could be due to tidal heating resulting from recent or ongoing tidal circularisation of an eccentric orbit, such as the highly eccentric orbits that typically result from scattering interactions. It will thus be important to determine more precisely the current orbital eccentricity by further high-precision radial velocity measurements or by timing the secondary eclipse, both to reduce the uncertainty on the planet's radius and to test tidal-heating models. Owing to its low surface gravity, WASP-17b's atmosphere has the largest scale height of any known planet, making it a good target for transmission spectroscopy.