[en] 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.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Anderson, D. R.; Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
Hellier, C.; Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
Gillon, Michaël ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astrophysique et traitement de l'image
Triaud, A H M J; Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
Smalley, B.; Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
Hebb, L.; School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK
Collier Cameron, A.; School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK
Maxted, P. F. L.; Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
Pollacco, D.; Astrophysics Research Centre, School of Mathematics & Physics, Queen's University, University Road, Belfast, BT7 1NN, UK
Ségransan, D.; Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
Udry, S.; Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
Wilson, D. M.; Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK ; Current address: Centre for Astrophysics & Planetary Science, University of Kent, Canterbury, Kent, CT2 7NH, UK.