WASP-117b: a 10-day-period Saturn in an eccentric and misaligned orbit^⋆,⋆⋆

¹ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bât. B5C, Liège 1, 4000 Belgium
e-mail: monika.lendl@ulg.ac.be
² Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
³ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
⁴ SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife KY16 9SS, UK
⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁶ Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, UK
⁷ N. Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
⁸ Kavli Institute for Astrophysics & Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received: 26 June 2014
Accepted: 31 July 2014

Abstract

We report the discovery of WASP-117b, the first planet with a period beyond 10 days found by the WASP survey. The planet has a mass of M_p = 0.2755 ± 0.0089 M_J, a radius of and is in an eccentric (e = 0.302 ± 0.023), 10.02165 ± 0.00055 d orbit around a main-sequence F9 star. The host star’s brightness (V = 10.15 mag) makes WASP-117 a good target for follow-up observations, and with a periastron planetary equilibrium temperature of K and a low planetary mean density () it is one of the best targets for transmission spectroscopy among planets with periods around 10 days. From a measurement of the Rossiter-McLaughlin effect, we infer a projected angle between the planetary orbit and stellar spin axes of β = −44 ± 11 deg, and we further derive an orbital obliquity of deg. Owing to the large orbital separation, tidal forces causing orbital circularization and realignment of the planetary orbit with the stellar plane are weak, having had little impact on the planetary orbit over the system lifetime. WASP-117b joins a small sample of transiting giant planets with well characterized orbits at periods above ~8 days.