WASP-39b: a highly inflated Saturn-mass planet orbiting a late G-type star^⋆

¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
e-mail: f.faedi@qub.ac.uk
² Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
³ School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK
⁴ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, France
⁵ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Observatoire de Haute-Provence, CNRS/OAMP, 04870 St Michel l’Observatoire, France
⁷ Observatoire astronomique de l’Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁸ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive Suite 102, CA 93117, USA
⁹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁰ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
¹¹ Department of Physics and Astronomy, The Open University, Milton Keynes, MK7 6AA, UK
¹² Laboratoire d’Astrophysique de Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹³ Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de Palma, Spain

Received: 7 February 2011
Accepted: 21 April 2011

Abstract

We present the discovery of WASP-39b, a highly inflated transiting Saturn-mass planet orbiting a late G-type dwarf star with a period of 4.055259 ± 0.000008 d, Transit Epoch T₀ = 2   455   342.9688 ± 0.0002 (HJD), of duration 0.1168 ± 0.0008 d. A combined analysis of the WASP photometry, high-precision follow-up transit photometry, and radial velocities yield a planetary mass of M_pl = 0.28 ± 0.03   M_J and a radius of R_pl = 1.27 ± 0.04   R_J, resulting in a mean density of 0.14 ± 0.02   ρ_J. The stellar parameters are mass M_⋆ = 0.93 ± 0.03   M_⊙, radius R_⋆ = 0.895 ± 0.23   R_⊙, and age  Gyr. Only WASP-17b and WASP-31b have lower densities than WASP-39b, although they are slightly more massive and highly irradiated planets. From our spectral analysis, the metallicity of WASP-39 is measured to be [Fe/H]  =  −0.12 ± 0.1 dex, and we find the planet to have an equilibrium temperature of  K. Both values strengthen the observed empirical correlation between these parameters and the planetary radius for the known transiting Saturn-mass planets.