WASP-21b: a hot-Saturn exoplanet transiting a thick disc star ^*,**

¹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Bd Arago, 75014 Paris, France e-mail: bouchy@iap.fr
² Observatoire de Haute-Provence, CNRS/OAMP, 04870 St. Michel l'Observatoire, France
³ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
⁴ SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK
⁵ Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de la Palma, Tenerife, Spain
⁶ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
⁷ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁸ Department of Physics and Astronomy, The Open University, Milton Keynes, MK7 6AA, UK
⁹ Astrophysics Research Centre, School of Mathematics and Physics, Queen's University, University Road, Belfast, BT7 1NN, UK
¹⁰ NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA
¹¹ Laboratoire d'Astrophysique de Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹² Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
¹³ Las Cumbres Observatory, 6740 Cortona Drive Suite 102, Goleta, CA 93117, USA
¹⁴ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK
¹⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK

Received: 19 April 2010
Accepted: 13 May 2010

Abstract

We report the discovery of WASP-21b, a new transiting exoplanet discovered by the Wide Angle Search for Planets (WASP) Consortium and established and characterized with the FIES, SOPHIE, CORALIE and HARPS fiber-fed echelle spectrographs. A 4.3-d period, 1.1% transit depth and 3.4-h duration are derived for WASP-21b using SuperWASP-North and high precision photometric observations at the Liverpool Telescope. Simultaneous fitting to the photometric and radial velocity data with a Markov Chain Monte Carlo procedure leads to a planet in the mass regime of Saturn. With a radius of 1.07 R_Jup and mass of 0.30 M_Jup, WASP-21b has a density close to 0.24 ρ_Jup corresponding to the distribution peak at low density of transiting gaseous giant planets. With a host star metallicity [Fe/H] of –0.46, WASP-21b strengthens the correlation between planetary density and host star metallicity for the five known Saturn-like transiting planets. Furthermore there are clear indications that WASP-21b is the first transiting planet belonging to the thick disc.