Research Note

A much lower density for the transiting extrasolar planet WASP-7^⋆,⋆⋆

¹ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
e-mail: jkt@astro.keele.ac.uk
² SUPA, University of St Andrews, School of Physics & Astronomy, North Haugh, St Andrews, KY16 9SS, UK
³ Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen Ø, Denmark
⁴ Centre for Star and Planet Formation, Geological Museum, Øster Voldgade 5, 1350 Copenhagen, Denmark
⁵ Department of Physics, Sharif University of Technology, PO Box 11155-9161 Tehran, Iran
⁶ Max-Planck-Institute for Solar System Research, Max-Planck Str. 2, 37191 Katlenburg-Lindau, Germany
⁷ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁸ Qatar Foundation, Doha, Qatar
⁹ Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, via Ponte Don Melillo, 84084 - Fisciano (SA), Italy
¹⁰ Deutsches SOFIA Institut, Universitaet Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany
¹¹ SOFIA Science Center, NASA Ames Research Center, Mail Stop N211-3, Moffett Field CA 94035, USA
¹² Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Napoli, Italy
¹³ Istituto Internazionale per gli Alti Studi Scientifici (IIASS), 84019 Vietri Sul Mare (SA), Italy
¹⁴ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹⁵ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
¹⁶ Institut d’Astrophysique et de Géophysique, Université de Liège, 4000 Liège, Belgium
¹⁷ Astronomisches Rechen-Institut, Zentrum für Astronomie, Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
¹⁸ Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK
¹⁹ Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester, M13 9PL, UK
²⁰ Dipartimento di Ingegneria, Università del Sannio, Corso Garibaldi 107, 82100- Benevento, Italy
²¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 23 November 2010
Accepted: 20 December 2010

Abstract

We present the first high-precision photometry of the transiting extrasolar planetary system WASP-7, obtained using telescope defocussing techniques and reaching a scatter of 0.68 mmag per point. We find that the transit depth is greater and that the host star is more evolved than previously thought. The planet has a significantly larger radius (1.330 ± 0.093 R_Jup versus  R_Jup) and much lower density (0.41 ± 0.10   ρ_Jup versus    ρ_Jup) and surface gravity (13.4 ± 2.6 m s^-2 versus  m s^-2) than previous measurements showed. Based on the revised properties it is no longer an outlier in planetary mass-radius and period-gravity diagrams. We also obtain a more precise transit ephemeris for the WASP-7 system.