Transiting exoplanets from the CoRoT space mission^*,**

XI. CoRoT-8b: a hot and dense sub-Saturn around a K1 dwarf
 


¹ Institut d'astrophysique spatiale, Université Paris-Sud 11 & CNRS (UMR 8617), Bât. 121, 91405 Orsay, France e-mail: pascal.borde@ias.u-psud.fr
² Institut d'astrophysique de Paris, Université Paris 6 & CNRS (UMR 7095), 98 bd Arago, 75014 Paris, France
³ Observatoire de Haute-Provence, CNRS & OAMP, 04870 St-Michel l'Observatoire, France
⁴ 
Laboratoire d'astrophysique de Marseille, Université de Provence & CNRS (UMR 6110), 38 rue F. Joliot-Curie, 13388 Marseille, France
⁵ Institute of Planetary Research, German Aerospace Center, Rutherfordstrasse 2, 12489 Berlin, Germany
⁶ Laboratoire de l'univers et de ses théories, Observatoire de Paris & CNRS (UMR 8102), 5 place Jules Janssen, 92195 Meudon, France
⁷ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁸ Department of Physics, Denys Wilkinson Building Keble Road, Oxford, OX1 3RH, UK
⁹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹⁰ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
¹¹ LESIA, UMR 8109 CNRS, Observatoire de Paris, UPMC, Université Paris-Diderot, 5 place J. Janssen, 92195 Meudon, France
¹² Physikalisches Institut Universität Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
¹³ Rheinisches Institut für Umweltforschung an der Universität zu Köln, Aachener Str. 209, 50931 Köln, Germany
¹⁴ Research and Scientific Support Department, European Space Agency, Keplerlaan, 2200AG, Noordwijk, The Netherlands
¹⁵ Institute for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180, Vienna, Austria
¹⁶ Institute of Astronomy, Geophysics and Atmospheric Sciences, University of São Paulo, Brasil
¹⁷ Thüringer Landessternwarte, 07778 Tautenburg, Germany
¹⁸ IAG, Université de Liège, Allée du 6 août 17, Liège 1, Belgium
¹⁹ Université de Nice-Sophia Antipolis, CNRS UMR 6202, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France
²⁰ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, Graz, Austria
²¹ Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
²² Center for Astronomy and Astrophysics, TU Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany

Received: 12 April 2010
Accepted: 21 June 2010

Abstract

Aims. We report the discovery of CoRoT-8b, a dense small Saturn-class exoplanet that orbits a K1 dwarf in 6.2 days, and we derive its orbital parameters, mass, and radius.

Methods. We analyzed two complementary data sets: the photometric transit curve of CoRoT-8b as measured by CoRoT and the radial velocity curve of CoRoT-8 as measured by the HARPS spectrometer.

Results. We find that CoRoT-8b is on a circular orbit with a semi-major axis of 0.063 ± 0.001 AU. It has a radius of 0.57 ± 0.02 R_J, a mass of 0.22 ± 0.03 M_J, and therefore a mean density of 1.6 ± 0.1 g cm^-3.

Conclusions. With 67% of the size of Saturn and 72% of its mass, CoRoT-8b has a density comparable to that of Neptune (1.76 g cm^-3). We estimate its content in heavy elements to be 47–63 , and the mass of its hydrogen-helium envelope to be 7–23 . At 0.063 AU, the thermal loss of hydrogen of CoRoT-8b should be no more than ~0.1% over an assumed integrated lifetime of 3 Ga.