Letters to the Editor

The retrograde orbit of the HAT-P-6b exoplanet^⋆,⋆⋆

¹ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
e-mail: hebrard@iap.fr
² Observatoire de Haute-Provence, CNRS/OAMP, 04870 Saint-Michel-l’Observatoire, France
³ UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, 38041 Grenoble, France
⁴ Laboratoire d’Astrophysique de Marseille, Univ. de Provence, CNRS (UMR6110), 38 rue F. Joliot Curie, 13388 Marseille Cedex 13, France
⁵ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Portugal

Received: 15 December 2010
Accepted: 26 January 2011

Abstract

We observed the transit of the HAT-P-6b exoplanet across its host star with the SOPHIE spectrograph (OHP, France). The resulting stellar radial velocities display the Rossiter-McLaughlin anomaly and reveal a retrograde orbit: the planetary orbital spin and the stellar rotational spin point in approximately opposite directions. A fit to the anomaly measures a sky-projected angle λ = 166° ± 10° between these two spin axes. All seven known retrograde planets are hot Jupiters with masses M_p < 3   M_Jup. About two thirds of the planets in this mass range, however, are prograde and aligned (λ ≃ 0°). In contrast, most of the more massive planets (M_p > 4   M_Jup) are prograde but misaligned. Different mechanisms may therefore be responsible for planetary obliquities above and below  ~3.5   M_Jup.