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

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 Mp< 3   MJup. 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 (Mp> 4   MJup) are prograde but misaligned. Different mechanisms may therefore be responsible for planetary obliquities above and below  ~3.5   MJup.