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 lambda = 166 degrees +/- 10 degrees 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 (lambda similar or equal to 0 degrees). 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 similar to 3.5 M-Jup.