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

摘要

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??4???MJup) are prograde but misaligned. Different mechanisms may therefore be responsible for planetary obliquities above and below ?~3.5???MJup.