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

TL;DR

This study reports the discovery of a retrograde orbit for the exoplanet HAT-P-6b, using spectroscopic observations to analyze its orbital alignment and compare it with other hot Jupiters.

Contribution

It provides the first measurement of a retrograde orbit for HAT-P-6b and discusses the correlation between planetary mass and orbital obliquity among hot Jupiters.

Findings

HAT-P-6b has a retrograde orbit with a sky-projected angle of 166 +/- 10 degrees.

Most retrograde hot Jupiters are less massive (< 3 M_Jup), while more massive ones tend to be prograde but misaligned.

Different mechanisms may influence planetary obliquities depending on planetary mass.

Abstract

We observed with the SOPHIE spectrograph (OHP, France) the transit of the HAT-P-6b exoplanet across its host star. 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 towards approximately opposite directions. A fit to the anomaly measures a sky-projected angle lambda = 166 +/- 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 ~ 0). By 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.