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.
@article{arxiv.1101.5009,
title = {The retrograde orbit of the HAT-P-6b exoplanet},
author = {G. Hebrard and D. Ehrenreich and F. Bouchy and X. Delfosse and C. Moutou and L. Arnold and I. Boisse and X. Bonfils and R. F. Diaz and A. Eggenberger and T. Forveille and A. -M. Lagrange and C. Lovis and F. Pepe and C. Perrier and D. Queloz and A. Santerne and N. C. Santos and D. Segransan and S. Udry and A. Vidal-Madjar},
journal= {arXiv preprint arXiv:1101.5009},
year = {2015}
}
Comments
4 pages, 4 figures, accepted for publication in A&A Letters