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

G. Hébrard, D. Ehrenreich, F. Bouchy, X. Delfosse, C. Moutou, L. Arnold, I. Boisse, X. Bonfils, R. F. Díaz, A. Eggenberger, T. Forveille, A.-M. Lagrange, C. Lovis, F. Pepe, C. Perrier, D. Queloz, A. Santerne, N. C. Santos, D. Ségransan, S. Udry

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.

planets and satellites: dynamical evolution and stability - techniques: radial velocities - planetary systems - stars: individual: HAT-P-6

Based on observations collected with the SOPHIE spectrograph on the 1.93-m telescope at Observatoire de Haute-Provence (CNRS), France, by the SOPHIE Consortium (program 10A.PNP.CONS).
SOPHIE radial velocities are only available in electronic form at the CDS via anonymous ftp to ( or via

Astronomy and Astrophysics
Volume 527, Number of pages L11_1
2011 March

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Fundação para a Ciência e a Tecnologia
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