Simon Albrecht, Joshua N. Winn, Geoffrey W. Marcy, Andrew W. Howard, Howard Isaacson, John A. Johnson
We measure the sky-projected stellar obliquities (lambda) in the multiple-transiting planetary systems KOI-94 and Kepler-25, using the Rossiter-McLaughlin effect. In both cases the host stars are well-aligned with the orbital planes of the planets: for KOI-94 we find lambda=-14+-13 deg, confirming a recent result by Hirano and coworkers; and for Kepler-25 we find lambda=2+-5 deg. There are now a total of five obliquity measurements for host stars of systems of multiple transiting planets, all of which are consistent with spin-orbit alignment. This alignment is unlikely to be the result of tidal interactions, because of the relatively large orbital distances and low planetary masses in the systems. In this respect the multiplanet host stars differ from hot-Jupiter host stars, which commonly have large spin-orbit misalignments whenever tidal interactions are weak. In particular the weak-tide subset of hot-Jupiter hosts have obliquities consistent with an isotropic distribution (p=0.6), but the multiplanet hosts are incompatible with such a distribution (p~10^-6). This suggests that high obliquities are confined to hot-Jupiter systems, and provides further evidence that hot Jupiter formation involves processes that tilt the planetary orbit.
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http://arxiv.org/abs/1302.4443
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