SPIN–ORBIT ALIGNMENT OF EXOPLANET SYSTEMS: ENSEMBLE ANALYSIS USING ASTEROSEISMOLOGY

Abstract

The angle ψ between a planet’s orbital axis and the spin axis of its parent star is an important diagnostic of planet formation, migration, and tidal evolution. We seek empirical constraints on ψ by measuring the stellar inclination i s via asteroseismology for an ensemble of 25 solar-type hosts observed with NASA’s Kepler satellite. Our results for i s are consistent with alignment at the 2 σ level for all stars in the sample, meaning that the system surrounding the red-giant star Kepler-56 remains as the only unambiguous misaligned multiple-planet system detected to date. The availability of a measurement of the projected spin–orbit angle λ for two of the systems allows us to estimate ψ . We find that the orbit of the hot Jupiter HAT-P-7b is likely to be retrograde ( ), whereas that of Kepler-25c seems to be well aligned with the stellar spin axis ( ). While the latter result is in apparent contradiction with a statement made previously in the literature that the multi-transiting system Kepler-25 is misaligned, we show that the results are consistent, given the large associated uncertainties. Finally, we perform a hierarchical Bayesian analysis based on the asteroseismic sample in order to recover the underlying distribution of ψ . The ensemble analysis suggests that the directions of the stellar spin and planetary orbital axes are correlated, as conveyed by a tendency of the host stars to display large values of inclination.