The misaligned orbit of the Earth-sized planet Kepler-408b

Abstract

Kepler-408 is one of the 33 planet-hosting {\it Kepler} stars for which asteroseismology has been used to investigate the orientation of the stellar rotation axis relative to the planetary orbital plane. The transiting "hot Earth," Kepler-408b, has an orbital period of 2.5 days and a radius of $0.86$~$R_\oplus$, making it much smaller than the planets for which spin-orbit alignment has been studied using the Rossiter-McLaughlin effect. Because conflicting asteroseismic results have been reported in the literature, we undertake a thorough re-appraisal of this system and perform numerous checks for consistency and robustness. We find that the conflicting results are due to the different models for the low-frequency noise in the power spectrum. A careful treatment of the background noise resolves these conflicts, and shows that the stellar inclination is $\is=42^{+5}_{-4}$ degrees. Kepler-408b is, by far, the smallest planet known to have a significantly misaligned orbit.