Stellar Oblateness versus Distant Giants in Exciting Kepler Planet Mutual Inclinations

Abstract

Abstract An overabundance of single-transiting Kepler planets suggests the existence of a subpopulation of intrinsically multiplanet systems possessing large mutual inclinations. However, the origin of these mutual inclinations remains unknown. Recent work has demonstrated that mutual inclinations can be excited soon after protoplanetary disk dispersal owing to the oblateness of the rapidly rotating host star, provided that the star is tilted. Alternatively, distant giant planets, which are common in systems of close-in Kepler planets, could drive up mutual inclinations. The relative importance of each of these mechanisms has not been investigated. Here, we show that the influence of the stellar oblateness typically exceeds that of an exterior giant soon after planet formation. However, the magnitude of the resulting mutual inclinations depends critically on the timescale over which the natal disk disperses. Specifically, we find that if the disk vanishes over a timescale shorter than ∼103–4 yr, comparable to the viscous timescale of the inner ∼0.2 au, the inner planets impulsively acquire misalignments that scale with the stellar obliquity. In contrast, if the disk disperses slowly, the inner planets remain coplanar. They first align with the stellar equator but subsequently realign with the distant giant’s plane as the star spins down. Our findings are consistent with recent observations that giants tend to be aligned with close-in multiplanet systems but misaligned with single-transiting planets. Stellar obliquity measurements offer a promising test of our proposed framework.