Stellar Oblateness versus Distant Giants in Exciting Kepler Planet Mutual Inclinations

TL;DR

This paper investigates how stellar oblateness and distant giant planets influence mutual inclinations in Kepler systems, finding that stellar oblateness generally dominates early on, with the effect depending on disk dispersal timescales.

Contribution

It compares the effects of stellar oblateness and distant giants on mutual inclinations, highlighting the importance of disk dispersal timescales in shaping planetary system architectures.

Findings

Stellar oblateness often exceeds the influence of distant giants shortly after planet formation.

Rapid disk dispersal (<10^3-4 years) leads to significant mutual inclinations.

Slow disk dispersal results in planets remaining coplanar or realigning with the giant's plane.

Abstract

An overabundance of single-transiting Kepler planets suggests the existence of a sub-population of intrinsically multi-planet 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 due to the oblateness of the rapidly-rotating host star, provided 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 upon the timescale over which the natal disk disperses. Specifically, we find that if the disk vanishes over a timescale shorter than 10^(3-4) years, comparable to the viscous timescale of the inner ~0.2AU, 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 multis but misaligned with singles. Stellar obliquity measurements offer a promising test of our proposed framework.