Spin-orbit Misalignment as a Driver of the Kepler Dichotomy

TL;DR

This paper proposes that stellar obliquity, excited during the disk-hosting stage, can explain the Kepler Dichotomy by inducing mutual inclinations in planetary systems, leading to more single-transiting planets, especially around more massive stars.

Contribution

It introduces a novel explanation for the Kepler Dichotomy based on spin-orbit misalignment caused by stellar quadrupole moments during early stellar evolution.

Findings

Stellar obliquity can excite mutual inclinations in planetary systems.

Single-transiting systems are more common around more massive stars.

Analysis of Kepler data supports the correlation between stellar mass and transit multiplicity.

Abstract

During its 5 year mission, the Kepler spacecraft has uncovered a diverse population of planetary systems with orbital configurations ranging from single-transiting planets to systems of multiple planets co-transiting the parent star. By comparing the relative occurrences of multiple to single-transiting systems, recent analyses have revealed a significant over-abundance of singles. Dubbed the "Kepler Dichotomy," this feature has been interpreted as evidence for two separate populations of planetary systems: one where all orbits are confined to a single plane, and a second where the constituent planetary orbits possess significant mutual inclinations, allowing only a single member to be observed in transit at a given epoch. In this work, we demonstrate that stellar obliquity, excited within the disk-hosting stage, can explain this dichotomy. Young stars rotate rapidly, generating a significant quadrupole moment which torques the planetary orbits, with inner planets influenced more strongly. Given nominal parameters, this torque is sufficiently strong to excite significant mutual inclinations between planets, enhancing the number of single-transiting planets, sometimes through a dynamical instability. Furthermore, as hot stars appear to possess systematically higher obliquities, we predict that single-transiting systems should be relatively more prevalent around more massive stars. We analyze the Kepler data and confirm this signal to be present.