The Kepler Dichotomy among the M Dwarfs: Half of Systems Contain Five or More Coplanar Planets

TL;DR

This study analyzes Kepler M dwarf systems, revealing that about half contain five or more coplanar planets and that a mixture of system types explains the observed distribution of transiting planets, confirming the Kepler dichotomy.

Contribution

It demonstrates that the Kepler dichotomy applies to M dwarfs and introduces a mixture model to explain the prevalence of single and multi-planet systems.

Findings

Approximately 55% of systems are single or have large mutual inclinations.

Most systems contain around 6 planets with low mutual inclinations.

The Kepler dichotomy is confirmed for M dwarf planetary systems.

Abstract

We present a statistical analysis of the Kepler M dwarf planet hosts, with a particular focus on the fractional number of systems hosting multiple transiting planets. We manufacture synthetic planetary systems within a range of planet multiplicity and mutual inclination for comparison to the Kepler yield. We recover the observed number of systems containing between 2 and 5 transiting planets if every M dwarf hosts 6.1+/-1.9 planets with typical mutual inclinations of 2.0 +4.0-2.0 degrees. This range includes the Solar System in its coplanarity and multiplicity. However, similar to studies of Kepler exoplanetary systems around more massive stars, we report that the number of singly-transiting planets found by Kepler is too high to be consistent with a single population of multi-planet systems: a finding that cannot be attributed to selection biases. To account for the excess singleton planetary systems we adopt a mixture model and find that 55 +23-12% of planetary systems are either single or contain multiple planets with large mutual inclinations. Thus, we find that the so-called "Kepler dichotomy" holds for planets orbiting M dwarfs as well as Sun-like stars. Additionally, we compare stellar properties of the hosts to single and multiple transiting planets. For the brightest subset of stars in our sample we find intriguing, yet marginally significant evidence that stars hosting multiply-transiting systems are rotating more quickly, are closer to the midplane of the Milky Way, and are comparatively metal poor. This preliminary finding warrants further investigation.