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

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.

Dynamically Hot Super-Earths from Outer Giant Planet Scattering

“…Lissauer et al (2011) first noted with preliminary Kepler data that when modeling the mutual inclination distribution as a Rayleigh function, they had difficulty reproducing the large observed ratio of single transiting systems to multiple transiting systems. This "problem" was later on referred to as the "Kepler dichotomy" in several other studies (Johansen et al 2012;Hansen & Murray 2013;Ballard & Johnson 2016). A wide range of studies into this problem have been undertaken with varying degrees of success (e.g., Johansen et al 2012;Moriarty & Ballard 2015;Ballard & Johnson 2016;Dawson et al 2016), but a consistent picture is still missing.…”

“…This "problem" was later on referred to as the "Kepler dichotomy" in several other studies (Johansen et al 2012;Hansen & Murray 2013;Ballard & Johnson 2016). A wide range of studies into this problem have been undertaken with varying degrees of success (e.g., Johansen et al 2012;Moriarty & Ballard 2015;Ballard & Johnson 2016;Dawson et al 2016), but a consistent picture is still missing.…”

A Search for Additional Bodies in the GJ 1132 Planetary System from 21 Ground-based Transits and a 100-hr Spitzer Campaign

“…They found that the total number of single transiting planet systems is incompatible with the number of systems with multiple transiting planets, assuming a single planet population distribution. Instead, half of all M dwarf planetary systems consist of a single planet, while half of M dwarf systems contain five or more planets with a low mutual inclination (Ballard & Johnson 2016). If GJ 1132 harbors additional planets, we can attempt to uncover them via transit and radial velocity observations.…”

“…Assuming a disk mass-tostellar mass ratio of 1%, these planets would account for nearly all of the non-hydrogen and helium disk mass, and Muirhead et al (2015) suggest that this may indicate a very high efficiency in the planet formation mechanism around M dwarfs and a lack of planets at large orbital periods around mid-M dwarfs. A study by Ballard & Johnson (2016) generated synthetic populations of exoplanets described by a range of planet multiplicity and mutual inclination for comparison to the Kepler M dwarf sample. They found that the total number of single transiting planet systems is incompatible with the number of systems with multiple transiting planets, assuming a single planet population distribution.…”

The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced*

“…In the Kepler multi-planet systems, multiple planets transit the star, resulting in measured orbital periods and planet-to-star radius ratios for each transiting planet. The observed and statistically inferred orbital properties in multi-planet systems have been used to deduce possible planet formation histories (Fang & Margot 2012;Hansen & Murray 2013;Malhotra 2015;Pu & Wu 2015;Steffen & Hwang 2015;Ballard & Johnson 2016;Xie et al 2016).…”