The Orbital Eccentricities of Planets Orbiting M Dwarfs

Exoplanet orbital eccentricities encode key information about planetary system formation and evolution. For M dwarf planets in particular, the proximity of the habitable zone to the host star makes eccentricity additionally important for understanding habitability. We investigate the underlying distribution of orbital eccentricities for planets around early-to-mid M dwarf host stars. We employ a sample of 163 planets around early- to mid-M dwarfs across 101 systems detected by NASA's Kepler Mission. We constrain the orbital eccentricity for each planet by leveraging the Kepler lightcurve with a stellar density prior, constructed using metallicity from spectroscopy and stellar parallax from ESA's Gaia mission. Within a Bayesian hierarchical framework, we extract the underlying eccentricity distribution for both single- and multi-transit systems. The data suggest the possibility of distinct dynamically warmer and cooler sub-populations within the single-transit distribution: the single-transit data prefer a mixture model composed of two distinct Rayleigh distributions, one preferring higher eccentricities and one lower eccentricities, over a single Rayleigh distribution with 7:1 odds. We contextualize our findings within a planet formation framework by comparing them to analogous results in the literature for planets orbiting FGK stars. By combining our derived eccentricity distribution with other M dwarf demographic constraints, we estimate the underlying eccentricity distribution for the population of early- to mid-M dwarf planets in the local neighborhood. Comparisons between the eccentricity distribution for singly- and multiply-transiting systems, as well as the distribution between M dwarfs and FGK dwarfs, help illuminate the dynamical states of M dwarf planets at a population level. Extending this work, we construct a flexible stellar age prediction model by combining stellar rotation information with age-correlated galactic kinematics information from Gaia for a calibration sample of roughly 50,000 stars. We use this model to estimate ages for Kepler M dwarfs, and constrain their metallicities using Gaia color-magnitude information and spectral data from the SDSS APOGEE survey. We investigate possible relationships between M dwarf stellar properties and the occurrence and dynamics of their planetary systems