The Orbital Eccentricities of Planets in the Kinematic Thin and Thick Galactic Disks

Abstract The orbital eccentricity distribution of exoplanets is shaped by a combination of dynamical processes, reflecting both formation conditions and long-term evolution. Probing the orbital dynamics of planets in the kinematic thin and thick Galactic disks provides insight into the degree to which stellar and Galactic environmental factors affect planet formation and evolution pathways. The classification of host stars in Galactic kinematic terms constitutes a potentially useful axis for the interpretation of orbital eccentricity, when included together with stellar metallicity and age. Leveraging the photoeccentric effect, we constrain orbital eccentricities for the sample of Kepler planets and candidates orbiting F, G, K, and M dwarf stars. With Gaia astrometry, inferred Galactic phase space information, and kinematic disk criteria calibrated on stellar chemical abundances, we implement a machine learning approach to associate each planet host with the kinematic thin or thick Galactic disks. We then fit the underlying eccentricity distributions for the single- and multitransit populations. We find that for single-transiting planets, kinematic thick-disk planets exhibit higher eccentricities than thin-disk planets, yet we find no such difference among multitransiting planets. We determine that the difference in eccentricity is unlikely to be caused solely by the effects of host stellar metallicity or giant planet occurrence. We situate these findings in the context of known eccentricity relations, including their relationships with planet multiplicity, radius, and metallicity. We suggest comprehensive analyses to disentangle these results from the effects of poorly understood star–planet relationships, such as those between stellar age and planetary orbital dynamics.