California-Kepler Survey. VIII. Eccentricities of Kepler Planets and Tentative Evidence of a High-metallicity Preference for Small Eccentric Planets

Abstract

Characterizing the dependence of the orbital architectures and formation environments on the eccentricity distribution of planets is vital for understanding planet formation. In this work, we perform statistical eccentricity studies of transiting exoplanets using transit durations measured via Kepler combined with precise and accurate stellar radii from the California-Kepler Survey and Gaia. Compared to previous works that characterized the eccentricity distribution from transit durations, our analysis benefits from both high-precision stellar radii (~3%) and a large sample of ~1000 planets. We observe that systems with only a single observed transiting planet have a higher mean eccentricity (e ~ 0.21) than systems with multiple transiting planets (e ~ 0.05), in agreement with previous studies. We confirm the preference for high- and low-eccentricity subpopulations among the single transiting systems. Finally, we show suggestive new evidence that high-e planets in the Kepler sample are preferentially found around high-metallicity ([Fe/H] > 0) stars. We conclude by discussing the implications on planetary formation theories.