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

Characterizing the dependence of the orbital architectures and formation environments on the eccentricitydistribution of planetsis vital for understanding planetformation. In this work, we perform statistical eccentricitystudies of transiting exoplanetsusing transit durations measured via Keplercombined with precise and accurate stellar radii from the California- KeplerSurvey and Gaia. Compared to previous works that characterized the eccentricitydistribution 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 planethave 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- eccentricitysubpopulations among the single transiting systems. Finally, we show suggestive new evidence that high-e planetsin the Keplersample are preferentially found around high-metallicity ([Fe/H] > 0) stars. We conclude by discussing the implications on planetary formation theories.