A SUPER-SOLAR METALLICITY FOR STARS WITH HOT ROCKY EXOPLANETS

Abstract

Host star metallicity provides a measure of the conditions in protoplanetary disks at the time of planet formation. Using a sample of over 20,000 Kepler stars with spectroscopic metallicities from the LAMOST survey, we explore how the exoplanet population depends on host star metallicity as a function of orbital period and planet size. We find that exoplanets with orbital periods less than 10 days are preferentially found around metal-rich stars ([Fe/H] similar or equal to 0.15 +/- 0.05 dex). The occurrence rates of these hot exoplanets increases to similar to 30% for super-solar metallicity stars from similar to 10% for stars with a sub-solar metallicity. Cooler exoplanets, which reside at longer orbital periods and constitute the bulk of the exoplanet population with an occurrence rate of greater than or similar to 90%, have host star metallicities consistent with solar. At short orbital periods, P < 10 days, the difference in host star metallicity is largest for hot rocky planets (< 1.7 R-circle plus), where the metallicity difference is [Fe/H] similar or equal to 0.25 +/- 0.07 dex. The excess of hot rocky planets around metal-rich stars implies they either share a formation mechanism with hot Jupiters, or trace a planet trap at the protoplanetary disk inner edge, which is metallicity dependent. We do not find statistically significant evidence for a previously identified trend that small planets toward the habitable zone are preferentially found around low-metallicity stars. Refuting or confirming this trend requires a larger sample of spectroscopic metallicities.