A Super-Solar Metallicity For Stars With Hot Rocky Exoplanets

Abstract

The host star metallicity provide 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]~ 0.15 +- 0.05 dex). The occurrence rates of these hot exoplanets increases to ~30% for super-solar metallicity stars from ~10% for stars with a sub-solar metallicity. Cooler exoplanets, that resides at longer orbital periods and constitute the bulk of the exoplanet population with an occurrence rate of >~ 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_Earth), where the metallicity difference is [Fe/H] =~ 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.