Dependence of Exoplanets on Host Stars' Metallicity and Mass

Abstract

We discuss dependence of extrasolar planetary systems on host stars' metallicity (abundance of heavy elements) and mass (equivalently, spectral type), through theoretical arguments and comparison with observational data. The theoretical arguments are based on the conventional giant planet formation scenario: core accretion followed by gas accretion onto the core. The large number (> 150) of extrasolar planets so far discovered is large enough for statistical discussion. Based on the most up-to-date theories of core and gas accretion, planet-disk interaction, and the observed distribution of disk mass, we simulate mass and semimajor axis distributions for systems around host stars with a range of metallicity and mass. Assuming that the fraction of heavy elements contained in host stars is identical to that in protoplanetary disks in which planets are formed, we evaluate the metallicity dependence of fraction of stars harboring planets that are detectable with current radial velocity survey. The detection probability around metal-rich stars would be greatly enhanced because protoplanetary cores formed in them can grow to ∼ 10 Earth masses, which are required for the cores to initiate rapid gas accretion and to transform into giant planets, prior to their depletion. The theoretically predicted metallicity dependence is consistent with the observation. We also extrapolate predictions to planetary systems around different mass stars. We show that around small-mass stars (M dwarf stars), close-in Neptune-mass ice-giant planets may be relatively common, while close-in Jupiter-mass gas-giant planets are relatively rare. The mass distribution of close-in planets generally has two peaks at about Neptune mass and Jupiter mass. The lower-mass peak takes the maximum frequency for M dwarfs. Around more massive solar-type stars (F, G, K dwarf stars), the higher-mass peak is much more pronounced. These predictions can be tested directly with observations in the near future.