Quantifying Orbital Migration from Exoplanet Statistics and Host Metallicities

We investigate how the statistical distribution of extrasolar planets may be combined with knowledge of the host stars' metallicity to yield constraints on the migration histories of gas giant planets. At any radius, planets that barely manage to form around the lowest metallicity stars accrete their envelopes just as the gas disk is being dissipated, so the lower envelope of planets in a plot of metallicity versus semimajor axis defines a sample of nonmigratory planets that will have suffered less than average migration subsequent to gap opening. Under the assumption that metallicity largely controls the initial surface density of planetesimals, we use simplified core accretion models to calculate how the minimum metallicity needed for planet formation varies as a function of semimajor axis. Models that do not include core migration prior to gap opening (type I migration) predict that the critical metallicity is largely flat between the snow line and a ≈ 6 AU, with a weak dependence on the initial surface density profile of planetesimals. When slow type I migration is included, the critical metallicity is found to increase steadily from 1 to 10 AU. Large planet samples that include planets at modestly greater orbital radii than present surveys therefore have the potential to quantify the extent of migration in both type I and type II regimes.