A Mass Function Constraint on Extrasolar Giant Planet Evaporation Rates

The observed mass function for all known extrasolar giant planets (EGPs) varies approximately as M −1 for mass M between ∼ 0.2 Jupiter masses (M J) and ∼ 5 M J. In order to study evaporation effects for highly-irradiated EGPs in this mass range, we have constructed an observational mass function for a subset of EGPs in the same mass range but with orbital radii < 0.07 AU. Surprisingly, the mass function for such highly-irradiated EGPs agrees quantitatively with the M −1 law, implying that the mass function for EGPs is preserved despite migration to small orbital radii. Unless there is a remarkable compensation of mass-dependent orbital migration for mass-dependent evaporation, this result places a constraint on orbital migration models and rules out the most extreme mass loss rates in the literature. A theory that predicts more moderate mass loss gives a mass function that is closer to observed statistics but still disagrees for M < 1 M J .