Dispersing the Gaseous Protoplanetary Disk and Halting Type II Migration

More than 30 extrasolar Jupiter-like planets have shorter periods than the planet Mercury. It is generally accepted that they formed farther out, past the "snow line" (~1 AU), and migrated inward. In order to be driven by tidal torques from the gaseous disk, the disk exterior to the planet had to contain about a planetary mass. The fact that the planets stopped migrating means that their outer disk was removed. We suggest, following the simulation by Kley and Bate et al., that the outer disk was accreted by the planet. This not only halts migration but removes the outer disk. For planets exterior to about 2 AU, the outer disk could have been removed by photoevaporation (Matsuyama et al.). Furthermore, as also shown by Bate et al., this process also provides an upper limit to planetary masses in agreement with the analysis of observed planetary masses by Zucker & Mazeh. In this scenario, the endgame is a race. The central star is accreting the inner disk and the planet, while the planet is accreting the outer disk. The planet survives if it accretes its outer disk before being accreted by the star. The winner is determined solely by the ratio of the mass of the outer disk to the local surface density of the disk. Some planets certainly lose the race and are eaten by the central star.