Saving the Earth: Orbital Migration in the Context of Disk History
A planet interacting with a gas disk by gravitation drives density perturbations in the disk that in turn exert a gravitational torque on the planet that cause it to migrate in orbital radius. Planets smaller than gas giants interacting with isothermal disks migrate inward to the central star on timescales shorter than the disk lifetime, raising the question of how terrestrial mass planets survive. However, recent models including explicit treatment of the heating and cooling of a radiatively thick disk yield nil or outward migration for opaque enough disks. Thus, terrestrial planets within opaque disks can survive. However, at late times, as disks lose mass, they will eventually become optically thin, allowing inward migration to occur. We here show that by the time this happens, the disk is so low density that the migration timescale far exceeds the remaining lifetime of the disk, so that terrestrial planets should avoid accretion by the central star.