Halting Planet Migration in the Evacuated Centers of Protoplanetary Disks

Precise Doppler searches for extrasolar planets find a surfeit of planets with orbital periods of 3-4 days and no planets with orbital periods less than 3 days. The circumstellar distance R 0 , where small grains in a protoplanetary disk reach sublimation temperatures (~1500 K), corresponds to a period of ~6 days. Interior to R 0 , turbulent accretion due to magnetorotational instability may evacuate the disk center. We suggest that planets with orbital periods of 3-4 days are so common because migrating planets halt once this evacuated region contains the sites of their exterior 2 : 1 Lindblad resonances.