  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 temperature (~1500 K) corresponds to a period of ~6
days. Interior to R_0, turbulent accretion due to magneto-rotational
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.
