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 ( \sim 1500 K ) corresponds to a period of \sim 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 .