Magnetospheric Truncation, Tidal Inspiral, and the Creation of Short and Ultra-Short Period Planets

Abstract

Sub-Neptunes around FGKM dwarfs are evenly distributed in log orbital period down to $\sim$10 days, but dwindle in number at shorter periods. Both the break at $\sim$10 days and the slope of the occurrence rate down to $\sim$1 day can be attributed to the truncation of protoplanetary disks by their host star magnetospheres at co-rotation. We demonstrate this by deriving planet occurrence rate profiles from empirical distributions of pre-main-sequence stellar rotation periods. Observed profiles are better reproduced when planets are distributed randomly in disks---as might be expected if planets formed in situ---rather than piled up near disk edges, as would be the case if they migrated in by disk torques. Planets can be brought from disk edges to ultra-short ($< 1$ day) periods by asynchronous equilibrium tides raised on their stars. Tidal migration can account for how ultra-short period planets (USPs) are more widely spaced than their longer period counterparts. Our picture provides a starting point for understanding why metal-rich stars tend to harbor more USPs, and why the sub-Neptune population drops at $\sim$10 days regardless of whether the host star is of type FGK or early M. We predict planet occurrence rates around A stars to also break at short periods, but at $\sim$1 day instead of $\sim$10 days because A stars rotate faster than lower mass stars (this prediction presumes that the planetesimal building blocks of planets can drift inside the dust sublimation radius).