Carving the Edges of the Rocky Planet Population

Abstract

Short-period planets provide ideal laboratories for testing star-planet interaction. Planets that are smaller than $\sim$2$R_\oplus$ are considered to be largely rocky either having been stripped of or never having acquired the gaseous envelope. Zooming in on these short-period rocky planet population, clear edges appear in the mass-period and radius-period space. Over $\sim$0.2--20 days and 0.09--1.42$M_\odot$, the maximum mass of the rocky planets stay below $\sim$10$M_\oplus$ with a hint of decrease towards $\lesssim$1 day, $\gtrsim$4 day, and $\lesssim 0.45 M_\odot$. In radius-period space, there is a relative deficit of $\lesssim$2$R_\oplus$ planets inside $\sim$1 day. We demonstrate how the edges in the mass-period space can be explained by a combination of tidal decay and photoevaporation whereas the rocky planet desert in the radius-period space is a signature of magnetic drag on the planet as it orbits within the stellar magnetic field. Currently observed catastrophically evaporating planets may have started their death spiral from $\sim$1 day with planets of mass up to $\sim$0.3$M_\oplus$ under the magnetic drag. More discoveries and characterization of small planets around mid-late M and A stars would be welcome to better constrain the stellar parameters critical in shaping the edges of rocky planet population including their UV radiation history, tidal and magnetic properties.