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  ∼2 R ⊕ are considered to be largely rocky, either having been stripped of or never having acquired the gaseous envelope. Zooming in on this short-period rocky planet population, clear edges appear in the mass–period and radius–period space. Over  ∼0.2–20 days and 0.09–1.42 M ⊙ , the maximum mass of the rocky planets stays below  ∼10 M ⊕ with a hint of decrease toward  ≲1 day,  ≳4 days, and  ≲0.45 M ⊙ . In radius–period space, there is a relative deficit of  ≲2 R ⊕ planets inside  ∼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  ∼1 day with planets of mass up to  ∼0.3 M ⊕ under the magnetic drag. More discoveries and characterization of small planets around mid- to 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 effects, and magnetic properties.