Carving the Edges of the Rocky Planet Population

TL;DR

This paper investigates the boundaries of the rocky planet population in short-period orbits, revealing how tidal decay, photoevaporation, and magnetic drag shape the observed edges in mass and radius distributions.

Contribution

It introduces explanations for the observed edges in the rocky planet population using models of tidal decay, photoevaporation, and magnetic drag effects.

Findings

Maximum mass of rocky planets stays below ~10 M⊕ in the specified period range.

A deficit of planets smaller than 2 R⊕ exists inside ~1 day orbital period.

Magnetic drag influences the distribution of evaporating planets and the rocky planet desert.

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.