A stellar-mass-dependent drop in planet occurrence rates

TL;DR

This study reveals that planet occurrence rates and orbital distributions vary significantly with stellar spectral type, showing higher frequencies around cooler stars and a stellar-mass-dependent inner boundary likely linked to planet formation or migration processes.

Contribution

It provides the first detailed analysis of how planet occurrence rates depend on stellar spectral type and mass, highlighting a stellar-mass-dependent inner boundary of planet populations.

Findings

Planets around M stars are twice as common as around G stars.

A drop in planet occurrence occurs inward of a 10-day orbit, scaling with stellar mass.

The inner boundary of planet occurrence correlates with the pre-main-sequence co-rotation radius.

Abstract

The Kepler Spacecraft has discovered a large number of planets up to one-year periods and down to terrestrial sizes. While the majority of the target stars are main-sequence dwarfs of spectral type F, G, and K, Kepler covers stars with effective temperature as low as 2500 K, which corresponds to M stars. These cooler stars allow characterization of small planets near the habitable zone, yet it is not clear if this population is representative of that around FGK stars. In this paper, we calculate the occurrence of planets around stars of different spectral types as a function of planet radius and distance from the star, and show that they are significantly different from each other. We further identify two trends: First, the occurrence of Earth to Neptune-sized planets is successively higher toward later spectral types at all orbital periods probed by Kepler; Planets around M stars occur twice as frequently as around G stars, and thrice as frequently as around F stars. Second, a drop in planet occurrence is evident at all spectral types inward of a 10 day orbital period, with a plateau further out. By assigning to each spectral type a median stellar mass, we show that the distance from the star where this drop occurs is stellar mass dependent, and scales with semi-major axis as the cube root of stellar mass. By comparing different mechanisms of planet formation, trapping and destruction, we find that this scaling best matches the location of the pre-main-sequence co-rotation radius, indicating efficient trapping of migrating planets or planetary building blocks. These results demonstrate the stellar-mass dependence of the planet population, both in terms of occurrence rate and of orbital distribution. The prominent stellar-mass dependence of the inner boundary of the planet population shows that the formation or migration of planets is sensitive to the stellar parameters.