Diverse outcomes of planet formation and composition around low-mass stars and brown dwarfs

TL;DR

This study models planet formation around low-mass stars and brown dwarfs, revealing that such systems tend to be compact with few planets, and that planet mass depends on stellar and disc mass, with water-rich planets being common.

Contribution

It introduces a population synthesis model for planet formation around low-mass stars, highlighting the conditions for forming planets above Mars and Earth mass, and characterizing typical system architectures.

Findings

Planets above Mars mass form around stars with M* ≥ 0.07 M_sun.

Planets larger than 5 M_earth do not form in the model.

Most planetary systems have 1-3 planets, with many systems having up to 7.

Abstract

The detection of Earth-size exoplanets around low-mass stars -- in stars such as Proxima Centauri and TRAPPIST-1 -- provide an exceptional chance to improve our understanding of the formation of planets around M stars and brown dwarfs. We explore the formation of such planets with a population synthesis code based on a planetesimal-driven model previously used to study the formation of the Jovian satellites. Because the discs have low mass and the stars are cool, the formation is an inefficient process that happens at short periods, generating compact planetary systems. Planets can be trapped in resonances and we follow the evolution of the planets after the gas has dissipated and they undergo orbit crossings and possible mergers. We find that formation of planets above Mars mass and in the planetesimal accretion scenario, is only possible around stars with masses $M_{\star} \ge 0.07 M_{sun}$ and discs of $M_{disc} \ge 10^{-2}~M_{sun} $. We find that planets above Earth-mass form around stars with masses larger than $0.15 M_{\oplus}$ while planets larger than $5 M_{\oplus}$do not form in our model, even not under the most optimal conditions (massive disc), showing that planets such as GJ 3512b form with another, more efficient mechanism. Our results show that the majority of planets form with a significant water fraction; that most of our synthetic planetary systems have 1, 2, or 3 planets, but those with 4, 5, 6, and 7 planets are also common, confirming that compact planetary systems with many planets should be a relatively common outcome of planet formation around small stars.