Rapid Formation of Super-Earths Around Low-Mass Stars

TL;DR

This study uses N-body simulations to explore how super-Earths form rapidly around low-mass M dwarf stars, revealing key influences of gas discs and initial conditions on planetary system outcomes.

Contribution

It provides new insights into the rapid formation and final configurations of super-Earths around low-mass stars, highlighting the limited impact of initial solid distribution profiles.

Findings

Planets form mostly within the first 1 Myr.

Gas discs reduce the number of final planets and cause inward migration.

Final planetary distributions do not reflect initial surface density slopes.

Abstract

NASA's TESS mission is expected to discover hundreds of M dwarf planets. However, few studies focus on how planets form around low-mass stars. We aim to better characterize the formation process of M dwarf planets to fill this gap and aid in the interpretation of TESS results. We use ten sets of N-body planet formation simulations which vary in whether a gas disc is present, initial range of embryo semi-major axes, and initial solid surface density profile. Each simulation begins with 147 equal-mass embryos around a 0.2 solar mass star and runs for 100 Myr. We find that planets form rapidly, with most collisions occurring within the first 1 Myr. The presence of a gas disc reduces the final number of planets relative to a gas-free environment and causes planets to migrate inward. We find that roughly a quarter of planetary systems experience their final giant impact inside the gas disc, suggesting that some super-Earths may be able to reaccrete an extended gaseous envelope after their final giant impact, though these may be affected by additional process such as photoevaporation. In addition, we find that the final distribution of planets does not retain a memory of the slope of the initial surface density profile, regardless of whether or not a gas disc is present. Thus, our results suggest that present-day observations are unlikely to provide sufficient information to accurately reverse-engineer the initial distribution of solids.