Formation of cold giant planets around late M dwarfs via core accretion and the fate of inner rocky worlds

TL;DR

This study demonstrates that cold giant planets can form around very low-mass M dwarfs through pebble and gas accretion in specific disk conditions, with potential survival of inner rocky planets, challenging previous assumptions.

Contribution

It shows that forming cold giants around low-mass stars does not require high dust masses, highlighting the roles of planet collisions, pebble accretion, and disk lifetime.

Findings

Cold giants can form with as little as 6 Earth masses of pebbles.

A 10% disk gas mass suffices for giant formation around a 0.1 solar mass star.

Inner rocky planets can survive if they reach the cavity before the outer giant.

Abstract

Modeling the formation of cold giant planets around M dwarfs is difficult because their disks may not contain enough solids to form massive cores and because forming giants are expected to migrate inward through disk interactions. It is also unclear whether inner rocky planets can survive in systems hosting a cold giant, with implications for the habitability of close-in worlds. We investigated the conditions that allow giant planets to form at 1-3 au around a 0.1 M$_\odot$ star and explored when a close-in rocky planet can survive. We perform N-body simulations in which embryos grow through pebble and gas accretion in a disk with a local turbulent viscosity of $\alpha_t = 10^{-4}$. Planet-disk interactions are included using a prescription that triggers outward migration when the planet-to-star mass ratio ($q$) exceeds 0.002. We find that a cold giant can form even in a disk with an initial pebble mass of 6 M$_\oplus$ if the disk gas mass is 10$\%$ of the stellar mass. This requires a compact 20 au disk with a dense inner region set by $\alpha_g = 10^{-4}$, the assembly of a $\sim$5 M$_\oplus$ core within 1 Myr, and a disk lifetime of 10 Myr. A close-in rocky planet can survive if it reaches the inner cavity before the outer body becomes a giant. Thus, giant planet formation around very low-mass stars does not require high dust masses as previously thought. A combination of planet-planet collisions, efficient pebble accretion, and a long disk lifetime plays a key role in enabling the formation of cold giant planets with masses between those of Saturn and Jupiter.