Collisional Fragmentation is Not a Barrier to Close-in Planet Formation

TL;DR

This study shows that collisional fragmentation does not prevent rocky planet formation close to stars, with simulations indicating formation possible down to about 1.1 times the Roche radius, despite tidal effects at smaller distances.

Contribution

The paper demonstrates through analytic arguments and N-body simulations that rocky planet formation can occur very close to stars, challenging previous assumptions about fragmentation barriers.

Findings

Planet formation possible down to 1.1 times the Roche radius.

Fragmentation is not a barrier at small orbital distances.

Tidal effects limit formation closer than the Roche radius.

Abstract

Collisional fragmentation is shown to not be a barrier to rocky planet formation at small distances from the host star. Simple analytic arguments demonstrate that rocky planet formation via collisions of homogeneous gravity-dominated bodies is possible down to distances of order the Roche radius ($r_\mathrm{Roche}$). Extensive N-body simulations with initial bodies ${\gtrsim}1700$ km that include plausible models for fragmentation and merging of gravity-dominated bodies confirm this conclusion and demonstrate that rocky planet formation is possible down to ${\sim}$1.1 $r_\mathrm{Roche}$. At smaller distances, tidal effects cause collisions to be too fragmenting to allow mass build-up to a final, dynamically stable planetary system. We argue that even differentiated bodies can accumulate to form planets at distances that are not much larger than $r_\mathrm{Roche}$.