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

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 $r_{\mathrm{Roche}}$.

15 pages, 9 figures, accepted for publication in AJ; revised version after addressing referee comments

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