2017AJ....154..175W

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_Roche). Extensive N-body simulations with initial bodies >=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 ∼1.1r_Roche. At smaller distances, tidal effects cause collisions to be too fragmenting to allow mass buildup 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_Roche.