The formation of retrograde planetary orbits by close stellar encounters

Abstract

We consider the growing number of observations of the RossiterMcLaughlin effect in transiting planets, which seem to suggest that ~30% of transiting planets are in highly inclined or retrograde orbits. We consider the dense cluster environment in which stars are born and investigate whether perturbations from passing stars can drive planetary systems into retrograde configurations. We find that fly-bys can result in significantly more inclination excitation than might naively be expected from impulse approximations, leading to several percent of stellar systems possessing planets in retrograde orbits.