The Dynamics of Planetary Ejection

The ubiquity of free-floating planets inferred from microlensing and direct imaging surveys suggests that planetary ejection—a process in which planets initially born encircling a stellar host become gravitationally unbound—is common. Four overarching mechanisms have been proposed to induce planetary ejection: close approaches of neighboring planets, instabilities in binary or multi-star systems, stellar and planetary flybys, and post-main-sequence stellar evolution. Here we review the mechanisms underlying planetary ejection, as well as predictions derived from each. Current and upcoming microlensing surveys offer the potential to test existing models and distinguish between planetary ejection mechanisms, providing further insight into the demographic-level architectures of exoplanets across stellar environments.