Free Floating or Merely Detached?

Abstract Microlensing surveys suggest the presence of a surprisingly large population of free-floating planets, with a rate of about two Neptunes per star. The origin of such objects is not known, neither do we know if they are truly unbound or are merely orbiting at large separations from their host stars. Here, we investigate planet–planet scattering as a possible origin through numerical simulations of unstable multiplanet systems. We find that planet ejection by scattering can be slow, often taking more than billions of years for Neptune-mass scatterers orbiting at a few astronomical units and beyond. Moreover, this process invariably delivers planets to orbits of hundreds of astronomical units that are protected from further scattering. We call these “detached” planets. Under the hypothesis that planet–planet scattering is the dominant source of free-floating planets, we estimate that about one-half of the reported “free-floating” Neptunes are not free but merely “detached.”