Free Floating or Merely Detached?

TL;DR

This study explores whether free-floating planets are truly unbound or just detached from their host stars, using simulations to show many such planets could be temporarily or permanently separated through planet-planet scattering.

Contribution

The paper demonstrates through simulations that planet-planet scattering can produce detached planets at large orbits, explaining the observed free-floating planet population.

Findings

Planet ejection via scattering can take over a billion years.

Many detached planets are on stable, distant orbits.

Approximately half of observed free-floating Neptunes may be detached, not unbound.

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 multi-planet 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 AU and beyond. Moreover, this process invariably delivers planets to orbits of hundreds of AU that are protected from further scattering. We call these ``detached" planets. Under the scattering hypothesis, we estimate that about half of the reported ``free-floating" Neptunes are not free but merely ``detached".