Planemos

Planemos, short for planetary-mass objects, are astronomical bodies with masses in the planetary range that are not stars. The term is typically used for objects with masses below the deuterium-burning limit, around 13 Jupiter masses, though exact boundaries can be uncertain. Planemos are often free-floating in space, not gravitationally bound to a star, and are sometimes described as rogue or free-floating planets. The label is descriptive and mass-based rather than tied to a specific formation path, and planemo status is not an official IAU category.

Planemos can form in several ways. Some arise through direct collapse of dense cloud fragments, similar to how brown dwarfs form, while others may originate in planetary systems and be ejected during early dynamical instabilities. As a result, planemos can be single objects wandering interstellar space or members of star-forming regions where they remain unbound from any host star.

Detection and study of planemos rely on indirect methods. Direct imaging in young, nearby star-forming regions has yielded candidates that are consistent with planetary masses. Gravitational microlensing surveys have also identified possible free-floating planets in the Milky Way, with masses in the planetary range. Estimating their numbers depends on models of formation and evolution, and current results carry significant uncertainties.

Planemos provide insight into the low-mass end of the initial mass function and into substellar formation processes. Their existence challenges simple distinctions between planets and brown dwarfs and informs theories of planetary system development and star formation.