free-floating planet

A free-floating planet is a planet that is not orbiting a star. Their existence is suggested by planetary migration theories and simulations which indicate planets kicked from a stellar system, i.e., accelerated beyond escape velocity in a gravitational encounter with one or more other planets. Any such planets are by definition exoplanets (beyond the solar system) and MACHOs. Some other terms used for such planets:

• rogue planet - a widely-used term.
• field planet.
• wandering planet.
• interstellar planet.
• PMO or planetary mass object (which is also sometimes used for objects orbiting stars: the word planet could be used, but the phrase perhaps suggests an absence of knowledge about the object.)
• isolated PMO or isolated planetary mass object (IPMO), clearly indicating it is not part of a stellar system.
• MACHO which is also meant to include brown dwarfs and perhaps any unsuspected populations of dim stars.
• sub-brown dwarf - within the mass-range of planets, but the term suggests it is star-like, the results of star formation producing a lower-mass object, rather than the results of typical planet formation within a protoplanetary disk.
• substellar object - may be meant to include brown dwarfs.

Like all exoplanets, they are challenging to detect and observe, and the effective exoplanet detection methods based upon observing the host star do not apply. Nearby candidates are observable IR sources, and gravitational microlensing can turn up candidates. With the ongoing improvement of observation equipment, candidates are being discovered, largely within the last decade or two. Invariably the question arises whether the candidate is actually a brown dwarf: there is always uncertainty regarding the object's mass and internal characteristics that would classify it. An example candidate is WISE 0855-0714.