Planetary-mass object

An astronomical object with a mass comparable to a planet, whether bound to a star or free-floating. The category spans objects whose origins and physical properties may differ.

Planet: A celestial body that orbits a star or stellar remnant, is massive enough for gravity to make it nearly round, and meets other classification criteria. Some planetary-mass objects orbit stars and qualify as planets; others do not meet the orbital criterion.
Core accretion: A planet-formation model in which a solid core grows by accretion and later gathers surrounding gas. It explains how some star-bound planetary-mass objects form within protoplanetary disks.
Gravitational microlensing: A lensing phenomenon in which a foreground object's gravity briefly magnifies light from a more distant background source. Microlensing can reveal isolated planetary-mass lenses that emit too little light for direct detection.
International Astronomical Union planet definition: The International Astronomical Union's 2006 definition of a planet in the Solar System, including orbital clearing as a criterion. Its orbit-based rule excludes free-floating bodies even when their masses resemble those of planets.
Brown dwarf: A substellar object too low in mass for sustained hydrogen fusion but capable of deuterium fusion at higher masses. The boundary between massive planetary-mass objects and low-mass brown dwarfs is debated.
Disk instability: A planet-formation mechanism in which a gravitationally unstable region of a disk collapses into a bound object. Disk fragmentation can produce objects near the upper planetary-mass range.
Direct imaging: An astronomical observation technique that separates an object's light from the glare of a nearby star. It detects young, luminous planetary-mass companions and measures their spectra.
Deuterium-burning limit: An approximate mass threshold above which an object's interior can burn deuterium for a limited time. Its value depends on composition, making it an imperfect dividing line between planets and brown dwarfs.
Free-floating planet: A planetary-mass object that travels through space without being gravitationally bound to a star. This is the planetary-mass category's most discussed unbound subset.
Molecular cloud: A cold, dense interstellar cloud of gas and dust where stars and other astronomical objects can form. Direct collapse within a cloud offers a star-like formation route for isolated low-mass objects.

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