Substellar object

A substellar object, sometimes called a substar, is an astronomical object whose mass is smaller than the smallest mass, approximately 0.08 solar masses, at which a star can sustain hydrogen fusion. This definition includes brown dwarfs; it also includes objects of planetary mass, regardless of their formation mechanism or whether or not they are associated with a primary star.

Assuming that a substellar object has at least the mass of Jupiter (approximately 10^-3 solar masses), its radius will be comparable to that of Jupiter (approximately 0.1 solar radii). This is because the center of a substellar object just below the hydrogen-burning limit is quite degenerate, with a density of ~10³ g/cm³, but this degeneracy lessens with decreasing mass until, at the mass of Jupiter, a substellar object has central density less than 10 g/cm³. The density decrease balances the mass decrease, keeping the radius approximately constant.

A substellar object just below the hydrogen-fusing limit may ignite hydrogen fusion temporarily at its center, but although this will provide some energy, it will not be enough to overcome the object's ongoing gravitational contraction; likewise, although an object above approximately 0.013 solar masses will be able to fuse deuterium for a time, this source of energy will be exhausted in approximately 10⁶ to 10⁸ years. Apart from these sources, the radiation of an isolated substellar object comes only from the release of its gravitational potential energy, which causes it to gradually cool and shrink. A substellar object in orbit about a star will shrink more slowly as it is kept warm by the star.

See also

• Brown dwarf
• Planemo