Asubstellar object, sometimes called asubstar, is anastronomical object, themass of which is smaller than the smallest mass at whichhydrogen fusion can be sustained (approximately 0.08solar masses). This definition includesbrown dwarfs and former stars similar toEF Eridani B, and can also includeobjects of planetary mass, regardless of their formation mechanism and whether or not they are associated with a primarystar.^[1][2][3][4]

Assuming that a substellar object has a composition similar to theSun's and at least the mass ofJupiter (approximately 0.001 solar masses), its radius will be comparable to that of Jupiter (approximately 0.1solar radii) regardless of the mass of the substellar object (brown dwarfs are less than 75 Jupiter masses). This is because the center of such a substellar object at the top range of the mass (just below thehydrogen-burning limit) is quitedegenerate, with adensity of ≈10³ g/cm³, but this degeneracy lessens with decreasing mass until, at the mass of Jupiter, a substellar object has a central density less than 10 g/cm³. The density decrease balances the mass decrease, keeping the radius approximately constant.^[5]

Substellar objects like brown dwarfs do not have enough mass to fuse hydrogen and helium, hence do not undergo the usualstellar evolution that limits the lifetime of stars.

A substellar object with a mass just below the hydrogen-fusing limit may ignite hydrogen fusion temporarily at its center. Although this will provide some energy, it will not be enough to overcome the object's ongoinggravitational contraction. Likewise, although an object with mass above approximately 0.013 solar masses will be able tofuse deuterium for a time, this source of energy will be exhausted in approximately 1–100 million years. Apart from these sources, the radiation of an isolated substellar object comes only from the release of itsgravitational potential energy, which causes it to gradually cool and shrink. A substellar object inorbit around a star will shrink more slowly as it is kept warm by the star, evolving towards anequilibrium state where it emits as much energy as it receives from the star.^[6]

Substellar objects are cool enough to have water vapor in their atmosphere.Infrared spectroscopy can detect the distinctivecolor of water ingas giant size substellar objects, even if they are not in orbit around a star.^[7]

William Duncan MacMillan proposed in 1918 the classification of substellar objects into three categories based on their density and phase state: solid, transitional and dark (non-stellar) gaseous.^[8] Solid objects include Earth, smallerterrestrial planets and moons; with Uranus and Neptune (as well as latermini-Neptune andSuper Earth planets) as transitional objects between solid and gaseous. Saturn, Jupiter and large gas giant planets are in a fully "gaseous" state.

A substellar object may be a companion of a star,^[9] such as anexoplanet orbrown dwarf that is orbiting a star.^[10] Objects as low as 8–23 Jupiter masses have been called substellar companions.^[11]

Objects orbiting a star are often called planets below 13Jupiter masses and brown dwarves above that.^[12] Companions at that planet-brown dwarf borderline have been calledSuper-Jupiters, such as that around the starKappa Andromedae.^[13] Nevertheless, objects as small as 8 Jupiter masses have been called brown dwarfs.^[14]

• Quoted asChabrier and Baraffe:Chabrier, Gilles; Baraffe, Isabelle (September 2000). "Theory of Low-Mass Stars and Substellar Objects".Annual Review of Astronomy and Astrophysics.38:337–377.arXiv:astro-ph/0006383.Bibcode:2000ARA&A..38..337C.doi:10.1146/annurev.astro.38.1.337.S2CID 59325115.

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