Brown dwarfs are substellar objects that occupy the mass range between the heaviest gas giants and the lightest stars, of approximately 13 to 75–80 Jupiter masses (MJ). Below this range are the sub-brown dwarfs, sometimes referred to as rogue planets, and above it are the lightest red dwarfs (M9 V). Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth. Unlike the stars in the main-sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen (1H) to helium in their cores. They are, however, thought to fuse deuterium (2H) and to burn lithium (7Li) if their mass is above a debated threshold of 13 MJ and 65 MJ, respectively. It is also debated whether brown dwarfs would be better defined by their formation processes rather than by their supposed nuclear fusion reactions. Stars are categorized by spectral class, with brown dwarfs designated as types M, L, T, and Y. Despite their name, brown dwarfs are of different colors. Many brown dwarfs would likely appear magenta to the human eye, or possibly orange/red. Brown dwarfs are not very luminous at visible wavelengths. Planets are known to orbit some brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b At a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary system of brown dwarfs discovered in 2013. DENIS-P J082303.1-491201 b is listed as the most-massive known exoplanet (as of March 2014) in NASA's exoplanet archive, despite having a mass (28.5±1.9 MJ) more than twice the 13-Jupiter-mass cutoff between planets and brown dwarfs.

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