Brown dwarf

Artist's concept of a T-type brown dwarf

Comparison: most brown dwarfs are slightly larger in volume than Jupiter (15–20%),^[1] but are still up to 80 times more massive due to greater density. Image is to scale, with Jupiter's radius being 11 times that of Earth, and the Sun's radius 10 times that of Jupiter.

Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars. Their mass is approximately 13 to 80 times that of Jupiter (M_J)^[2][3]—not big enough to sustain nuclear fusion of ordinary hydrogen (¹H) into helium in their cores, but massive enough to emit some light and heat from the fusion of deuterium (²H). The most massive ones (> 65 M_J) can fuse lithium (⁷Li).^[3]

Astronomers classify self-luminous objects by spectral type, a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M, L, T, and Y.^[4][5] As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age.

Their name comes not from the color of light they emit but from their falling between white dwarf stars and "dark" planets in size. To the naked eye, brown dwarfs would appear in different colors depending on their temperature.^[4] The warmest ones are possibly orange or red,^[6] while cooler brown dwarfs would likely appear magenta or black to the human eye.^[4][7] Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.^[8]

Though their existence was initially theorized in the 1960s, it was not until the mid-1990s that the first unambiguous brown dwarfs were discovered. As brown dwarfs have relatively low surface temperatures, they are not very bright at visible wavelengths, emitting most of their light in the infrared. However, with the advent of more capable infrared detecting devices, thousands of brown dwarfs have been identified. The nearest known brown dwarfs are located in the Luhman 16 system, a binary of L- and T-type brown dwarfs about 6.5 light-years (2.0 parsecs) from the Sun. Luhman 16 is the third closest system to the Sun after Alpha Centauri and Barnard's Star.