Brown Dwarf
A substellar object intermediate in mass between giant gas planets and low-mass stars, incapable of sustaining stable core hydrogen fusion.
Governing formula
13 M_J ≤ M < 80 M_J (~0.013 to 0.08 M_☉)
SI unit
Jupiter Masses (M_J) or M_☉
In depth
Brown dwarfs can briefly fuse deuterium or lithium, but lack sufficient mass (<0.08 M_☉) to reach core temperatures needed for sustained proton-proton hydrogen fusion. They cool gradually over cosmic time.
Examples in the real world
Gliese 229B, a cool brown dwarf companion detected via infrared astronomy.