A brown dwarf is a substellar object that bridges the gap between the largest planets and the smallest stars: too massive to be a planet but too light to sustain ordinary hydrogen fusion in its core. Above about 13 times Jupiter's mass it can briefly fuse deuterium; below about 80 it never ignites hydrogen and slowly cools instead. Brown dwarfs have cloudy, weather-driven atmospheres, and the coldest known are barely warmer than Earth.