Brown dwarfs are a special type of low-mass star (approximately 13-70 Jupiter masses) that never fuse hydrogen into helium in their cores, as do stars on the main sequence.

Early in their development most brown dwarf stars do have lithium and deuterium fusion in their cores, and a lack of lithium is a test for low-mass objects that are suspected of being brown dwarfs. Brown dwarfs continue to glow in the red and infrared after their deuterium is exhausted. This glow is from the leftover heat generated by their formation and by the earlier deuterium and lithium fusion. The atmospheres of known brown dwarfs range over temperatures from about 2300 to 700°C. All brown dwarfs cool over time; more massive objects cool more slowly than lower mass objects.

A few potential brown dwarf candidates have been detected, and they are thought to be the most common type of star in the Milky Way galaxy. Gas giant planets that form directly from a collapsing nebula rather than accreting from a protostellar disk like other planets are more properly termed brown dwarf stars.

Recent observations of known brown dwarf candidates have revealed a pattern of brightening and dimming of infrared emissions that suggests relatively cool, opaque cloud patterns obscuring the hot interior and stirred by winds. The weather on such bodies is thought to be extremely violent, comparable to but far exceeding Jupiter's famous storms.