Last updated September 9, 2025

Brown Dwarfs: Between Stars and Giant Planets

What is a Brown Dwarf?

Brown dwarfs are substellar objects whose mass is too low to sustain hydrogen fusion in their core like a typical star. Their mass ranges approximately between 13 and 80 times that of Jupiter (≈0.012 to 0.08 M☉). They represent an intermediate between stars and giant planets.

Physical Characteristics

Mass: 13 to 80 M_J (≈0.012 to 0.08 M☉)
Radius: ~0.1 to 0.15 R☉, close to that of Jupiter
Effective temperature: 500 to 2,500 K, cooler than M-type stars
Luminosity: very low, decreases rapidly with age
Lifespan: cool gradually without ever becoming true stars

Internal Structure

Core: no stable hydrogen fusion, but sometimes deuterium or lithium fusion for the most massive
Atmosphere: mainly composed of hydrogen and helium with traces of molecules like CH4, H2O, CO
Convective: energy transport by convection dominates throughout the star

Known Examples

Main Brown Dwarfs Studied
Object	Spectral Type	Mass (M☉)	Radius (R☉)	Temperature (K)	Remarks
WISE 0855−0714	Y	≈0.01	0.11	250	Very cold brown dwarf, close to Jupiter
Gliese 229B	T7	≈0.03	0.1	950	Companion of the star Gliese 229
2MASS J0415−0935	T8	≈0.03	0.1	800	T-type brown dwarf
LP 944-20	M9	≈0.06	0.1	2,300	Brown dwarf near the M-type limit
WISE J1049−5319B	L7.5	≈0.03	0.1	1,300	Companion of the L7.5 dwarf in the Luhman 16 binary system
2MASS J1507−1627	L5	≈0.04	0.11	1,600	L-type brown dwarf close to the Sun
DENIS-P J0255−4700	L8	≈0.03	0.1	1,400	Late L-type brown dwarf
SDSS J1416+1348	T7.5	≈0.03	0.1	900	Example of a cold T-type brown dwarf

Sources: NASA – Brown Dwarfs Data, International Astronomical Union.