About the image. Drawn as a Jupiter-sized ball with dark magenta and brown cloud bands, glowing faintly from its own heat. The colour follows models and spectra of brown dwarfs; the cloud pattern is imagined.
Between the largest planets and the smallest stars lies a class of objects that took decades to find. Brown dwarfs have between about 13 and 80 times the mass of Jupiter. They form like stars, from collapsing clouds of gas, but never become heavy enough to sustain the hydrogen fusion that powers a true star.
Not quite failed
Brown dwarfs are often called failed stars, but that is not quite fair. Those above about 13 Jupiter masses do fuse deuterium, a heavy form of hydrogen, for a while. After that they shine only from the heat left over from their formation, slowly cooling and dimming over billions of years. A young brown dwarf glows like a dim red star; an old one is darker and cooler than some planets.
Strangely, they are all about the same size: roughly as wide as Jupiter. Adding mass squeezes them tighter instead of making them bigger.
As they cool, their spectra change through the classes astronomers call M, L, T and Y. The coolest Y dwarfs are below 600 K (327 °C). The coldest known, WISE 0855−0714, only 7.4 light-years away, is about 285 K (12 °C), around room temperature on a cool day.
Found at last
The term was coined by Jill Tarter in 1975, in her doctoral thesis; she chose “brown” for a colour between red and black. Brown dwarfs had been predicted long before, but they are so faint that the first ones were not confirmed until 1995: Teide 1, in the Pleiades, and Gliese 229B, a companion to a nearby red dwarf 18.8 light-years away. Gliese 229B showed methane in its spectrum, like a giant planet. In 2024 it was found to be two brown dwarfs orbiting each other every 12 days.
More than 1 800 are now known, and the Milky Way may hold 25 to 100 billion.
Weather on a brown dwarf
The nearest brown dwarfs are a pair called Luhman 16, 6.5 light-years away, the third-closest system to the Sun. They have weather. In 2014 Ian Crossfield and colleagues used the brightness changes of Luhman 16B as it rotated to make a map of its patchy clouds, the first such map of a brown dwarf. In 2024 the James Webb Space Telescope found water, carbon monoxide and methane in their atmospheres, and clouds of tiny silicate grains.
How hot is it?
Earth (average)288 K · 15 °C
Water boils373 K · 100 °C
Venus surface737 K · 464 °C
Iron melts1 811 K · 1 540 °C
Sun's surface5 772 K · 5 500 °C
Brown dwarf285 K · 12 °C
Brown dwarf: WISE 0855−0714, the coldest known. Logarithmic scale from 100 K to 10 000 K. Reference values: NASA, IAU.
How long to get there?
Brown dwarf is about 6.51 light-years away (Luhman 16, the nearest brown dwarfs).
Light299 792 km/s6.5 years
Breakthrough Starshotplanned light sail, 20% of light speed33 years
Parker Solar Probefastest spacecraft so far, 192 km/s10 165 years
Voyager 1leaving the Solar System, 17 km/s114 803 years
At Voyager 1's speed the trip would take about 1 435 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Gallery
A dim, dark magenta ball with faint swirling clouds glowing against black space, an impression of a brown dwarf. AI-generated painting.Two banded reddish-brown balls side by side in black space, one larger than the other, an impression of a pair of brown dwarfs. AI-generated painting.Brown dwarf, 3D artist's impression, rendered from the interactive model.Brown dwarf, 3D artist's impression, from another angle, rendered from the interactive model.Brown dwarf, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
1975Jill Tarter coins the term brown dwarf.
1995Teide 1 and Gliese 229B are confirmed.
2013Luhman 16, the nearest pair, is discovered.
2014The first cloud map of a brown dwarf, Luhman 16B.
2024Gliese 229B turns out to be two brown dwarfs.
Questions people ask
Is a brown dwarf a star or a planet?
Neither, strictly. It forms like a star but is too light to sustain hydrogen fusion.
What colour is a brown dwarf?
Not really brown. Tarter chose the word for a colour between red and black: young ones glow dull red, and older ones are darker still.