What Makes Brown Dwarfs So Weird?

Meet the brown dwarf: bigger than a planet, and smaller than a star. A category of its own, it’s one of the strangest objects in the universe.

Brown dwarfs typically are defined to have masses anywhere from 12 times the mass of Jupiter right up to the lower limit for a star. And despite their names, they are not actually brown. The largest and youngest ones are quite hot, giving off a steady glow of radiation. In fact, the largest brown dwarfs are almost indistinguishable from red dwarfs, the smallest of the stars. But the smallest, oldest, and coldest ones are so dim they can only be detected with our most sensitive infrared telescopes.

Unlike stars, brown dwarfs don’t generate their own energy through nuclear fusion, at least not for very long. Instead they emit radiation from the leftover heat of their own formation. As that heat escapes, the brown dwarf continues to dim, sliding from fiery red to mottled magenta to invisible infrared. The greater the mass at its birth, the more heat it can trap and the longer it can mimic a proper star, but the ultimate end fate is the same for every single brown dwarf, regardless of its pedigree.

At first it may seem like brown dwarfs are just extra-large planets, but they get to do something that planets don’t. While brown dwarfs can’t fuse hydrogen in their cores – that takes something like 80 Jupiter masses to accomplish – they can briefly partake in another kind of fusion reaction.

In the cooler heart of a brown dwarf, deuterium, which is a single proton and neutron, can convert into Helium-3, and in the process release energy. This process doesn’t last long; in only a few million years even the largest brown dwarfs use up all their available deuterium, and from there they will just cool off.

As for their size, they tend not to be much larger in diameter than a typical gas giant like Jupiter. That’s because unlike a star, there isn’t an additional source of energy, and thereby pressure, to prop themselves up. Instead, all that’s left is the exotic quantum force known as degeneracy pressure, which means that you can only squeeze so many particles into so small a volume. In this case, brown dwarfs are very close to the limit for degeneracy pressure to maintain their size given their mass.

This means that despite outweighing Jupiter, they won’t appear much larger. And unlike Jupiter, they are briefly capable of nuclear fusion. After that, however, they spend the rest of their lives wandering the galaxy, slowly chilling out.