Brown dwarfs are strange little things. Bridging the gap between stars and planets, they’ll never be hot enough to fuse hydrogen the way the Sun does. Instead they’re only massive enough to burn deuterium, glowing like a hot coal sitting at the edge of a bonfire. There’s been a lot of talk lately about these curious sub-stars. They’ve provided some combination of entertainment and confusion for astronomers ever since their incontrovertible discovery in 1995.
By toeing the line between star and planet, brown dwarfs can be elusive, being quite easily mistaken for either massive planets or diminutive stars. They tend to have between 13 Jupiter masses and 0.075 Solar masses, but that isn’t always helpful. You don’t always know how heavy they are.
They certainly do act like stars. They seem to form like stars. They form binary pairs with each other like stars. Indeed, some even have planets of their own. One of the first direct exoplanet images was of a planet orbiting a brown dwarf.
Mind you, if you know what to look for, spotting the difference between a brown dwarf and a red dwarf is actually fairly easy. The nuclear fusion in fully fledged stars, even red dwarfs, will steadily destroy lithium over time. You can actually estimate the age of a young star by measuring how much lithium its spectrum shows. Even the youngest stars will have already burned off a lot of lithium though, so if the star’s spectrum shows plenty of the stuff, it’s a good sign you’re looking at a brown dwarf. Another spectral clue is seeing absorptions from molecular methane. The coolest red dwarfs can start to show signs of C-H molecular stretches, but only brown dwarfs are cool enough for CH4 to form in any great quantity.
It’s not just Methane that’s been found, either. Phosphine and hydrogen sulfide have been seen in brown dwarf atmospheres, as well as oxides, iron and silicates. In the coolest brown dwarfs, clouds of water vapour are expected!
Spotting the difference between brown dwarfs and big planets, however, is a little more troublesome. Brown dwarfs emit most of their radiation well into the infrared, without giving out very much in the visible. Giant planets also tend to be quite hot. Both Jupiter and Saturn, for instance, give out more heat than they receive from the Sun. Some giant exoplanets have been found because they glow so brightly in infrared. The best way to tell the difference would be to look inside (not that that’s possible for us, sadly). Brown dwarfs are thought to be fully convective, just like red dwarfs. Giant planets, on the other hand, are expected to have a large solid core.
Some now believe that brown dwarfs could make up around two thirds of all stars in our neighbourhood. That’s a lot of brown dwarfs. They can happily slip by unnoticed because they’re so dim. Take a look, for example, at this image. That’s a Hubble Space Telescope image of the cool red dwarf, Gliese 229 (class M1V — which is smaller than Proxima). The visually tiny object to its right is its brown dwarf companion Gliese 229B. Comparing it directly this way can really give you a good idea of exactly how dim these objects are. We have trouble spotting red dwarfs that are too far away because they’re too dim. Scarce wonder we haven’t found a great many brown dwarfs.
All of this makes me take recent news about brown dwarfs not being found as companions to larger stars with a pinch of salt, especially given that most of the brown dwarfs I’ve heard about in the past have been in orbit around other stars (although several lone brown dwarfs are known). I haven’t looked at any of the publications associated with the study, but surely there’s a chance that any brown dwarfs were simply drowned out in the glare, the same way planets are. Many stars have low mass red dwarf companions at large separations (typically hundreds of AU). Perhaps some have low mass brown dwarf companions which are simply not bright enough to be obvious.
In closing, perhaps the most interesting consideration with brown dwarfs is, as has been speculated on Centauri Dreams at least a couple of times, what if a brown dwarf exists closer to us than Alpha Centauri? It’s not impossible. Such a rogue brown dwarf could conceivably go undetected. For now it’s simply an interesting thought experiment, but hopefully the question will be answered soon enough, by NASA’s WISE mission, scheduled to launch and map the infrared sky in fine detail next November. Then perhaps we might find out a bit more about the tim’rous beasties…