Ever speculate on where else, besides Earth, we might find life in the universe? All kinds of attention has been flowing to the idea of an ‘Earth 2,’ a terrestrial world orbiting a star much like our own. But there are places in the stars that are nothing like this, which is why so-called ‘brown dwarfs’ get to be so interesting. A good science fiction writer can work wonders with one of these odd objects, too small to sustain nuclear fusion like stars, but too large to be considered planets, and capable of hosting planets of their own. For when something grows about thirteen times as massive as Jupiter, interesting things happen.
A brown dwarf — we could call it a ‘failed star’ — tries to be star-like by burning an isotope of hydrogen, deuterium, in its core. But it’s too small to ignite true hydrogen fusion, which is what powers up the stars we see in the night sky. We’ve only been identifying these objects for a couple of decades now, though they were thought to exist as incredibly dim objects mixed in among the stellar population. These days we’re finding them all over the place. The closest one currently known is called WISE 1049-5319, a binary brown dwarf that draws its numerical designation from WISE, the Wide-field Infrared Survey Explorer satellite.
WISE 1049-5319 is fully 6.5 light years away, But because brown dwarfs are so faint and emit so little heat compared to normal stars, we haven’t had a good read on how common they are, and in fact we’re still trying to work out the answer. For a time there was a flurry of speculation that we might find a brown dwarf closer to us than Proxima Centauri, the nearest of all known stars. Maybe a good deal closer. And that raised the possibility that the daunting journey to another star might not stretch out a lengthy 4.2 light years but perhaps something much less, perhaps a distance that would be within reach of a future long-haul spacecraft.
The WISE mission turned out to be an ideal way of looking for brown dwarfs because it observes the universe not at visible wavelengths but in the infrared, where these dim objects are much more likely to be spotted. Essentially, it’s looking for heat. And despite digging through abundant WISE data for some time now, astronomers have been unable to find any closer brown dwarfs. It turns out Proxima Centauri may well be the closest star to us.
But keep brown dwarfs in your thinking if you’re interested in the possibility of extraterrestrial life. For although they’re faint, various studies have shown that they do produce enough heat to warm a ‘habitable zone’ in which planets could give birth to living things. By habitable zone, most astronomers mean a zone around a star (or brown dwarf) that is warm enough for liquid water to exist on the surface. Brown dwarfs cool off over time rather than brightening, like the Sun, but it’s a long, slow fade, and billions of years of habitability might exist for any local organisms on the planets that could form here.
Brown dwarfs are exotic locales for future exploration that could vastly increase the real estate for astrobiology, the study of life in the universe. We used to think that they were about as common as normal stars in the galaxy, and that would mean as many as 200 billion of them could exist, but the WISE mission found — at least within a sphere stretching out 26 light years from the Sun — only 33 brown dwarfs. Because 211 normal stars can be identified within the same realm, the early data point to about one brown dwarf for every six normal stars. That would make 30 billion or so in the galaxy.
It will take a lot more research to find out whether these numbers stand up, for we’re just getting into finding and classifying these tricky objects. But 33 billion is a big number for a population of failed stars we didn’t know existed until the early 1990s.
Imagine a planet hugging a brown dwarf close enough to stay warm. If life can form here as well as around the warmer red dwarfs and relatively bright G-class stars like our own Sun, we could be looking at a galaxy where living things are common. But it will take new space-based instruments to study the atmospheres of planets around other stars before we’ll know whether the chemical signatures of life exist anywhere other than our own Earth.