Artist drawing of a solitary brown dwarf in space, glowing feebly in visible light. Credit: NASA/JPL-Caltech
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Artist drawing of a solitary brown dwarf in space, glowing feebly in visible light. Credit: NASA/JPL-Caltech

Are we overlooking a lot of nearby brown dwarfs?

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Apr 17, 2018

You may already know that the closest star system to the Sun is Alpha Centauri. You may also know that the closet star to the Sun is Promixa Centauri, a dim bulb of a red dwarf that makes up 1/3 of the Alpha Centauri trinary system, just 4.24 light-years away.

It's incredibly unlikely any stars are closer to us than that. Even the dimmest possible star — defined (more or less) as an object massive enough to have sustained nuclear fusion in its core — would be bright enough to have been found by now.

But there are objects that have lower mass than stars, but tend to be much more massive than planets. We call these beasties brown dwarfs. They had been theorized to exist for decades, but they're so faint the first one, Teide 1, was only found in 1995 (I actually did some work on Hubble observations of the second one ever found, Gliese 229b). Once we knew they were out there and what they looked like, many more were found. Something like 3,000 have been discovered so far. There are probably upwards of a hundred billion of them in the Milky Way!

Because they're so faint, we really only find the ones nearest to us. Unlike stars, too, brown dwarfs tend to fade with age. They are born hot, hot enough to glow, but without a source of energy in their core they get dimmer over time. A really old one could be very hard to detect, even with current technology.

Put all this together, and I have to wonder: Could there be a brown dwarf closer to the Sun than Proxima Centauri?

The answer is: We don't know. It's possible, though how likely is hard to say. There have been all-sky surveys that would have found any above a certain brightness, but who knows what lurks below our visibility limits?

That's why I read this paper with such interest. It describes a survey of the sky called Sondage Infrarouge de Mouvement Propre (SIMP). This uses a pair of telescopes (one each in the northern and southern hemisphere) equipped with an infrared detector (where brown dwarfs tend to emit most of their light) and looks repeatedly at the same parts of the sky, looking for objects that have large proper motions — literally, are actually moving rapidly across the sky.

Now, "rapidly" is relative; you still have to watch them for years to see enough movement to measure. Every star we can see is orbiting the center of the galaxy in a similar way to planets orbiting the Sun. They all move at different speeds, so over time they move relative to one another. Most stars are so far away that this motion is too tiny to measure easily. But for the same reason high proper motion stars tend to be close to the Sun. It's like driving down a road and watching nearby trees whiz by while more distant mountains look like they're moving slowly.

Most brown dwarfs are identified by their high proper motion. However, there are other criteria used to figure out what they are, like color (literally, how much light they emit at different wavelengths). The authors of the SIMP paper note that many brown dwarfs SIMP detects would be excluded from other surveys due to unusual colors or other characteristics.

So they broadened their own parameters somewhat, easing up on the restrictions. Over the course of about three years, SIMP surveyed 28% of the sky, observing millions of stars. It generated 115,000 separated observations, which were taken with short exposures to maximize sky coverage. Given their criteria, they favored finding stars less than about 150 light-years away.

And they got results! They found 165 new objects, most very faint actual stars and a whole passel of brown dwarfs of different varieties. And because they opened up their parameters, a lot of these objects are weird. Some are redder or bluer than expected, possibly because of an unseen companion object or unusual chemistry in their atmospheres.

That's actually important. With stars, the lower mass they are the cooler they are, and the redder they get. But with brown dwarfs that's not always the case. If they cool past a certain point, for example, molecules like titanium oxide and hydrogen sulfide (rotten egg gas) can form. These strongly absorb light at different wavelengths, which can change the colors we see from the brown dwarf significantly. Allowing a wider range of features to be used when looking for them can really open up the skies to finding lots more.

A secondary result is that they found previously discovered objects that were near star clusters. It can be hard to know if a star is a member of a cluster or not; it might be a lot closer or farther away and just happen to lie along the line of sight. One such object was rejected by a previous study as a member of the Hyades cluster (the V that makes up the head and horns of the bull of Taurus), because it was classified as a star; if it were a red dwarf then due to its faintness it must be too far away to actually be in the Hyades. But the new study reclassified it as a brown dwarf. Because they are dimmer, it must actually be closer than previously thought, and more likely to be in the Hyades. That's pretty cool.

After all this, does this help any in knowing whether there may be something more proximate to us than Proxima? Maybe! While they didn't find anything that close, this survey shows that there are plenty of objects not too far away that we missed in previous surveys. That doesn't mean there is such an object less than 4.2 light-years away, but it does make me wonder even more if there might be.

And it certainly shows us that we need to look at the sky even more carefully. Space is vast and deep, and many objects in it faint. What amazing discoveries could be lurking out there, just waiting for us to look at them the right way?