Subscribers glow brightly in the infrared

I do so love brown dwarfs. These are a weird kind of astronomical object, with masses between that of a planet and a true star. They have characteristics of both but also of neither — they are more than a dozen times the mass of Jupiter, but aren’t really much bigger, so they’re very dense even though they have thick gaseous atmospheres. They don’t have sustained fusion in their cores like stars, but can temporarily fuse lighter elements like deuterium and lithium.

And they’re so faint they’re hard to detect, even when they’re nearby. Luhman-16 is a binary brown dwarf system (two orbiting each other) that was discovered only in 2013 despite being the third closest system to the sun!

Another issue with finding them is that they generally don’t glow much in visible wavelengths of light (the kind we see). Because they can’t generate energy in their cores, they tend to form and then cool off over time. As they do their emitted light reddens, until eventually most of the light they give off is in the infrared.

We know of about 2,900 brown dwarfs right now, some discovered in infrared surveys, others in observations by, say, JWST. In those cases their colors (different emitted wavelengths of infrared light) are used to nail them down.

But there are other ways. Because the ones we find are so dim, they tend to be close to the sun (within a couple of hundred light-years) or else they’d be too faint to detect. Like everything else in the Milky Way, they orbit the center of the galaxy, which means they move in the sky relative to more distant, background stars. If we take images sufficiently separated in time (a few years, say) then that motion (called proper motion) can be detected. The two images can be blinked one after another, for example, and the nearby objects will appear to flicker back and forth. Or, one image can be subtracted from the other; stars will more or less disappear, but anything moving enough will be seen as a pair of objects close together, one positive (white) and one negative (black).

That’s a lot of work. How do you do it? 

You ask a huge team of dedicated and enthusiastic volunteers to join in! That’s the point of citizen science^* : get lots of people to do small tasks that can then be combined to create a much larger wealth of knowledge. This has been a huge boon in astronomy, with many projects set up to help train non-scientists on the task, test their ability, and then let them loose on the data. Studies have shown the results tend to be really accurate, which is amazing. 

One such effort is Backyard Worlds: Planet 9, part of the much larger Zooniverse project. It uses observations from the venerable Wide-field Infrared Survey Explorer (or WISE) mission to find objects that have appeared to move over time. There have been thousands — yes thousands — of volunteers who have looked at the data, and there have been over 100,000 individual submissions of candidates (many of which overlap; that is, many people seeing the same target). This generates a huge database of observations, which is then examined by professional astronomers to see if they qualify for the catalog.

The upshot: the project has found 3,006 “motion-confirmed” brown dwarfs. Three thousand! That literally more than doubles the known number of these objects, a huge score [link to journal paper].

A big part of understanding a new kind of object is simply finding as many of them as you can. As you do, trends start to show up; how bright they are versus mass, for example, or their distribution in ages or distance. All of these tell you something about the greater population, and the more you have the better the statistics get, so you can be more certain you’re seeing real trend and not weird outliers (which is exactly what happened when the first exoplanets were discovered; the ones found first were the ones easiest to find — bigger ones with short orbits — which may not be representative of the whole collection).

Mind you, the first legit brown dwarf found — Teide 1 — was discovered only in 1994 and announced in ‘95! Another, Gleise 229b, was discovered that same year, and is now known to be a binary system of two brown dwarfs orbiting a red dwarf. I spent some time studying that one, which is a big reason I love these objects so much.

I’m very impressed by this new work. The science is amazing, and the addition to our knowledge of brown dwarfs hugely valuable. But it also shows — once again — that these kinds of crowdsourcing work very well, and make valuable contributions to our understanding of the universe.

You can be a part of it, too! Just go to the Zooniverse site and see what projects are open. SciStarter is a GREAT site that has tons of such projects across lots of different fields of science, not just astronomy. I wrote about them back in Issue 329 when I was on a NASA panel about these projects, too.

Who knows? Join up, and you might make a real contribution to real science.

^* I’ve been told this term is falling out of favor, since the word “citizen” is rather loaded. Fair enough, but the one I see being used more now is crowdsourcing, which, sure, but is a bit generic. It fits, though, so if it catches on, fine with me.

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