Hey, this is so cool: the brown dwarf Gliese 229B really is a binary system. It’s not one brown dwarf it’s two, orbiting each other!

I wrote about this possibility back in January 2024 in BAN issue #665. The backstory is there, but to catch y’all up… 

Brown dwarfs are objects more massive than planets but not massive enough to be true stars (that is, to ignite nuclear fusion in their cores like the Sun does). They’re peculiar objects, between roughly 13 and 77 times the mass of Jupiter. Their cores are so compressed the matter in them displays a bizarre quantum mechanic effect called degeneracy, and don’t behave as you might expect. For example, add more mass to a brown dwarf and it gets smaller, not bigger. This make them incredibly dense. Even though they’re only about the same size as Jupiter they can be far denser than iron!

The first known brown dwarf, Teide 1, was discovered in 1995. Gliese 229B (abbreviated Gl229B) was actually discovered in 1994 but not confirmed until later in 1995. Gl229B orbits a low-mass red dwarf star (called Gliese 229A), close enough together that despite being pretty close to us, less than 19 light-years, they appear as one object except in very large telescopes. 

But it didn’t take long to find a problem. Brown dwarfs don’t generate energy, but instead form very hot and cool over the eons by emitting infrared light. The more massive they are the brighter they are, for example, and younger ones should be brighter than older ones.

Gl22B was measured to have a mass of 71 times Jupiter. But according to all the models of how brown dwarfs behave, it should be far brighter than it is for that mass. Why was it so much dimmer than expected?

There were two possibilities. One is that the models were wrong. That seemed unlikely; they do a pretty good job with most brown dwarfs, giving consistent answers that make sense. The other possibility is that Gl229B isn’t a brown dwarf. It’s actually two.

In Issue 665 I wrote about some earlier research that supported this idea; if you suppose that it’s actually two brown dwarfs in a tight orbit, so tight they look like a single object from Earth, the numbers start to make sense. The brightness of a brown dwarf increases sharply with mass, so if you have two lower-mass ones instead of one, even together they’d be fainter than what you’d expect for a single object. 

But that’s theoretical. What we need are observations. And that’s just what we have now.

In the new work [link to journal paper], astronomers observed Gl229B in two different ways. In one, they used the Very Large Telescope (or VLT) as an interferometer, combining the light from the four massive 8.2-meter telescopes. This doesn’t make an image, really, but can be used to make extremely high-resolution observations. What they found is that the data don’t match a single object, but instead make much more sense if Gl229B is actually two separate objects. Over the five nights of observations they even can tell the two objects have moved relative to each other — that’s expected, if they’re so close together they should be orbiting each other rather rapidly.

They also used just one of the VLT telescopes to take spectra, splitting the incoming light into thousands of separate colors. Different molecules — water, methane, and the like — in the atmospheres of the brown dwarfs absorb light at specific colors, so you see less light at those wavelengths. By plotting the brightness versus color you can tell a lot about an object, like what’s in it, how rapidly it’s moving toward or away from you, and more). This part is tricky: As the two objects orbit around reach other, the light from each gets Doppler shifted. But they show opposite effects! As one is headed toward us, the other is headed away, and vice-versa depending on where they are in their orbits.

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