One of the closest objects in the entire Universe to the Sun is the binary brown dwarf Luhman 16, aka WISE 1049 — only the Alpha Centauri system and Barnard’s star are closer. The pair is 6.4 light-years away, but so faint that it was only discovered in 2013.

Brown dwarfs are objects intermediate in mass between planets and stars. Planets have “normal” matter in their cores, but brown dwarfs are so dense that matter in their cores changes into a weird quantum mechanical state called degeneracy, where it behaves in bizarre ways (I talk about this a bit in the link above). On the other hand, brown dwarfs aren’t dense enough to create the pressure needed to ignite nuclear fusion, so they aren’t true stars either. This makes them they’re own thing, a whole class of objects to study.

Like planets, they can have thick atmospheres, but they’re generally hotter than planets. They don’t generate any heat on their own like stars do, and simply cool after they form. But that takes a long time, billions of years, so even old brown dwarfs are pretty hot in human terms. The two making up Luhman 16 are no older than the Sun, though they could be much younger. This all means they are incredibly faint in visible light, but glow in the infrared, making them an excellent target for JWST, which is designed to see in that part of the spectrum.

Here’s a fun bit: using JWST, astronomers watched the pair over the course of about 15 hours [link to journal paper]. During that time they took lots of spectra, breaking the incoming light into individual colors, which can reveal the presence of various elements and molecules in the brown dwarfs’ atmospheres. Unsurprisingly they found things like water, methane, and carbon monoxide, which are common in brown dwarfs.

What’s cool here is that over the timespan of the observations, the spectra changed. Like, the shape of the spectra changed measurably, indicating changes in the composition of the atmospheres. It’s unlikely the air changed overall — like the composition changed — so almost certainly what they were seeing was clouds.

Imagine you’re orbiting Earth and you only have a peephole to see through that doesn’t let you get a good picture. But you can measure the color of the light you see. Mostly it’s brown and green as you pass over land, but then suddenly it becomes brighter and white. You can infer you saw a cloud (it could be ice, but if you know you were over, say, Africa at the time that’s unlikely).

Similar thing here.

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