Space News Deep Dive: The Possible Brown Dwarfs of the Small Magellanic Cloud

Anybody who reads the Spacing Out newsletter knows how much I adore a pretty picture of space. The Webb and Hubble Space Telescopes are, of course, always a good bet for generating those. A recent stunner from Webb is no exception. Except it turns out this one, with a little assist from Hubble, has some pretty amazing science behind it, because Webb may have just found the first brown dwarfs ever detected outside the Milky Way.

If you don’t know what a brown dwarf is, that’s not terribly surprising. They’re a bit of a niche astronomy topic, and not nearly as charismatic as things like planets or stars or galaxies. But they’re also weird, fun things out there in the darkness and the fact that Webb may have found ones outside the Milky Way is really an impressive achievement.

I’ll explain what brown dwarfs are (can I also confess how much I want to call these things brown dwarves rather than brown dwarfs, because that’s how Tolkien pluralizes the word dwarf?) and just what it is that Webb found, but first just look at this incredible image!

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Star vs. Planet

The term most used to describe a brown dwarf is a “failed star”, which is a bit of an unfair term. You might as well call it a “superior planet”, you’d sound just as judgmental, be pretty much just as accurate, and not sound nearly as negative.

Essentially brown dwarfs occupy that liminal space between small stars and big planets. Planets can, after all, become pretty big balls of gas. I know we think of Jupiter as big (and it is, don’t get me wrong) but there are worlds out there much larger than our own system’s biggest planet. We know of a couple over twice Jupiter’s size, making them at least a few thousand Earths big.

At the same time, stars can get pretty small. In fact, we know of stars smaller than Jupiter. The wee little red dwarf EBLM J0555-57Ab, 600 light years away, is only just a little bigger than Saturn. Clearly size is not what makes a planet or a star (or a brown dwarf for that matter). It’s all about mass.

Our Earthly brains often think of size and mass interchangeably, but they are two different concepts. To brazenly steal a quote from my coworker Caity, “size is how much space you take up, mass is how much stuff you’re made of.” So EBLM J0555-57Ab takes up less space than Jupiter, but it’s got a lot of stuff packed into that smaller space, making it far more massive than Jupiter (about 85 times as much, in fact). 

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That means, as small as EBLM J0555-57Ab is, it’s massive enough to force hydrogen atoms to fuse together within its core. This is called stellar fusion, and it’s what makes a star shine. If you can do this, you’re a star. Period. That seems to happen at around 80 Jupiter masses, so EBLM J0555-57Ab didn’t make the cut by much. But it did, so it’s a star.

Planets, on the other hand, cannot generate fusion within their cores. They just don’t have the mass to force atoms together that don’t want to be together (hydrogen atoms really don’t want to be together, it’s why you can get so much energy out when you make them). If you’re a great big gas world but not massive enough to start fusion in your core, you’re a planet. Where this stops is a bit fuzzier, which I’ll discuss more in a bit.

In-Between

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You’d think that would be pretty much it, right? Hydrogen fusion = star, no hydrogen fusion = planet. But of course it can’t be that simple. Because there are some things out there that can’t fuse normal hydrogen in their cores but can fuse a certain isotope of hydrogen called deuterium. This is a hydrogen with an extra neutron thrown in. 

The presence of this neutron has an effect on the strong nuclear force (one of the fundamental forces of nature, but one that only affects very small things like protons and neutrons, so most folks don’t think about it very often) and means deuterium can fuse under much lower temperatures and pressures than normal hydrogen does.

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If you can’t fuse normal hydrogen, but you can fuse deuterium, congratulations, you are a brown dwarf. You don’t emit bright visible light like a star, but you have a nice glow in infrared.  Despite being called a brown dwarf, you’re possibly a lovely shade of dark mauve. And because you’re so dim you’re not easy to find. Brown dwarfs were first theorized in the 1960s, but we didn’t actually confirm discovery of any until the mid-1990s. Today we know of roughly 3,000 of them. 

Exactly how much mass it takes to start fusing deuterium isn’t definitively known, making the lower mass threshold for brown dwarfs a bit fuzzy. The generally accepted number is that if something is less than 13 Jupiter masses, you can call it a planet. Above that, it’s a brown dwarf.

Enter Webb

Remember how I said brown dwarfs glow in infrared light? Infrared is the current in which the Webb Telescope swims. Its entire design is optimized to make it exquisitely sensitive to things emitting infrared light.

Webb is responsible for the discovery of the smallest known free-floating brown dwarf. It has found brown dwarfs with protoplanetary disks, suggesting it may be possible for them to have solar systems of their own. Webb even was able to observe storm clouds in the atmospheres of the two brown dwarfs closest to our solar system. And now it has gone beyond the Milky Way itself.

Webb (along with some assisting visible light images from Hubble) looked at a star-forming cluster called NGC 602, which is not in the Milky Way itself but in the Small Magellanic Cloud, a satellite galaxy of our own. That puts NGC 602 about 200,000 light years away from us. Let’s take another look at that lovely cluster, this time with the region Webb was paying the most attention to marked off. We deserve a treat.

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Astronomers were able to use this remarkably gorgeous image to map out the masses of the objects being formed in this cluster. They found 64 objects that appear to be in or very near the mass range associated with brown dwarfs. These objects ranged in mass from 50 to 84 Jupiter masses: well above that fuzzy lower limit and some enough above the theoretical upper limit that they may actually be small stars.

Figuring out a brown dwarf’s age is tricky business, but because these dwarf candidates are all mixed in with the stars of the cluster, it’s very likely that they formed at the same time—and astronomers are very good at figuring out how old stars are. Astronomically speaking, they’re total babies, probably no more than 3 million years old.

We cannot, at this time, be certain these things are brown dwarfs, which is why they’re called “candidates” (and as I already said, some of them may very well turn out to be puny stars instead). But it seems highly likely that some of them must, in fact, be the first brown dwarfs ever seen outside the Milky Way.

Where Brown Dwarfs Come From

Discoveries like this are important for figuring out exactly how brown dwarfs form. As they sit in the gap between stars and planets, it’s not entirely understood if brown dwarf formation follows a stellar or planetary pattern. Stars, you see, coalesce out of interstellar clouds of gas and dust. Planets, on the other hand, form around young stars, from protoplanetary disks that collapse around the baby stars as they roar to life.

Webb’s observations of NGC 602 offer up evidence in favor of the star model. None of these candidates appear to have formed from a disk around any of the young stars making up the cluster. They appear more likely to have formed directly from the gas cloud around them, just as their stellar cousins did. It’s not definitive or final proof of anything, but it’s a big step along the path to understanding these weird, liminal objects that lurk out in the dark. And seriously, how can you not love something that comes with a picture this pretty??

And you know what, I’ve mentioned Hubble’s contribution a couple of times but we haven’t looked at it yet, so let’s take one more look at NGC 602 the way Hubble sees it. Come on, once more for the road.

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