This episode of SciShow is supported by NordVPN.

Head over to nordVPN.com/space to learn more about virtual private networks and internet security. [INTRO ♪]. As kids, we're often taught that the objects in space can be divided into neat, orderly categories.

Like, if you have a big, hot, shining thing, that's a star. And if you have a small, cool ball of rock and gas, that's a planet. It all sounds pretty straightforward, and it's a great way for a kid to learn what's out there.

But in reality, the universe is a lot more interesting than that. Like, starting in the late 1990s, astronomers began to discover objects that were neither stars nor planets. Instead, they were somewhere in-between.

These objects are called brown dwarfs, and they're more than just cosmic oddities. As we learn more about them, they could have a significant influence on our understanding of the universe. If you look at a picture of a brown dwarf — or at least, what a NASA artist imagines them to look like — you might think these things are planets.

Maybe even gas giants, like Jupiter. They're not, though, because based on our current understanding of them, they don't form the same way. Planets seem to form when particles of dust, ice, or rock collide and stick together.

Eventually, some might grow big enough to attract an envelope of gas, but they still likely started with a dusty or rocky core. Brown dwarfs don't begin like this. Instead, they form like stars, when a huge cloud of gas collapses in on itself due to gravity.

Except... brown dwarfs aren't stars, either. See, when a gas cloud collapses to form a star, the atoms smash together and release a huge amount of energy — so much that it starts nuclear fusion. In this process, hydrogen is turned to helium, and a bunch of energy is released.

That's what gives stars their glow. Brown dwarfs form from smaller gas clouds, and those aren't quite big enough to start real hydrogen fusion. Early on, these objects can have enough energy for another type of fusion, where a form of hydrogen called deuterium fuses with a proton to make a different kind of helium.

But deuterium is in comparatively short supply, and it can get used up within a few million years. So, long story short, it doesn't count. This all means brown dwarfs aren't part of the Planets Club, and not part of the.

Stars Club, either. Instead, they're oddballs somewhere in between. At first glance, that might just make them seem like a curiosity — a kind of non-planet failed star, thing.

But there's a lot to learn about them and a lot they could teach us. For example, some scientists propose that brown dwarfs could help us understand the squishy gray area between stars and planets. They could help us answer questions like, “What's the lightest a star can get?” That's actually something that came up in 2018, when a brown dwarf was discovered that seemed to be just a little too massive for its own good.

Right now, most evidence suggests that brown dwarfs can get up to about 75 times the mass of Jupiter — because past that point, gas clouds are big enough to ultimately cause nuclear fusion. In 2018, though, one was found that seemed to be slightly past that cutoff. And none of our existing models could explain how it was possible; they predicted you'd get a star with that kind of mass.

This suggests that there's something about tiny star formation that we just don't understand, and with more research, brown dwarfs could help us figure it out. A few scientists, though, have proposed that these objects could do even more than that. They suggest that, maybe, brown dwarfs could help us understand the structure of the universe itself.

See, when researchers look at distant galaxies, they often notice that the motion of those objects can't be explained just by what they see. Instead, the galaxies need to have some extra, invisible mass. Astronomers believe that mass comes from what they call dark matter, which is often thought to be entirely different, undetectable kind of stuff.

But a handful of scientists have suggested that this missing mass could, in fact, be brown dwarfs. It sounds extreme, but it kind of makes sense. After all, brown dwarfs only give off a tiny pinpoint of infrared light, so they're really hard to detect, especially as they get older and dimmer.

Also, they have only slightly less mass than a star. So if there were enough of them, they could play a major role in keeping galaxies moving the ways they do. A recent survey even suggests there could be up to a hundred billion brown dwarfs in the Milky Way alone.

That's one for every hydrogen-burning star, so they could have a significant effect! Of course, considering that other studies have found thousands of brown dwarfs — not millions — this hypothesis might be a long shot. But, hey.

There's not exactly a ton of conclusive evidence about what dark matter really is, so it couldn't hurt to keep an open mind. Besides, even if they can't explain dark matter, brown dwarfs are already shaping the way we view the universe. They've proven that the line between stars and planets is fuzzier than you'd think, and since we've only been discovering them for a few decades, there's likely a lot more to learn.

So for now, they'll continue to remind us how varied and surprising space is, and how things don't usually exist in black-and-white categories. And in the future? Well, maybe someday, they could help us answer some of our biggest astronomical questions.

Speaking of big questions, here's one for you:. How's your Internet security these days? I'm not talking about sharing your passwords in chat rooms or anything.

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They're compatible with most operating systems, have apps for Android and iOS, and have thousands of servers in dozens of countries. If you want to learn more about them or about what it means to have a virtual private network, you can go to NordVPN.com/space. If you decide to sign up there, you'll get seventy percent off a three-year plan.

So for less than $3.50 a month, you can browse and shop securely online on all of your devices. For a short time, if you use the code “space” during that sign-up process, you'll also get an extra month for free! Again, that's NordVPN.com/space.

Thanks for considering it, and thanks as always for supporting SciShow Space! [Outro ♪].