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Dark Matter is the most abundant form of matter in the known universe, so what's keeping it from forming into planets?

Hosted by: Hank Green
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[ intro ] Over the last three decades, astronomers have cataloged some pretty weird worlds beyond our solar system.

There’s a planet whose atmosphere molecules are ripped apart by the heat of every sunrise, and a planet that’s dark pink. But what about planets made from the most abundant kind of matter in the universe, which we can’t even see?

Could a planet ever be made out of dark matter? It’s a question astronomers have pondered, and some have proposed ways for us to find out. Now we still do not know what dark matter is exactly, but all the leading candidates for dark matter particles have one thing in common: they do not interact with the electromagnetic force.

Since light is basically just another word for electromagnetic radiation, dark matter is rendered completely invisible. But what does this mean for planetary formation? Dark matter interacts with gravity just fine.

It’s matter. That’s how astronomers know it’s out there. And do not planets form when gravity causes a bunch of matter like rock and ice, and gas to collapse into a big ball?

Unfortunately for dark matter, it’s a little more complicated than that. When the building blocks of planets start to come together, they are literally the size of pebbles, and they have to rely on electrostatic cling to actually stick to each other. It’s only after enough of them have piled up that gravity can take over the planet forming process.

Now, electrostatic cling is just another way the electromagnetic force operates. So dark matter can’t do that. But maybe dark matter planets don’t form like most planets do.

Maybe they share an origin story with stars. Instead of building up from tiny pebbles, maybe dark matter starts as a giant cloud and collapses down. Imagine two particles of dark matter attracting one another with their mutual gravity.

That attraction gets stronger as the two get closer, and they speed up as they get closer, too. That speed usually sends them zooming past each other, and they get farther and farther until gravity slows them down, then pulls them back together. And the process repeats forever, unless the dark matter can find a way to shed some of its energy along the way.

In other words, for dark matter to condense into a planet, it has to cool down, first. But in the near vacuum of space, the best way to do that is to emit electromagnetic radiation. Since dark matter can’t do that, it can’t cool down much at all.

That’s why astronomers have observed evidence of massive spherical halos of dark matter surrounding galaxies like the Milky Way, where the regular matter has mostly collapsed into star and planet-filled disks. However, some proposed versions of dark matter can interact with a different fundamental force in the universe: the weak nuclear force. Through those interactions, dark matter actually could collide with matter, and lose energy that way.

But in 2017, one science duo proposed something that’s a little more out there: if there’s a light side of the electromagnetic force, maybe there’s a dark side, too. Or, at least, a completely separate dark variety of electromagnetism. If that sounds like something straight out of science fiction to you, you’re not alone.

In fact, the scientist who first came up with the idea was trying to prove dark matter planets couldn’t exist. But after playing around with the math, he ended up convincing himself it might work, after all. Teaming up with another researcher, the two created their preliminary model of dark electromagnetism.

In this model, there are two kinds of dark matter particles. The paper calls them H and L, for heavy and light, but they’re basically the dark version of a proton and electron. Part of the work they had to do was play around with the different masses these two particles could have, and how strong the dark electromagnetic force could be, given pre-existing ideas on dark matter and the current state of the universe.

And together, the math suggests the two particles could team up to make the dark equivalent of atomic hydrogen. Now, in regular hydrogen, an electron can hang out at different levels around its proton. To lose energy and drop to a lower level, it can emit a particle of light called a photon.

So this hypothetical dark atom can perform a similar action if it needs to, shedding a dark photon to lose energy. And when the parameters are just right, clumps of dark matter with masses under 100 million Suns could collapse, while larger masses would stay in those giant fluffy halos. That collapsing dark matter would go on to fracture into smaller and smaller clumps, including, hypothetically, those the size of planets While the math works on paper, dark electromagnetism is pretty much a thought experiment at this point.

If they want to move forward, the duo will have to propose a way to test some kind of observable result. But over the past few years, another team of dark matter planet hunters has made proposals of their own. According to their models of the early universe, dark matter could form planet-sized clumps less than 380,000 years after the Big Bang.

We’re talking back when regular matter was still jumbled up with photons in a plasma soup. But since dark matter was free to ignore what light was doing, it could do its own thing… g planets that range in mass from that of an asteroid to Neptune. These papers don’t propose exactly how dark matter planets form, but over billions of years, gravity would make them settle into the overall web of galaxy clusters stretching across the universe.

The team estimated that our galaxy alone could host a million billion dark matter planets. As for how scientists might actually be able to detect these planets, the team proposes they would also accumulate regular matter, like hydrogen and helium gas. And if enough of that gas gets concentrated at a high enough pressure and temperature, it could trigger nuclear fusion, and create light in the process.

But even for the planets that aren’t massive enough to do that, the gas would still be warm enough to glow, creating light just like anything with a measurable temperature does. So some of the light out there in the universe may come from the regular matter captured by planets made predominantly of dark matter. Now the team has to propose a way to figure out which light was made that way.

Maybe one day we’ll find direct evidence that dark matter planets really can form, or that it has its own suite of dark forces. We won’t know until scientists come up with testable hypotheses, but that’s what science is all about. Coming up with questions, and trying to answer them And if you have questions about the universe you’d like answered, head on over to

If you support us at any level, you’ll unlock access to an exclusive inbox where you can submit your questions about dark matter and more! [ outro]