We know that dark matter exists, but, irritatingly, we don’t know what it is.
One way to figure that out is to look for signs of it here on Earth, using subatomic particle detectors. But a new idea just published in a scientific journal is that we need to go bigger. A lot bigger: Using entire exoplanets as detectors.
I give them points for thinking originally, for sure.
Dark matter is a form of matter that has mass and gravity, but doesn’t emit light or interact with normal* matter directly. It affects the way galaxies rotate, the way galaxies behave in clusters, the way clusters affect the light of objects behind them, and a host of other things, too. We know it exists. And over the decades people have looked for it, but almost everything that could possibly work has been eliminated. It’s not teeny black holes, or rogue planets, or cold gas. Nothing made of normal matter works, leaving only “exotic” subatomic particles like axions as candidates. Attempts have been made to look for those, too, but so far zip.
Here’s where the new idea comes in.
Most theoretical models of dark matter as subatomic particles show that our Milky Way Galaxy is embedded in a vast halo of it hundreds of thousands of light years across, but this halo is not homogeneous. It’s denser toward the galactic center, and less dense out here in the suburbs 26,000 light years from the core.
Also, one theoretical type of dark matter (generically called WIMPs, for weakly interacting massive particles) can interact with normal matter but does so, well, weakly. If one of these encounters an electron or proton, it can bounce off it, what physicists call scattering. The critical part here is that if this happens, the dark matter particle loses energy — think of it as the particle slowing down.
Now picture a big old planet out there in space. It has a lot of electrons and protons in it, so tons of chances for a dark matter particle to scatter. If the particle slows enough, the gravity of the planet might be enough to capture it, so it becomes part of the planet. In a sense the planet provides friction to slow the particle enough to stop, and this generates heat — just like your brake pads on a car or bike get hot when you use them.
Also, these same kind of dark matter particles may self-annihilate; that is, if two of them come together they turn into pure energy (like when matter and antimatter collide). This also generates a lot of heat. So first they can heat a planet by being captured, and then as they collect inside the planet they can annihilate and generate more heat.
It takes a lot of dark matter to heat a planet appreciably, of course. But models of the galactic halo show it gets pretty dense toward the galaxy’s center. Running the numbers in their paper, the scientists find that the there may be enough dark matter in the galactic center that it could be detected by looking for extra hot planets.
The amount of heating depends on two things: how hot a planet is in general, and where it is in the galactic dark matter halo. The colder an object is, the easier it is for dark matter heating to outperform the object’s own non-dark-matter induced internal heat. For example, Jupiter is still hot leftover from its formation, and it cools with time. Looking for an exoplanet that’s really old but still hot would be supportive evidence for this idea. A rogue planet — one in space not orbiting a star — would be even better since a star won’t interfere with the observations And looking for one near the galactic center would help since there’s more dark matter there.
The best bet, they find, is a two-step process. The first step is to look for Jupiter-mass exoplanets in our local neighborhood to see if they are warmer than expected, because if they’re close by it’ll be easier to see even though there’s less dark matter to heat them up.
The second step is to look for more massive ones — technically brown dwarfs, objects more than about a dozen times Jupiter’s mass up to about 80 times (any more massive and they become stars) — closer in to the galactic center, where dark matter is more dense. Some models indicate an otherwise cold brown dwarf could be heated to over 1000° C just by dark matter interactions!
The key to looking for this is not so much looking for hot brown dwarfs, but looking for cold ones. Hot ones are expected if they’re young anyway, but if you find cold ones in the galactic center that falsifies (or at least weakens) the hypothesis. The scientists propose using James Webb Space Telescope or the upcoming Nancy Roman Space Telescope to look in the infrared for both nearby Jupiters and more distant brown dwarfs.
I’ll note there’s a lot of ifs between the hypothesis and actually finding these objects. It’s an interesting idea, but the odds are a tad long. Still, given how elusive dark matter has been, it’s probably worth a shot, especially if they can use other observations these telescopes were making anyway and search the data for their target exoplanets. The scientists involved have more papers planned with details.
And I have to note: Incredibly, one of the two scientists who wrote the paper, Juri Smirnov, says he got inspired to investigate brown dwarfs as possible dark matter detectors from Crash Course Astronomy! This is a series of videos I made with Hank and John Green’s Complexly production company that’s an introductory course into astronomy.
Smirnov is a particle physicist and was in the Ohio State Astronomy department’s journal club — a common practice, where grad students and faculty get together informally every morning (they call it Astro Coffee) for a half hour to discuss recent papers. He said he wasn’t familiar with all the objects and phenomena discussed, so he looked online and found Crash Course, specifically the episode on brown dwarfs. That got him thinking about using them as particle detectors, and yada yada yada, he and his colleague Rebecca Leane did the work and wrote the paper.
So. I’ll be honest: I’m rooting for them to detect dark matter this way because 1) the scientific importance of it is huge, and b) it’s a cool idea and would be fun if this method worked.
But also, I would be the guy that inspired the discovery.
I’m OK with that.
*As I’ve pointed out before, dark matter outnumbers “normal” matter by a ratio of around 5:1, so it makes you wonder which flavor of matter is normal.