May 3, 2021 8:00 AM

Where’s the Dark Matter? Look for Suspiciously Warm Planets

Physicists calculated that these mysterious particles will betray their location with heat. To prove it, they’ll need the most powerful telescopes in the cosmos.

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We're bathing in an uncertain universe. Astrophysicists generally accept that about 85 percent of all mass in the universe comes from exotic, still-hypothetical particles called dark matter. Our Milky Way galaxy, which appears as a bright flat disk, lives in a humongous sphere of the stuff—a halo, which gets especially dense toward the center. But dark matter’s very nature dictates that it's elusive. It doesn’t interact with electromagnetic forces like light, and any potential clashes with matter are rare and hard to spot.

Physicists shrug off those odds. They’ve designed detectors on Earth made out of silicon chips, or liquid argon baths, to capture those interactions directly. They’ve looked at how dark matter may affect neutron stars. And they’re searching for it as it floats by other celestial bodies. “We know we have stars and planets, and they're just peppered throughout the halo,” says Rebecca Leane, an astroparticle physicist with SLAC National Accelerator Laboratory. “Just moving through the halo, they can interact with the dark matter.”

For that reason, Leane is suggesting that we look for them in the Milky Way’s vast collection of exoplanets, or those outside our solar system. Specifically, she thinks we should be using large sets of gas giants, planets like our own Jupiter. Dark matter can get stuck in planets’ gravities, as if in quicksand. When that happens, particles can collide and annihilate, releasing heat. That heat can accumulate to make the planet piping hot—especially those near a galaxy’s dense center. In April, Leane and her coauthor, Juri Smirnov from Ohio State University, published a paper in Physical Review Letters which proposed that measuring an array of exoplanet temperatures toward the Milky Way’s center could reveal this telltale trace of dark matter: unexpected heat.

Their paper was based on calculations, not observations. But the temperature spikes Leane and Smirnov predict are noticeably large, and we’ll soon have a cutting-edge thermometer: NASA’s new James Webb Space Telescope is expected to launch this fall. The JWST is an infrared telescope, and the most powerful space telescope ever built.

“It's a very surprising and inventive approach to detecting dark matter,” says Joseph Bramante, a particle physicist with Queen’s University and the McDonald Institute in Ontario, who was not part of the study. Bramante has previously studied the possibility of detecting dark matter on planets. He says that detecting unusually hot planets pointing toward the Milky Way’s center “would be a very compelling smoking gun signature of dark matter.”

It’s been less than 30 years since astronomers detected the first exoplanets. Because they’re much dimmer than the stars they orbit, they are hard to see on their own; they usually reveal themselves by just barely obscuring the light from those stars. Astronomers also find and size up exoplanets with tricks like micro-lensing. (One star’s gravity warps our view of a further star’s light, and a planet between the two creates a blip in that effect.) The exoplanet tally now sits at 4,375, but some 300 billion could be out there.

Dark matter usually moves freely among these islands of “normal” matter, meaning that it slides past objects without interacting. But when one dark matter particle happens to nudge ordinary particles like protons, it slows down by a smidgeon. “Just like billiard balls,” Leane says. “It just comes in, literally hits it, and then bounces off. But it can bounce off with less energy.”

Accumulating enough of these collisions slows them too much to escape a planet’s gravity. Physicists expect that when this “scattering” and capture happens, dark matter particles can collide and annihilate each other. The once-energetic dark matter decays into other particles—and heat. “When they smash together,” Leane says, “it puts energy into the planets.”

Other researchers have examined how dark matter might flow heat into neutron stars, planets, and the moon. Bramante has studied heat flow limits on Earth and Mars. But Leane says there’s no better laboratory for this process than old gas giant exoplanets. While neutron stars are super dense, which may come in handy for trapping dark matter, exoplanets could outnumber them a thousand-fold. They’re also far larger, thus easier to spot: Neutron stars average about 20 kilometers across, compared to anywhere from 50,000 to 200,000 kilometers for the planets that interest Leane. And old gas giants should be cold, so any heat from annihilation would stand out. Brown dwarfs, small failed stars which fall into the sort of blurred line between stars and gas giants, also fit the bill.

So if these dark matter collisions theoretically occur, and billions of planetary dipsticks are out there—how might we even detect them? Uncertainty pervades the cosmos, so isolated hot spots aren’t out of the question. “In astrophysics, there are lots of anomalies,” Leane says. “So it's totally plausible that you could have a planet that's just arbitrarily too hot.” Leane and Smirnov wanted to trace a trend—a pattern of weird temperatures that could warrant such an extravagant explanation.

So they went all-in on dark matter density. Dark matter is most dense toward the center of the galaxy. More dark matter should mean more collisions. And with more collisions, there should be more heat. They calculated how planets as massive as many Jupiters would respond to this effect under different dark matter densities. They used variables like mass, radius, typical temperature, and escape velocity to relate the internal heat flow of a hypothetical exoplanet (or brown dwarf) to its dark matter “capture rate.” That equation let them convert existing predictions about dark matter distribution in the galaxy into their own predictions about how the temperatures of planets should trend.

Exoplanets closest to the center of the Milky Way should trend hotter, they show. In fact, according to their calculations, Jupiter-like exoplanets—which we’d otherwise expect to have surfaces at below-freezing temperatures—may be broiled to thousands of degrees. The surface of a planet within one parsec of the Milky Way’s center could reach over 5,700 kelvin, as hot as the sun’s surface, just from dark matter traffic. (Unlike stars, while the surfaces of these planets would get hot, their cores would not reach the high temperatures needed to start nuclear fusion.)

Leane and Smirnov propose two experiments to prove their theory: local and distant. The local test would detect dark matter by using infrared telescopes to read the surface temperatures of many gas giants in our galactic vicinity, then comparing results to heat flow models. (Astronomers have discovered hundreds of such giants, and they expect the Gaia telescope to catalog tens of thousands in the next decade.)

The distant test would use surface temperatures from brown dwarfs and rogue planets, which float freely outside of a solar system—unobscured by neighboring bright stars—to hunt for progressive warming. Finding unexpectedly high temperatures with an infrared telescope like JWST would be a huge win for our understanding of nature, and finding a warming trend would map the distribution of dark matter in our galactic backyard.

Leane and Smirnov calculate that their focus on big planets would detect more lightweight matter than any other existing method. Planets with relatively cold cores (compared to stars) should be better at trapping dark matter, because a hot core could give dark matter enough thermal energy to escape. This makes detecting lighter blobs of dark matter easier too—lighter particles flee more easily.

“This opens up a brilliant new window onto certain classes of dark matter which are otherwise quite difficult to detect,” says Bramante. “It pushes beyond prior limitations.”

Before any groundbreaking analyses take place, though, they need to see the planets. NASA’s James Webb infrared scope is expected to start thermometer duty later this year. Leane and Smirnov hope to choose candidates from the growing exoplanet catalog and use the telescope to prove their hypothesis. In their report, they estimate that it will be sensitive enough to see planets warmer than 650 kelvin, reaching depths just 100 parsecs from the Milky Way’s center.

But not everyone is sure that this instrument can resolve Leane’s dark matter hypothesis. “It's not super doable,” says Beth Biller, an astronomer at the University of Edinburgh who specializes in exoplanet searches and was not involved with the study. Biller is leading one of the first JWST exoplanet observation programs, and she points out that analyzing planets is particularly hard when they’re cold, dim, and close to stars. JWST will use devices called coronagraphs to mask out neighboring starlight. But many of the exoplanets Leane wants to study are too close to their star to work with JWST’s tightest coronagraphs, Biller says.

Leane concurs with Biller’s caution. “I totally agree; it won't work for all exoplanets,” she says. “You just have to choose the right candidate.” She adds that exoplanet discovery is rapidly increasing: “You need to find like 1,000 good candidates, and this is definitely within the scope of what we should be able to do within the next five to 10 years.”

Scanning the sky with JWST long enough to get reliable data would also be a hard sell to the panel of scientists who are allocating telescope time: One temperature read would take about 24 hours of continuous scanning. Plus, Biller adds, a scan designed solely for this dark matter research would have to compete for time with the search for habitable planets. “I think the panel would look at it and say, ‘Wow, that's a lot of time,’” she predicts. But for Jupiter-like exoplanets closer to home, Biller expects it’ll be possible down the road to use temperature data in the works from other telescopes. “That aligns with the goals of the exoplanet community anyway,” she says. “And if they are much, much hotter than expected, that will be very notable.”

Leane says she has been working with exoplanet scientists to explore next steps. She expects that JWST data from other searches will be sufficient for her analyses, without having to apply for any solo telescope time. “There are going to be a lot of surveys that just look into the Milky Way center for different reasons,” she says, adding that many scans will already be quite long. “We can potentially piggyback off other searches.” She hopes to have the data she needs within about five years of the telescope’s launch.

If a warming trend appears in the data, it’ll be difficult to find an explanation that doesn’t include dark matter, Leane says. But if the theory doesn’t hold? That’s fine too, she says. “We might really learn something new about the universe. We also might not. But you never know until you look.”

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