“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.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.
Dark matter can get stuck in planets’ gravities, as if in quicksand.
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.“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.”.Other researchers have examined how dark matter might flow heat into neutron stars, planets, and the moon.
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.
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.
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.
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.
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.
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.
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.
Plus, Biller adds, a scan designed solely for this dark matter research would have to compete for time with the search for habitable planets?If a warming trend appears in the data, it’ll be difficult to find an explanation that doesn’t include dark matter, Leane says.
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