Dark matter is a mysterious substance that makes up some 85 percent of the total matter in our universe. Although we cannot see it, we can see its gravitational fingerprints on the way galaxies move and the way massive objects bend light around themselves. And now, researchers may have found a way to turn giant exoplanets into sensitive detectors for dark matter particles, say researchers — and a new study of our own planet Jupiter shows how the method would work.
The paper was published June 27 in Physical Review Letters.
Because dark matter permeates space, the researchers say that the massive gravity of gas giant planets should draw dark matter particles into an invisible cloud surrounding and permeating them. There, collisions between dark matter particles in their upper atmospheres could produce light, as well as an increase in the amount of trihydrogen (H3+) there.
H3+ can be created in a planet’s atmosphere many ways, including through interactions with cosmic rays, solar wind, and lightning. In this case, though, the researchers examined observations from the Cassini spacecraft of Jupiter’s nightside, made as the spacecraft came within 6.2 million miles (10 million kilometers) of the planet in 2000 and 2001, while on its way to the Saturn system. They reasoned that looking at the nightside, particularly latitudes and times when the jovian aurora was not likely to be active, could reveal signs of dark matter annihilation around the giant planet.
They found no detection of such a glow in Jupiter’s upper atmosphere — however, like many “null results” in science, this isn’t bad news. Instead, the researchers used their lack of a detection to calculate a maximum size for dark matter particles.
Putting a size limit on dark matter
A leading theory of dark matter is that it made up of so-called weakly interacting massive particles, or WIMPS. Just what the particles could be is heavily disputed. The WIMP model states that dark matter particles have some mass — as opposed to particles with zero mass, like photons — and must be about the size of a proton.
These dark matter particles must be flowing through all planets of the solar system all the time, but usually without interacting.
According to University of Chicago particle astrophysicist Dan Hooper, who wasn’t involved in the study, a back-of-the-envelope calculation suggests that up to 2.2 pounds (1 kilogram) of dark matter is passing through Jupiter every second.
The planet’s intense gravity means some of those particles must have formed an invisible cloud within and around Jupiter that has built up over billions of years; and collisions between those invisible particles could create radiation. And in theory, that radiation would create an excess of H3+ that could be detected as an infrared glow in Jupiter’s upper atmosphere, Hooper says.
That glow is what the authors of the latest study had hoped to detect.
But “when Cassini pointed it instruments towards the night side of Jupiter, it saw exactly nothing,” says dark matter physicist Carlos Blanco of Princeton University, the study’s co-author.
Such a lack of detection — known as a “null result” in science — is akin to Sherlock Holmes solving a mystery because a dog didn’t bark.
Blanco and his co-author, astroparticle physicist Rebecca Leane at Stanford University, calculate that dark matter particles must have cross section less than 10-38 cm2. In physics, the cross section of a particle is related to how likely it is that the particle will interact with others. And if dark matter particles were any larger, they would interact more often and create a detectable signal.
That cross section is smaller than the dark matter dection experiments here on Earth can spot, and the researchers say it shows the value of their approach.
“The kind of detectors that you can put in a laboratory on Earth are really only sensitive to dark matter particles that are heavier than hydrogen, and they start losing sensitivity very quickly if the dark matter is lighter,” Blanco says. “What we discovered is that, in general, looking at signatures that dark matter might leave on molecules like H3+ is a good way of breaching that threshold and going to lighter and lighter particles.”
One step closer
The null result at Jupiter is only the beginning.
The authors suggest that the infrared glow created by collisions between dark matter particles could be easier to detect in the atmospheres of exoplanets with more mass than Jupiter, particularly in regions with more dark matter — near the center of our galaxy, for example.
“What you really want is something big, like a ‘super-Jupiter’ [a planet more massive than Jupiter], that’s closer to the galactic center where there’s a higher concentration of dark matter,” he says. “That way you would expect a higher signal.”
Currently, however, it isn’t possible to detect such glows from exoplanets, but Blanco hopes such technology will soon be developed. But he cautions that astronomers would have to improve the sensitivity of the relevant instruments by two or three times to be effective.
“That’s a long way off,” says cosmologist Joseph Silk of Johns Hopkins University, who wasn’t involved in the study.
Still, it’s one more possible way that atstronomers may someday see the unseeable and uncover the true nature of the mysterious dark matter that pervades our universe.
Editor’s note: This story has been updated to reflect Blanco’s status as study co-author, rather than lead author.
