Meet the latest dark matter detector: Jupiter’s Night Side

The matter that makes up animals, planets, and stars is only a small fraction of the matter that scientists believe exists. An estimated five-sixths of all the matter in the universe is made up of an invisible substance called dark matter. We don’t know what it is because it doesn’t interact with light, only with gravity and the weak nuclear force.

This force is responsible for nuclear decay and, in the case of dark matter, it could lead to the annihilation of the substance. This process should release ionizing radiation: light that can strip electrons from their molecules – and that’s where Jupiter comes in as a dark matter detector.

One of the most common ions in the universe is the trihydrogen cation (H₃⁺). This is a molecule made up of three hydrogen atoms that have lost an electron. Now imagine that you have a large reservoir of hydrogen, massive enough to interact with the elusive dark matter: you could theoretically measure the amount of trihydrogen cations and determine the properties of the dark matter.

“We emphasize that dark matter (DM) can produce an additional source of H₃⁺ in planetary atmospheres,” the study authors wrote in a paper on the subject. “This will occur if DM disperses and is captured by planets, and therefore annihilates, producing ionizing radiation.”

Scientists Carlos Blanco, of Princeton University and Stockholm University, and Rebecca Leane, of the Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory and Stanford University, studied six hours of data on Jupiter, three hours either side of the planet’s night. The data was collected by the Cassini mission during its passage over the night side of the planet in 2000.

Jupiter contains a lot of hydrogen. It is a very massive object, the second heaviest in the solar system. Looking to the night side, they looked at the hidden part of the Sun. Sunlight can create these intriguing ions, so this approach reduces this contribution. The team found a signal – there is a certain amount of H₃⁺ in the atmosphere of the far side of Jupiter.

Now, this doesn’t immediately mean that the signal is entirely caused by dark matter – it could be dark matter or other sources that could be causing it. But they can limit certain properties of the substance.

The researchers think future observations could do even better. The European Space Agency’s JUICE mission will be able to collect more sensitive measurements when it arrives at Jupiter in the 2030s. It might also be possible to see the signal from more massive planets, closer to the center of the Milky Way, where more dark matter is found.

An article discussing the result is published in the journal Physical Review Letters.