American scientists have conducted the most stringent test to date of the hypothesis that ultraviolet glow in the atmospheres of giant planets may arise partly from indirect interactions between dark matter and ordinary matter.
Ultraviolet Airglow in the Atmospheres of Giant Planets
The findings of a research team led by Carlos Blanco of Princeton University establish some of the strictest limits to date on the strength of interactions between dark matter and ordinary matter. The study also supports the hypothesis that the giant planets of our Solar System may be used as natural dark matter detectors. This was reported by phys.org.
When viewed through Earth’s atmosphere, the night sky is never completely dark. After molecules in the atmosphere are ionized by solar radiation during the day, they may recombine and emit photons—a process that continues throughout the night.
This phenomenon, known as airglow, was first explained more than 2,000 years ago. In 2024, however, Blanco and his colleague Rebecca Leane of the SLAC National Accelerator Laboratory proposed that another type of atmospheric glow could arise when hypothetical dark matter particles annihilate inside giant planets.
The scientists suggested that, as the Solar System orbits the Galaxy, giant planets such as Jupiter may capture significant numbers of dark matter particles. When these particles annihilate, the released energy may ionize nearby hydrogen molecules. The resulting ions react rapidly, forming triatomic hydrogen, or H₃⁺. As it transitions to lower-energy states, it emits infrared light.
By comparing predictions of this infrared emission with observations of Jupiter’s upper atmosphere, Blanco and Leane established some of the first reliable constraints on whether dark matter interactions could produce a detectable atmospheric signal. However, this infrared signal depends on H₃⁺, making it much more specific to particular planetary atmospheres.
“The natural next question was whether there is a signal that works for all giant planets,” Blanco says. “The answer turned out to be ultraviolet airglow—a phenomenon people have contemplated since the time of Aristotle.”
Searching for Mysterious Radiation Produced by Dark Matter
In their latest study, Blanco, Leane, and their colleagues examined another possible consequence of dark matter annihilation. Instead of searching for infrared emission from H₃⁺, they investigated whether energetic electrons produced during ionization could directly excite molecular hydrogen and cause it to emit ultraviolet light.
“Along with ionizing photons, ionizing electrons can also make molecular hydrogen glow in the ultraviolet—a signal we can search for on every giant planet at the same time,” Blanco explains.
Because any glow produced by dark matter would probably be overwhelmed by sunlight, the team focused on observations of the planets’ night sides. At the extremely low light levels predicted by the theory, suitable measurements are available only from spacecraft flybys. To date, such data have been provided by Voyager 1, Voyager 2, and New Horizons during their encounters with Jupiter, Saturn, Uranus, and Neptune.
Because the atmospheres of giant planets already produce faint ultraviolet nightglow through natural processes, the researchers searched for any additional emission that could be attributed to dark matter. By requiring that the predicted dark matter signal not exceed the ultraviolet light actually observed by the spacecraft, they established new constraints on the theory, obtaining some of the strictest limits to date on interactions between dark matter and ordinary matter.
Planets as Dark Matter Detectors
The team’s results strengthen the case for using the Solar System’s four giant planets as natural dark matter detectors. In particular, they could probe regions of dark matter parameter space that are inaccessible to underground experiments on Earth, including very light particles and strongly interacting dark matter that would be stopped before reaching terrestrial detectors.
“Dark matter sensitivity peaks near the proton mass, which transfers energy most efficiently to the hydrogen that makes up these planets,” Blanco explains. “Because the four giant planets differ in size, temperature, and composition, each of them probes different dark matter masses and models.”
At the same time, the predicted sensitivity raises new questions about the behavior of dark matter after it is captured by a planet. In particular, planetary heat may allow the lightest dark matter particles to escape before they produce an observable atmospheric signal, reducing the sensitivity of this method.
The researchers hope that many of these questions can be addressed by future missions, including ESA’s JUICE spacecraft, which is expected to enter orbit around Jupiter in 2031.
