Using exoplanets to study dark matter

RIVERSIDE, Calif. -- More than 5,000 planets have been discovered beyond our solar system, allowing scientists to explore planetary evolution and consider the possibility of extraterrestrial life. Now, a UC Riverside study published in Physical Review D suggests that these "exoplanets" could also serve as tools to investigate dark matter.

The researchers examined how dark matter, which makes up 85% of the universe’s matter, might affect Jupiter-sized exoplanets over long periods of time. Their theoretical calculations suggest dark matter particles could gradually collect in the cores of these planets. Although dark matter has never been detected in laboratories, physicists are confident it exists.

“If the dark matter particles are heavy enough and don’t annihilate, they may eventually collapse into a tiny black hole,” said paper first author Mehrdad Phoroutan-Mehr, a graduate student in the Department of Physics and Astronomy who works with Hai-Bo Yu, a professor of physics and astronomy. “This black hole could then grow and consume the entire planet, turning it into a black hole with the same mass as the original planet. This outcome is only possible under the superheavy non-annihilating dark matter model.”

According to the superheavy non-annihilating dark matter model, dark matter particles are extremely massive and do not destroy each other when they interact. The researchers focused on this model to show how superheavy dark matter particles are captured by exoplanets, lose energy, and drift toward their cores. There, they accumulate and collapse into black holes.

“In gaseous exoplanets of various sizes, temperatures, and densities, black holes could form on observable timescales, potentially even generating multiple black holes in a single exoplanet’s lifetime,” Phoroutan-Mehr said. “These results show how exoplanet surveys could be used to hunt for superheavy dark matter particles, especially in regions hypothesized to be rich in dark matter like our Milky Way’s galactic center.”

Phoroutan-Mehr was joined in the study by Tara Fetherolf, a postdoctoral researcher in the Department of Earth and Planetary Sciences.

Phoroutan-Mehr explained that, so far, astronomers have only detected black holes with masses greater than our sun. He said most existing theories suggest that black holes must be at least that massive. 

“Discovering a black hole with the mass of a planet would be a major breakthrough,” he added. “It would support the thesis of our paper and offer an alternative to the commonly accepted theory that planet-sized black holes could only form in the early universe.”

According to Phoroutan-Mehr, exoplanets have not been used much in dark matter research largely because scientists did not have enough data about them. 

“But in recent years, our knowledge of exoplanets has expanded dramatically, and several upcoming space missions will provide even more detailed observations,” he said. “With this growing body of data, exoplanets can be used to test and challenge different dark matter models.”

Phoroutan-Mehr said in the past scientists investigated dark matter by observing objects like the sun, neutron stars, and white dwarfs, since different models of dark matter can affect these objects in different ways. For example, some models suggest that dark matter can heat up neutron stars. 

“So, if we were to observe an old and cold neutron star, it could rule out certain properties of dark matter, since dark matter is theoretically expected to heat them up,” he said.

He added that many exoplanets (and Jupiter in our solar system) not having collapsed into black holes can help scientists rule out or refine dark matter models such as the superheavy non-annihilating dark matter model.

“If astronomers were to discover a population of planet-sized black holes, it could offer strong evidence in favor of the superheavy non-annihilating dark matter model,” Phoroutan-Mehr said. “As we continue to collect more data and examine individual planets in more detail, exoplanets may offer crucial insights into the nature of dark matter.”

Phoroutan-Mehr noted that another possible effect of dark matter on exoplanets — and possibly on planets in our solar system — is that it could heat them or cause them to emit high-energy radiation. 

“Today’s instruments aren’t sensitive enough to detect these signals,” he said. “Future telescopes and space missions may be able to pick them up.”

The title of the paper is “Probing Superheavy Dark Matter With Exoplanets.”

The University of California, Riverside is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment is more than 26,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual impact of more than $2.7 billion on the U.S. economy. To learn more, visit www.ucr.edu.