Stars like our Sun are powered by nuclear fusion, but a bizarre type of stars called “dark dwarfs” could actually be powered by elusive dark matter.
It’s a bold new idea in astrophysics, and one that would make the universe even more intriguing than it is now. But there’s only one problem: we haven’t yet seen a dark dwarf.
Our universe is made of what?
Grossly speaking, the universe is primarily composed of three things: ordinary matter (everything we see around us, planets, stars), dark matter, and dark energy. Ordinary matter is just 5% of everything in the universe, and the rest is… well we don’t really know what it is.
“We think that 25% of the universe is composed of a type of matter that doesn’t emit light, making it invisible to our eyes and telescopes. We only detect it through its gravitational effects. That’s why we call it dark matter,” explains Jeremy Sakstein, Professor of Physics at the University of Hawai‘i and one of the study’s authors.
Despite decades of searching, we haven’t uncovered much about exactly what dark matter is. Now, Sakstein and colleagues have a daring hypothesis: we may find dark matter hiding in the hearts of brown dwarfs.
Here’s the idea in simple terms: a brown dwarf forms like a star, but never gets hot enough in its core to sustain hydrogen fusion. Over time, it cools down. So far, nothing new.
But in regions where dark matter is densely packed (like the center of our galaxy, for instance), these particles might get captured by the dwarf’s gravity. When enough dark matter accumulates, the particles collide and annihilate, releasing energy. That heat keeps the dwarf from cooling completely. Eventually, it reaches a stable state where it’s powered solely by dark matter.
The researchers call these objects “dark dwarfs.” They’re different from stars, yet not quite dead. They’re something in between. And once formed, they could last longer than the age of the universe.
“These objects collect the dark matter that helps them become a dark dwarf. The more dark matter you have around, the more you can capture,” Sakstein explains. “And, the more dark matter ends up inside the star, the more energy will be produced through its annihilation.”
But how do you find one?
It’s a beautiful idea, but as Richard Feynman once said it doesn’t matter how beautiful your idea is; it needs to live up to experiment. We’ve not seen any such object before, how would we even find one?
Oddly enough, the answer is lithium.
In normal stars, lithium doesn’t last. It burns up in the intense heat. But in dark dwarfs, the internal temperatures would be cooler, even if the object is massive enough that, by standard physics, it should have destroyed its lithium. So, if astronomers find a heavy, old object near the galactic center that still has its original lithium, that could be a telltale sign.
Tools like the James Webb Space Telescope might already be able to detect this signature. But, according to Sakstein, there’s another possibility: “The other thing you could do is to look at a whole population of objects and ask, in a statistical manner, if it is better described by having a sub-population of dark dwarfs or not.”
Finding dark dwarfs won’t be easy. But if they do exist, they’re worth the effort. This isn’t just about finding a new type of star, but about finding a clue to one of the biggest puzzles in our universe. A dark dwarf could serve as a cosmic laboratory where you could test different hypotheses about the nature and origin of dark matter. They could illuminate one of the darkest mysteries in all of physics.
The research is available on preprint server arXiv and will be published in the Journal of Cosmology and Astroparticle Physics.
