Scientists may have discovered a whole new class of black holes. These may be far tinier than what scientists have found before.

New research shows scientists might have been missing an entire class of black holes that they didn’t know existed.

In a study published today in the journal Science, astronomers offer a new way to search for black holes. They suggest there’s a class of black holes smaller than the smallest known black holes in the universe.

“We’re showing this hint that there is another population out there that we have yet to really probe in the search for black holes,” said Todd Thompson, a professor of astronomy at The Ohio State University and lead author of the study.

Black holes are the corpses of what used to be massive stars that explode and ultimately collapse in on themselves. But not all stars turn into black holes. When some stars die and collapse, they form neutron stars, which are small and incredibly dense.

The search for black holes and neutron stars helps researchers piece together how the universe works and how stars live and die.

But if astronomers only focus on supermassive black holes and neutron stars they’re missing whole demographics of the population, the researchers said.

“People are trying to understand supernova explosions, how supermassive black stars explode, how the elements were formed in supermassive stars. So if we could reveal a new population of black holes, it would tell us more about which stars explode, which don’t, which form black holes, which form neutron stars. It opens up a new area of study.”

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Astronomers typically search for black holes in our own galaxy by measuring X-rays emitted when black holes gravitationally attract material from nearby stars. In distant galaxies, on the other hand, researchers look for gravitational waves produced by the merging of two black holes or from a collision of neutron stars.

For years, the black holes astronomers knew about were all between about five and 15 times the mass of the sun. As for neutron stars, they are generally no bigger than about 2.1 times the mass of the sun—if they were above 2.5 times the sun’s mass, they would collapse into a black hole.

But in 2017 things changed. The Laser Interferometer Gravitational-Wave Observatory (LIGO) spotted two black holes merging into one in a galaxy 1.8 million light-years away. The two giant black holes were 31 times the mass of the sun and 25 times its mass, respectively.

“Immediately, everyone was like ‘wow,’ because it was such a spectacular thing,” Thompson said. “Not only because it proved that LIGO worked, but because the masses were huge. Black holes that size are a big deal—we hadn’t seen them before.”

LIGO’s discovery proved that black holes could be larger.

Finding The Tiny Black Hole

But what about those that were smaller, existing between the boundary of neutron stars and black holes?

Thompson and his colleagues began analyzing data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), which collected light from 100,000 stars in the Milky Way.

Then they spotted something strange: a giant red star that seemed to be orbiting something. Shifts in the wavelengths of the star could show that the star was orbiting an unseen companion. Based on their calculations, it had to be smaller than any observable black holes found in the Milky Way. But way bigger than most known neutron stars.

After more calculations and additional data from the Tillinghast Reflector Echelle Spectrograph and the Gaia satellite, they realized they had found a low-mass black hole, likely about 3.3 times the mass of the sun.

“What we’ve done here is come up with a new way to search for black holes, but we’ve also potentially identified one of the first of a new class of low-mass black holes that astronomers hadn’t previously known about,” Thompson said. “The masses of things tell us about their formation and evolution, and they tell us about their nature.”

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