Closest Black Hole to Earth, 'The Unicorn,' Lies 1,500 Light-Years Away

Researchers say they have discovered what may be the closest known black hole to Earth, which they have dubbed "the Unicorn."

The black hole is about three times the mass of our sun, meaning it is also among the smallest of these celestial objects on record.

According to findings published in the journal Monthly Notices of the Royal Astronomical Society, the black hole is located around 1,500 light-years away from Earth, within the Milky Way galaxy.

The black hole is relatively tiny, given that some of these objects are on the order of billions of solar masses. In fact, scientists have only identified a handful of black holes in the universe that are in a similar size range to the Unicorn.

The authors of the study chose this name because of the black hole's unique characteristics, as well as the fact that it is located in the constellation Monoceros, which is Greek for "unicorn."

Trying to find black holes is a difficult task because they are essentially invisible to the tools astronomers use to measure light and other wavelengths.

Most black holes that have been discovered to date were identified due to their interactions with a nearby companion star. As these black holes strip material off of their companion, the interactions create large quantities of X-rays that astronomers can detect.

But scientists believe there are many more black holes in the universe that have yet to be identified, in particular, those that are not interacting with a star and exhibit very little X-ray activity.

"From theoretical models, we know that there are thousands of these non-interacting black holes in the galaxy," lead author of the study, Tharindu Jayasinghe—a doctoral student in astronomy at The Ohio State University—said in a statement. "Scientists from all over the world are very interested in finding these non-interactive black holes."

In the latest study, Jayasinghe and colleagues attempted to find a black hole using a different method, focusing their search on an unusual object—a red giant star in the Monoceros constellation.

This star had already been well documented by astronomers, but the authors of the latest study decided to analyze the available data on the object in a different way.

"When we looked at the data, this black hole—the Unicorn—just popped out," Jayasinghe said.

The researchers realized that the gravity of a nearby object was distorting the shape of the red giant in a phenomenon that astronomers call "tidal disruption."

One potential explanation for what the scientists were observing was the presence of a black hole. But such a black hole would have to be very small—less than five times the mass of our sun. Until recently, astronomers had not even considered it a possibility that black holes like these could exist.

"When you look in a different way, which is what we're doing, you find different things," Kris Stanek, a co-author on the study and astronomy professor at Ohio State, said in a statement.

"Tharindu looked at this thing that so many other people had looked at and instead of dismissing the possibility that it could be a black hole, he said, 'Well, what if it could be a black hole?'"

Eventually, the researchers came to the conclusion a small black hole was likely present near the red giant star, based on the available evidence.

"Just as the moon's gravity distorts the Earth's oceans, causing the seas to bulge toward and away from the moon, producing high tides, so does the black hole distort the star into a football-like shape with one axis longer than the other," Todd Thompson, another co-author of the study, from Ohio State, said in a statement.

"The simplest explanation is that it's a black hole—and in this case, the simplest explanation is the most likely one."

However, the authors note that other explanations may also be plausible, including a pair of neutron stars, or a white dwarf-neutron star binary.

Neutron stars are incredibly dense, compact objects that are left behind as remnants when massive stars explode as supernovae. White dwarfs, on the other hand, are the small, dense remnants of stars nearing the end of their lifecycles.

In 2020, another team of scientists described a black hole located around 1,000 light-years from Earth. But since the findings were reported, other researchers have since questioned the existence of a black hole in this location.

A black hole
Stock image showing an artist's illustration of a black hole. Scientists say they have discovered the closest known black hole to Earth. iStock