Astronomers Spot First Lone, Free-Wheeling Black Hole in the Milky Way Galaxy

The massive void, zipping at 28 miles per second, may have been blasted into space by a supernova explosion

An image of an isolated black hole lensing stars in space
Out of four known types of black holes, stellar-mass black holes are the most common and have masses less than 100 times the Sun. NASA via Wikimedia Commons under Public Domain

Black holes are so massive that nothing, not even light, can escape their gravitational pull. On their own, these lightless voids are virtually invisible in the vast darkness of outer space, so they are easier to find when interacting with other objects, like neutron stars or another black holes.

Now, for the first time, scientists may have identified a lone, freewheeling black hole in the Milky Way, according to a study released last month on the preprint server arXiv. The study, which has been submitted to the Astrophyical Journal, has not yet undergone peer review.

Located about 5,000 light-years away, the rogue flyer is roughly seven times the mass of the sun, which means it is a stellar-mass black hole, reports Scientific American's Jonathan O'Callaghan. The discovery may help researchers understand how isolated black holes form and estimate their abundance among the galaxies, Elizabeth Gibney reports for Nature.

Stellar-mass black holes form when a single star over 20 solar masses reaches the end of its life and collapses in a stunning supernova. Estimates show that there could be as many as 10 million to one billion stellar-mass black holes drifting in the Milky Way alone, reports Michelle Starr for Science Alert.  "There should be 100 million such black holes in the galaxy, they should be everywhere, but it's very hard to find them," says Kailash Sahu, the study's lead author, and astronomer at the Space Telescope Science Institute, to Nature. 

Astronomers caught the solo black hole using gravitational microlensing, which occurs when an object with a gravitational field passes almost in front of a distant star, bending and brightening the light like a magnifying glass. Big black holes with large gravitational fields will brighten the light for longer periods of time.

On two separate microlensing surveys, the astronomers recorded an event dubbed MOA-2011-BLG-191/OGLE-2011-BLG-0462, where a star began glowing brighter than usual for 270 days, Charles Choi reports for Space.com. Using equations derived by Albert Einstein in 1915, the team estimates the force manipulating this light is about seven times more massive than the sun, reports Nature.

"It took two years of planning followed by six years of observing with Hubble, but it was very satisfying to see the incredible results," Sahu tells Space.com. "It was immediately clear as daylight that it's a black hole. There was nothing else that could cause the deflections we measured."

The team calculated that the black hole is traveling 28 miles per second, or roughly 100,000 miles per hour. The reason for its speed may have been a "natal kick," meaning after its star underwent supernova, the explosion may have been lopsided and blasted the black hole hurling through space, Nature reports.

For future studies, researchers are planning on using X-rays to see if the stellar-mass black hole is accreting any material, per Science Alert.