NASA’s Hubble Space Telescope has, for the first time, identified what might be an isolated black hole roaming through the Milky Way galaxy, zipping around like it was shot from a cannon. Astronomers estimate it lies about 5,000 light years away and is traveling between 67,000 and 100,000 miles per hour, writes CNN’s Ashley Strickland.
“Our discovery of a black hole is consistent with the theoretical calculations which suggest that there should be about 100 million black holes in our Galaxy,” Kailash Sahu, an astronomer at the Space Telescope Science Institute, tells Newsweek’s Ed Browne. “Then, assuming black holes follow [a] similar distribution as stars, one expects, statistically, that the nearest black hole may be about 80 light years away.”
But the mysterious object may not be a black hole at all. Two teams—one from the University of California, Berkeley and another from the Space Telescope Science Institute in Baltimore—analyzed the Hubble data and both found a compact object. But the Baltimore team estimated the object’s mass is seven solar masses (one solar mass is the weight of our sun), which means it’s a black hole, per a NASA statement. The Berkeley team’s results, between 1.6 and 4.4 solar masses, were more ambiguous, perhaps pointing to the discovery of a neutron star instead.
“As much as we would like to say it is definitively a black hole, we must report all allowed solutions,” Jessica Lu of the Berkeley team says in the statement. “This includes both lower mass black holes and possibly even a neutron star.”
Roaming black holes form when huge stars—ones at least 20 times larger than the sun—explode. Gravity crushes the star’s core and it collapses in on itself, forming a gravitational field so strong that not even light can escape. But, in the case of a roaming star, the implosion isn’t symmetrical, so the black hole zooms through the galaxy “like a blasted cannonball,” per NASA. Black holes and neutron stars both form from the explosion and core collapse of stars, just of different size stars—black holes from more massive ones, neutron stars from smaller ones.
Black holes are invisible, so the discovery is rare, writes Elena Kadvany for the San Francisco Chronicle. The teams used a phenomenon called gravitational microlensing to estimate the mass of the object. As an object passes through space in front of a star, gravity causes the light to warp. Depending on how the light is altered, scientists can estimate the object’s size. The first image of a black hole was produced only in 2019, using a different technique called interferometry, which requires snapshots from a series of telescopes positioned all over the planet.
Regardless of whether the find is a black hole or a star, it’s still notable, per a Berkeley statement.
“This is the first free-floating black hole or neutron star discovered with gravitational microlensing,” Lu says in the statement. “With microlensing, we’re able to probe these lonely, compact objects and weigh them. I think we have opened a new window onto these dark objects, which can’t be seen any other way.”