Yes, there really is a black hole on the loose in Sagittarius

For the first time, astronomers have confirmed the existence of a lone black hole — one with no star orbiting it.

It’s “the only one so far,” says Kailash Sahu, an astronomer at the Space Telescope Science Institute in Baltimore.

In 2022, Sahu and his colleagues discovered the dark object coursing through the constellation Sagittarius. A second team disputed the claim, saying the body might instead be a neutron star. New observations from the Hubble Space Telescope now confirm that the object’s mass is so large that it must be a black hole, Sahu’s team reports in the April 20 Astrophysical Journal.

The discovery made headlines three years ago because all previously known stellar-mass black holes have companion stars that betray their presence. In each case, the visible star races around an invisible object with more than three times the sun’s mass, indicating a black hole rather than a neutron star.

While solitary black holes should be common, they are hard to find. The one in Sagittarius revealed itself when it passed in front of a dim background star, magnifying the star’s light and slowly shifting its position due to the black hole’s gravity.

This passage occurred in July 2011, but the star’s position is still changing. “It takes a long time to do the observations,” Sahu says. “Everything is improved if you have a longer baseline and more observations.”

The original discovery relied on precise Hubble measurements of star positions from 2011 to 2017. The new work incorporates Hubble observations from 2021 and 2022 as well as data from the Gaia spacecraft.

The upshot: The black hole is about seven times as massive as the sun, give or take 0.8 solar masses. “The uncertainty is about half of what we had before,” Sahu says. Moreover, the second research team revised its assessment in 2023 and agreed that the object is a black hole, finding a mass six times that of the sun but with a larger uncertainty, so the result is consistent with Sahu’s.

Located 5,000 light-years from Earth, this black hole is much closer than the supermassive one at the Milky Way’s center, which also lies in Sagittarius but about 27,000 light-years from us. The star-rich region around the galactic center provides an ideal hunting ground for solitary black holes passing in front of stars.

Sahu hopes to find additional lone black holes by using the Nancy Grace Roman Space Telescope, slated for launch in 2027.