These are boom times for astronomers hunting black holes. The biggest ones—supermassive black holes that can weigh billions of suns—have been found at the centers of most every galaxy, and we have even managed to image one. Meanwhile, researchers now routinely detect gravitational waves rippling through the universe from smaller merging black holes. Closer to home, we have witnessed the dramatic celestial fireworks produced when the Milky Way’s own supermassive black hole and its more diminutive cousins feed on gas clouds or even entire stars. Never before, though, have we seen a long-predicted phenomenon: an isolated black hole drifting aimlessly through space, born and flung out from the collapsing core of a massive star.
Until now.
Scientists have announced the first-ever unambiguous discovery of a free-floating black hole, a rogue wanderer in the void some 5,000 light-years from Earth. The result, which appeared January 31 on the arXiv preprint server but has not yet been peer-reviewed, represents the culmination of more than a decade of ardent searching. “It’s super exciting,” says Marina Rejkuba from the European Southern Observatory in Germany, a co-author on the paper. “We can actually prove that isolated black holes are there.” This discovery may be just the start; ongoing surveys and upcoming missions are expected to find dozens or even hundreds more of the dark, lonely travelers. “It’s the tip of the iceberg,” says Kareem El-Badry from the Harvard-Smithsonian Center for Astrophysics, who was not involved in the paper.
In 1919, the British astronomer Arthur Stanley Eddington performed a famous experiment. Einstein’s theories of special and general relativity had postulated that massive objects should cause a dent in spacetime, bending nearby rays of light in a process known as gravitational lensing. Eddington proved this to be true during a total solar eclipse, when the sun’s glare was minimized so that background stars adjacent to it in the heavens could be seen. Using a technique known as astrometry, he carefully noted these stars’ positions before and during the eclipse, revealing a subtle change in their apparent locations in the sky due to their light being warped by our star’s considerable gravitational pull. “The apparent position of the stars had a tiny shift,” says Feryal Özel from the University of Arizona, who was also not involved in the paper.
In the subsequent decades, scientists realized a novel use for this technique. Stars greater than about 20 times the mass of our sun should form black holes at the end of their lives, when their heavy cores collapse under their own weight following the exhaustion of their thermonuclear fuel. The birth of such a stellar-mass black hole—a city-sized sphere containing up to dozens of times our sun’s mass—is often accompanied by a bright supernova from the enormous energies released by the core collapse. These forces can be so great they sometimes kick the newborn black hole right out of its womb on an endless interstellar cruise. That cosmic wanderlust—plus the black holes’ small sizes and inherent darkness—should make them almost impossible to see. Eddington’s work, however, suggested these outcasts could be found by observing their lensing effects—typically a telltale transient brightening of any background stars the black holes flit across within our field of view. The odds of seeing such an event for an isolated black hole were slim, but given that millions of stellar-mass black holes are predicted to be drifting through our galaxy, some might turn up in sufficiently broad and deep surveys of the sky.
Several projects now search for these and other so-called microlensing events, including the Optical Gravitational Lensing Experiment (OGLE), run by the University of Warsaw in Poland, and the Microlensing Observations in Astrophysics (MOA) survey run by researchers in New Zealand and Japan. In June 2011, these two surveys spotted something of note: A suddenly brightening star 20,000 light-years away towards the densely packed galactic bulge in the center of the Milky Way. Could this have been a microlensing event from a rogue black hole? Astronomers raced to find out.
Among them was Kailash Sahu from the Space Telescope Science Institute in Baltimore, the lead author on the arXiv preprint detailing the object’s discovery. Using the Hubble Space Telescope, he and his colleagues zoomed in on the star within weeks of its brightening, then returned to it again and again over the next six years. They were able to confirm the star’s light had been magnified, pointing to the presence of an unseen lensing object, but they found something even more important. The star’s apparent position in space had shifted by a minuscule amount. The effect was “1,000 times smaller than what Eddington measured,” says Sahu, and was near the limits of Hubble’s capabilities. Something hidden had amplified and warped the light from the star. The best candidate? An invisible stellar-mass black hole, 7.1 times the mass of our sun.
“There was no possibility other than a black hole,” Sahu says. Two things were needed to confirm that to be the case. “The first criterion was there should be no light coming from the lens,” Sahu says, to rule out more prosaic objects such as a failed star known as a brown dwarf. The second was that the magnification effect should have a long duration, given the expansive size of a black hole’s gravitational sphere of influence. Lasting for about 300 days, the June 2011 event fit the bill. “It’s a pretty thorough and careful analysis,” El-Badry says. “They’ve done their due diligence.”
The amount of lensing and deflection of light from the star then allowed Sahu and his collaborators to peg the suspected black hole’s mass at just over seven solar masses. That places it “smack in the middle” of what we would expect for stellar-mass black holes, Özel says. The team was also able to calculate its speed. “It’s moving at about 45 kilometers per second,” Sahu says. This is relatively fast compared to nearby stars—the exact sort of thing one would expect if the black hole had been given an ejecting kick from a dying massive star. It is not clear when that event would have happened, but it “may be somewhere close to 100 million [years ago],” Sahu says. “We can’t really tell because we don’t know where exactly it came from.”
This is not, however, the first observational hint of microlensing from rogue stellar-mass black holes; several other candidates predate this one. What is different now is the successful measurement of the lensing object’s gravitational deflection of the star’s light, rather than its mere amplification, allowing the lensing object’s mass—and thus its true nature—to be conclusively surmised. “There have been detections of black hole candidates before, but they didn’t have these astrometric measurements,” says David Bennett at NASA’s Goddard Space Flight Center, a co-author with Sahu and others on the discovery paper. “This technique is the best one to use for isolated stellar-mass black holes. This is the first attempt to do it. All the black holes that have been found before have been found because they’re not isolated.”
This black hole’s mass offers further evidence that astrophysicists’ formation models are correct—that solitary black holes can rise from the ashes of especially hefty stellar progenitors. It is possible, though, that these black holes can also form in binary systems too before becoming nomads in the void. For this particular object, it is not possible to say with certainty which origin story occurred. What is certain, though, is that finding more isolated black holes will allow researchers to probe and refine those models in more detail. “We’ve never been able to study black holes that are by themselves,” Özel says. “So, this new way of finding them, and being able to determine their mass, is definitely exciting. Are they forming differently? Is their mass distribution different?”
Answers to such questions could arrive quite soon. The European Space Agency’s Gaia telescope is currently mapping the positions of billions of stars in our Milky Way. In 2025, scientists on the project will release lensing data from its observations, expected to contain evidence for many more stellar-mass singletons bolting around our galaxy. “Gaia’s data will be of similar or even better quality than Hubble’s,” says Łukasz Wyrzykowski from Warsaw University, a co-author on this latest discovery paper who also hunts for rogue black holes with Gaia. The forthcoming lensing data, he estimates, will contain dozens of additional candidates.
The Vera C. Rubin Observatory in Chile, which is scheduled to begin a 10-year survey of the night sky next year, is also expected to harvest its own crop of rogue black holes, as is NASA’s Nancy Grace Roman Space Telescope, set to launch in 2027. Rubin and Roman alike have very wide fields of view, allowing each to capture panoramic star-filled vistas in which vast numbers of free-floating black holes must lurk. “The expectation is that this data will be there,” El-Badry says. “The hope is that [Rubin and Roman] will be able to measure this astrometric shift for many [stars].”
For now, this dark discovery forecasts a bright future for the search. Rogue stellar-mass black holes, long predicted but only now observationally confirmed, might well be sufficiently common in our galaxy to support demographic studies of their population. Pinning down their true abundance, masses and other properties could shore up our still-incomplete theories of stellar evolution—or reveal important new gaps in our understanding. “We’ve been waiting for this discovery for many, many years,” Wyrzykowski says. “It shows this method works. Gravitational microlensing is the way to find these isolated black holes.”