When a star 20 times as massive as our sun dies, it can explode in a supernova and squeeze back down into a dense black hole (with the help of gravity). But that explosion is never perfectly symmetrical, so sometimes the resulting black holes shoot into space. These wandering objects are often referred to as “rogue black holes” because they float around freely, untethered by other celestial bodies.
But that name could be a “misnomer”, according to… Jessica Luc, associate professor of astronomy at the University of California Berkeley. She prefers the term “free-floating” to describe these black holes. “Rogue,” she says, implies that the nomads are rare or unusual — or not good at all.
That is certainly not the case. Astronomers estimate that there are as many as 100 million such black holes that roam our galaxy. But because they are solitary, they are extremely hard to find. Until recently, these so-called rogue black holes were known only through theory and calculations.
“They’re ghosts, so to speak,” says Lu, who has made it her mission to find the free-floating black holes in the Milky Way.
[Related: We’re still in the dark about a key black hole paradox]
Earlier this year, two teams of space researchers separately revealed detections of what may well be one of these wandering black holes. One of those teams was led by Casey Lam, a graduate student in Lu’s lab. The other was led by Kailash C. Sahu, an astronomer at the Space Telescope Science Institute. Both teams placed their papers in a free open-access journal without expert review.
The scientists will get more data from the Hubble Space Telescope in October that Lu says should help “solve the mystery of whether this is a black hole or a neutron star.” “There is still a lot of uncertainty about how stars die and about the ghostly remains they leave behind,” she notes. When stars much more massive than our sun run out of nuclear fuel, they are believed to collapse into a black hole or neutron star. “But we don’t know exactly which ones die and turn into neutron stars or die and turn into black holes,” Lu added. “We don’t know when a black hole is born and a star dies, is there a violent supernova explosion? Or will it collapse directly into a black hole and maybe give it a little burp?”
With star material making up everything we know in the world, understanding the afterlife of stars is the key to understanding how we came to be.
How to spot a stray black hole?
Black holes are invisible by nature. They catch all the light they encounter, therefore there is nothing for the human eye to perceive. So astronomers have to get creative to detect these dense, dark objects.
They usually look for anomalies in gas, dust, stars and other material that could be caused by the intense gravity of a black hole. If a black hole rips material away from another celestial body, the resulting disk of debris surrounding the black hole can be clearly visible. (For example, astronomers took the first direct image of a in 2019 and an image of the black hole at the center of the Milky Way earlier this year.)
But if a black hole’s gravity doesn’t cause chaos, there’s hardly anything to discover. That is often the case with these moving black holes. So astronomers like Lu use another technique called astrometric or gravity microlensing†
“What we’re doing is waiting for the accidental alignment of one of these free-floating black holes and a background star,” explains Lu. “When the two align, the light from the background star is distorted by the black hole’s gravity [in front of it]† It appears as a brightening of the star [in the astronomical data]† It also makes it take a little jaunt in the air, a little wobbling, so to speak.”
The background star doesn’t actually move, but appears to shift off course as the black hole or other compact object passes in front of it. That’s because the black hole’s gravity distorts the fabric of spacetime, according to Albert Einstein’s general theory of relativitythat changes the starlight.
Astronomers use microlenses to study all kinds of temporal phenomena in the universe, from supernovas to exoplanets orbiting their stars. But it’s tricky to do with ground-based telescopes, because Earth’s atmosphere can blur the images.
“In astrometry you try to measure the position of something very precisely and you need very sharp images,” explains Lu. So astronomers rely on telescopes in space, like Hubble, and a few ground-based instruments with sophisticated systems to adapt to atmospheric interference. “There are really only three facilities in the world that can do this astrometric measurement,” says Lu. “We’re working at the cutting edge of what our technology can do today.”
The first rogue black hole?
It was that brightening, or a “gravity lens event,” as Lu calls it, that both she and Sahu’s teams saw in 2011 Hubble Space Telescope data. Something, they suspected, must be passing in front of that star.
To find out what caused the wobble and change of intensity in a star’s light, two measurements are needed: luminosity and position. Astronomers observe that same spot in the sky over time to see how the light changes as the object passes in front of the star. This gives them the data they need to calculate the mass of that object, which in turn determines whether it is a black hole or a neutron star.
“We know the thing that the lenses do is heavy. We know it’s heavier than your typical star. And we know it’s dark,” Lu notes. “But we’re still a little unsure about how heavy and precise how dark.” If it’s just a little bit massive, let’s say one and a half times the mass of our sun, it could actually be a neutron star. But if it’s three to ten times as massive as our sun, then it could be a neutron star. black hole, explains Lu.
When the two teams collected data from 2011 to 2017, their analyzes clearly revealed different masses for that compact object. Sahu’s team determined that the wandering object has a mass seven times that of our sun, which would put it squarely in the area of a black hole. But Lam and Lu’s team calculated that it is less massive, somewhere between 1.6 and 4.4 solar masses, encompassing both possibilities.
[Related: Black holes can gobble up neutron stars whole]
The astronomers can’t be sure which calculation is correct until they get a chance to know how bright the background star normally is and how it is in the sky if something isn’t passing in front of it. They weren’t focused on that star before noticing its uncharacteristic brightness and wobble, so they’re only now getting a chance to make those basic observations because the lens effect has faded, Lu explains. Those observations will come from new Hubble data in the fall.
What they do know is that the object in question is in the Carina-Sagittarius Spiral Arm from the Milky Way, and is currently about 5,000 light-years from Earth. This detection also suggests that the nearest wandering black hole could be less than 100 light-years away, Lu says. But that’s no cause for concern.
“Black holes are a drain. If you get close enough, they’ll devour you,” says Lu. “But you have to get really close, much closer than I think we usually imagine.” The boundary around a black hole that marks the line where light can still escape its gravity, called the event horizon, usually has a radius of less than 20 miles.
The chances that a wandering black hole could pass through our celestial environment and disrupt life on Earth is “astronomically small,” Lu says. “That’s the size of a city. So a black hole could pass through the solar system and we would hardly notice it.”
But she doesn’t rule it out. “I’m a scientist,” she says. “I can’t say there’s no chance.”
Whether the first teams have detected a wandering black hole or a neutron star, “the real revolution that these two papers show is that we can now find these black holes using a combination of luminosity and position measurements.” This opens the door to discoveries from more light-catching nomads, especially as new telescopes come online, including the Vera C. Rubin Observatory currently under construction in Chile and the Nancy Grace Roman Space Telescope due to launch later this decade.
As Lu sees it, “the next chapter of black hole research in our galaxy has already begun.”