‘Runaway’ black hole detected by the James Webb telescope adds a strange new chapter to our universe’s story
Last year, astronomers were fascinated by a runaway comet crossing our solar system from somewhere far beyond. It was moving at about 68 kilometers per second, a little more than double Earth speed around the Sun.
Imagine if it were something much bigger and faster: a black hole traveling at over 3,000 km per second. We wouldn’t see it happen until its intense gravitational forces start moving around the orbits of the outer planets.
This may seem a bit ridiculous, but over the past year several lines of evidence have come together to show that such a visitor is not impossible. Astronomers have seen clear signs of fleeing supermassive black holes ripping apart other galaxies and have discovered evidence that smaller, undetectable fugitives likely exist as well.
Runaway black holes: the theory
The story begins in the 1960s, when New Zealand mathematician Roy Kerr found a solution to Einstein’s equations of general relativity which described rotating black holes. This led to two crucial discoveries about black holes.
First, the “theorem without hair“, which tells us that black holes are distinguished by only three properties: their mass, their spin and their electric charge.
For the second, we must think about of Einstein famous formula E = MC² which says that energy has mass. In the case of a black hole, Kerr’s solution tells us that up to 29% of a black hole’s mass can be in the form of rotational energy.
English physicist Roger Penrose deducted 50 years ago that this rotational energy of black holes can be released. A rotating black hole is like a battery capable of releasing large amounts of rotational energy.
A black hole can contain about 100 times more extractable energy than a star of the same mass. If two black holes merge into one, much of this vast energy can be released in a matter of seconds.
It took two decades of careful computer calculations to understand what happens when two rotating black holes collide and merge, creating gravitational waves. Depending on how black holes spin, gravitational wave energy can be released much more strongly in one direction than others, sending the black holes shooting like a rocket in the opposite direction.
If the rotations of the two colliding black holes are aligned in the right direction, the final black hole can be propelled by a rocket to speeds of several thousand kilometers per second.
Learning from real black holes
It was all just theory, until the LIGO and Virgo gravitational-wave observatories began detecting the screams and chirps of gravitational waves emitted by pairs of colliding black holes in 2015.
One of the most exciting discoveries was that of black hole ringdowns: a tuning fork-like ringing of newly formed black holes that tells us about their rotation. The faster they spin, the longer they ring.
Better observations of merging black holes revealed that some pairs of black holes had randomly oriented spin axes and that many of them had very high spin energy.
All of this suggested that runaway black holes were a real possibility. Moving at 1% the speed of light, their trajectories through space would not follow the curved orbits of stars in galaxies, but would instead be almost straight.
Runaway black holes spotted in nature
This brings us to the final step in our sequence: the true discovery of runaway black holes.
It is difficult to search for relatively small runaway black holes. But a runaway black hole of a million or billion solar masses will create huge disturbances in the stars and gas around it as it travels through a galaxy.
They are predicted to leave star trails in their wake, forming from interstellar gas in the same way that cloud trails form in the wake of a jet plane. Stars form from the collapse of gas and dust attracted by the passing black hole. This is a process that would last tens of millions of years, the time it takes for the fleeing black hole to pass through a galaxy.
In 2025, several papers showed images of surprisingly straight star streaks within galaxies, like the image below. This appears to be compelling evidence for runaway black holes.
One paper, led by Yale astronomer Pieter van Dokkum, describes a very distant galaxy photographed by the James Webb Telescope with a surprisingly bright contrail. 200,000 light years long. The contrail showed the expected pressure effects of gravitational compression of gas as a black hole passes: in this case, it suggests a black hole with a mass 10 million times that of the Sun, moving at nearly 1,000 km/s.
Another describes a long straight streak passing through a galaxy called NGC3627. This is probably caused by a black hole about 2 million times the mass of the Sun, moving at 300 km/s. Its trail is approximately 25,000 light years long.
If these extremely massive runaways exist, their smaller cousins should also exist, because observations of gravitational waves suggest that some of them come together with the opposite rotations needed to create powerful kicks. The speeds are fast enough that they can travel between galaxies.
So runaway black holes that pass through and through galaxies are a new ingredient in our remarkable universe. It is not impossible that such a planet could appear in our solar system, with potentially catastrophic consequences.
We should not lose sleep over this discovery. The chances are tiny. It’s just another way for the history of our universe to become a little richer and a little more exciting than it was before.
This edited article is republished from The conversation under Creative Commons license. Read the original article.
