Last year, astronomers were fascinated by a runaway asteroid passing through our solar system from somewhere far beyond. It was moving at around 68 kilometres per second, just over double Earth’s speed around the Sun.
Now imagine something far more extreme: a black hole racing through space at nearly 3,000 km per second. We would not see it coming until its gravity began disturbing the orbits of the outer planets.
That scenario may sound far-fetched. But over the past year, several strands of evidence have converged to show such visitors are not impossible. Astronomers have spotted clear signs of runaway supermassive black holes tearing through distant galaxies, and uncovered clues that smaller, undetectable runaways may also exist.
The story traces back to the 1960s, when New Zealand mathematician Roy Kerr solved Einstein’s equations for spinning black holes. His work revealed two crucial insights. One was the “no-hair theorem”, which says black holes are defined only by mass, spin and electric charge. The other followed from Einstein’s famous E = mc²: energy has mass. Kerr showed that up to 29% of a black hole’s mass can exist as rotational energy.
English physicist Roger Penrose later demonstrated that this spin energy can be extracted. A rotating black hole, he argued, is like a vast battery. A black hole can contain around 100 times more extractable energy than a star of the same mass. When two black holes merge, much of this energy can be released in seconds.
Decades of supercomputer modelling eventually showed what happens during such mergers. Depending on how the black holes spin, gravitational waves can be emitted far more strongly in one direction. The result is a recoil: the newly formed black hole is shot off like a rocket.
If the spins are aligned just right, the final black hole can be propelled at thousands of kilometres per second. For years, this remained theoretical. That changed in 2015, when the LIGO and Virgo observatories began detecting gravitational waves from colliding black holes.
One key discovery was the “ringdown” — a tuning-fork-like vibration of newborn black holes that reveals their spin. Faster-spinning black holes ring longer. Over time, observations showed that many merging black holes have large spin energies and randomly oriented spin axes, making powerful kicks likely.
This implied that runaway black holes should exist. Travelling at up to 1% of the speed of light, they would move in near-straight lines rather than the curved orbits of stars.
Finding smaller runaways is difficult. But supermassive black holes — millions or billions of times heavier than the Sun — leave dramatic signatures as they plough through galaxies. They are predicted to create long “contrails” of stars, formed as gas is compressed by the passing black hole.
In 2025, a study led by Yale astronomer Pieter van Dokkum described a 200,000-light-year contrail seen by the James Webb Space Telescope,likely caused by a black hole 10 million times the Sun’s mass moving at nearly 1,000 km per second. Another, in the galaxy NGC 3627, points to a two-million-solar-mass black hole travelling at 300 km per second.
If such giants exist, smaller runaway black holes should too. Gravitational-wave data suggest the kicks needed to eject them are common, and fast enough for them to roam between galaxies.
The chance of one passing through our Solar System is vanishingly small. But runaway black holes are now part of the universe’s story — another reminder that it is stranger, richer and more dynamic than once imagined.
The Conversation