Last year, astronomers were captivated by the sight of a runaway asteroid zipping through our Solar System, originating from a distant, unknown region of space. Traveling at an astonishing speed of 68 kilometers per second, it moved at more than twice the speed of Earth’s orbit around the Sun. But what if it had been something far larger and faster, like a black hole hurtling through space at 3,000 kilometers per second? Such an event would remain undetected until its immense gravitational forces began to disrupt the orbits of the outer planets.
This scenario, while seemingly far-fetched, is not beyond the realm of possibility. Over the past year, multiple lines of evidence have emerged suggesting that runaway supermassive black holes are indeed tearing through other galaxies. Furthermore, astronomers have found evidence indicating that smaller, undetectable runaway black holes may also exist.
Runaway Black Holes: The Theory
The concept of runaway black holes can be traced back to the 1960s when New Zealand mathematician Roy Kerr discovered a solution to Einstein’s general relativity equations that described spinning black holes. This breakthrough led to two pivotal discoveries about black holes.
The first is the “no-hair theorem,” which posits that black holes can be characterized solely by three properties: their mass, their spin, and their electric charge. The second discovery involves Einstein’s renowned equation E = mc², which implies that energy possesses mass. Kerr’s solution revealed that up to 29% of a black hole’s mass could exist as rotational energy.
English physicist Roger Penrose, over 50 years ago, deduced that this rotational energy could be harnessed. A spinning black hole, he suggested, functions like a colossal battery capable of releasing vast amounts of spin energy. In fact, a black hole can contain about 100 times more extractable energy than a star of the same mass. When two black holes merge, a significant portion of this energy can be unleashed in mere seconds.
It took decades of meticulous supercomputer calculations to understand the dynamics of colliding spinning black holes, which generate gravitational waves. Depending on their spin alignment, the energy from these waves can be emitted more strongly in one direction, propelling the black holes in the opposite direction like a rocket.
Learning from Real Black Holes
This theoretical framework remained speculative until 2015, when the LIGO and Virgo gravitational wave observatories began detecting the distinct signals of gravitational waves produced by colliding black holes. One of the most thrilling discoveries was the detection of black hole “ringdowns,” a phenomenon akin to the ringing of a tuning fork that provides insights into the spin of newly formed black holes. The faster they spin, the longer they resonate.
As observations of coalescing black holes improved, it became evident that some pairs had randomly oriented spin axes and possessed substantial spin energy. These findings suggested that runaway black holes could indeed exist. Traveling at 1% of the speed of light, their paths through space would be almost linear, unlike the curved orbits of stars within galaxies.
Runaway Black Holes Spotted in the Wild
The final piece of the puzzle was the actual observation of runaway black holes. Although detecting relatively small runaway black holes is challenging, a runaway black hole with a mass of millions or billions of solar masses would cause significant disruptions to the surrounding stars and gas as it traverses a galaxy.
These massive black holes are predicted to leave trails of stars in their wake, formed from interstellar gas, akin to the contrails left by jet planes. As the black hole moves through a galaxy, stars form from collapsing gas and dust attracted to it—a process that could persist for tens of millions of years.
In 2025, several studies presented images of unexpectedly straight streaks of stars within galaxies, providing compelling evidence for runaway black holes. One study, led by Yale astronomer Pieter van Dokkum, described a distant galaxy captured by the James Webb Telescope, featuring a bright contrail stretching 200,000 light-years. This contrail exhibited pressure effects indicative of gravitational compression, suggesting a black hole with a mass 10 million times that of the Sun, traveling at nearly 1,000 kilometers per second.
Another study highlighted a long, straight contrail cutting across the galaxy NGC3627, likely caused by a black hole with a mass approximately 2 million times that of the Sun, moving at 300 kilometers per second. Its contrail extended about 25,000 light-years.
If these colossal runaway black holes exist, their smaller counterparts likely do too. Gravitational wave observations suggest that some black holes merge with the opposing spins necessary to generate powerful kicks, enabling them to travel between galaxies.
Thus, runaway black holes tearing through and between galaxies add a new dimension to our understanding of the universe. Although the prospect of one entering our Solar System is remote, the potential consequences would be catastrophic.
Despite this discovery, there is no need for sleepless nights. The odds of such an event are minuscule. Instead, it serves as a reminder of the ever-expanding and exhilarating story of our universe.
