Black holes the size of an atom that contain the mass of an asteroid may be flying through the inner solar system about once a decade. Theoretically created just after the big bang, these examples of so-called primordial black holes could explain the missing dark matter thought to dominate our universe. And if they sneak by the moon or Mars, scientists should be able to detect them, a new study shows.
Such black holes could have arisen easily right after the universe was born, when space is thought to have expanded hugely in a fraction of a second. During this expansion, tiny quantum fluctuations in space’s density would have grown larger, and some spots might have become so dense that they collapsed into black holes scattered throughout the cosmos. If dark matter were fully explained by such black holes, their most likely mass, according to some theories, would range from 1017 to 1023 grams—about that of a large asteroid, packed into the size of an atom.
If primordial black holes are responsible for dark matter, one probably zips through the solar system about every 10 years, according to a recent study in Physical Review D. And if such a black hole comes near a planet or a large moon, it should nudge the body off course enough for the change to be measurable by current instruments. “As it passes by, the planet starts to wobble,” says study co-author Sarah R. Geller, a theoretical physicist now at the University of California, Santa Cruz. “The wobble will grow over a few years but eventually it will damp out and go back to zero.”
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Study lead author Tung X. Tran, then an undergraduate student at the Massachusetts Institute of Technology, built a computer model of the solar system to see how the distance between Earth and nearby objects would change after a black hole flyby. He found that such an effect would be most noticeable for Mars, whose distance from Earth scientists know within about 10 centimeters. A black hole in the middle of the predicted mass range weighing, 1021 grams, would produce one meter of variation in 10 years, Tran says—“way above the threshold of precision that we can measure.” The Earth-Mars distance is particularly well tracked because scientists have been sending generations of probes and landers to the Red Planet.
If scientists detect a disturbance, they must determine whether the planet was pushed by a black hole or just a plain old asteroid. By tracking the wobble pattern over time, they can trace the culprit’s trajectory and predict where it will head in the future. “We actually get really rich information from the pattern of perturbations,” says study co-author Benjamin V. Lehmann of M.I.T. “We’d need to convince ourselves that it’s really a black hole by telling observers where to look.” If the object is an asteroid, telescopes should be able to see it. Plus, most asteroids come from within the solar system and therefore orbit on the same plane as the planets. A primordial black hole, in contrast, would be coming from far away and would likely have a different trajectory than that of an asteroid.
Another potential way to look for primordial black holes in the solar system would be to analyze the fine movements of asteroids such as Bennu, which has been tracked very precisely by the ongoing space mission OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer). After reading the new study, “I think we can try to dig into OSIRIS-REx data to see if we can see this effect,” says Yu-Dai Tsai, a particle physicist at Los Alamos National Laboratory. “I think it’s a promising direction to look at.” Tsai and his colleagues studied how the probe’s Bennu measurements could be used to look for other forms of dark matter in a paper published on September 20 in the journal Communications Physics.
Primordial black holes are an increasingly appealing solution to the puzzle of dark matter, an invisible form of mass that physicists think makes up most of the matter in our universe. Because they can “see” this substance only through its gravitational effects on regular matter, its identity has remained elusive as many favored theories have failed to pan out. For decades physicists thought dark matter was likely to take the form of so-called weakly interacting massive particles (WIMPs). Yet generations of ever more sensitive experiments meant to find these particles have come up empty, and particle accelerators have also seen no sign of them. “Everything is on the table because WIMPs have been put in such a corner, and they were the dominant paradigm for decades,” says astrophysicist Kevork N. Abazajian of the University of California, Irvine, who wasn’t involved in the Physical Review D study. “Primordial black holes are really gaining popularity.”
Physicists are also recognizing that dark matter may not interact with regular matter through any force other than gravity. Unlike WIMPs, which could also touch regular matter through the weak nuclear force, black holes would be detectable only through their gravitational pull. “Given that we are still searching for the correct way to detect dark matter interacting with ordinary matter, it is particularly important to explore probes based on the gravitational force it produces, which is the only interaction of dark matter whose strength is already known and the only interaction we are sure exists,” says theoretical physicist Tim M. P. Tait of U.C. Irvine, who was also not involved in the M.I.T. team’s new research. “This is a really interesting idea and one that is timely.”
That same issue of Physical Review D also happened to include a paper about a different team’s search for signs of primordial black holes flying near Earth. The researchers’ simulations found that such signals could be detectable in orbital data from Global Navigation Satellite Systems, as well as gravimeters that measure variations in Earth’s gravitational field. The two papers are complementary, says David I. Kaiser of M.I.T., a co-author of the study on interplanetary distance measurements.
Although these black holes could be passing relatively nearby, the chances that one could move through a human body are incredibly low. If that were to happen to you, though, it wouldn’t be fun: as the tiny black hole moved through you, it would tug everything toward it, crushing cells together in a deadly fashion. Its minuscule volume, however, would at least prevent you from getting sucked in.
A version of this article entitled “Mini Monsters” was adapted for inclusion in the December 2024 issue of Scientific American. This text reflects that version, with the addition of some material that was abridged for print.