For years, scientists have been scanning the outer reaches of the solar system for signs of Planet Nine, a world that could be about 10 times as massive as Earth. While this speculative planet has not been directly detected, its presence is inferred by the gravitational tug that astronomers think it is exerting on smaller objects near it.
But what if this mysterious gravitational source isn’t a planet at all? What if it is actually a black hole? But not just any black hole—a black hole from the dawn of the universe.
This tantalizing possibility, which would have dramatic implications for particle physics and cosmology, is explored in a paper published on the arXiv preprint server this week. According to co-authors Jakub Scholtz and James Unwin, a hypothetical object called a primordial black hole (PBH) could account for the odd orbits observed in the distant solar system.
“A solution with an ordinary planet and a solution with an exotic compact object like a primordial black hole are very similar,” said Unwin, who is an assistant professor and theoretical particle physicist at the University of Illinois at Chicago.
“However, the search strategy you need to identify [a black hole] is drastically different,” he added. “We're not currently using all the tools in our toolbox to search for this thing.”
Unlike normal black holes, which form when stars collapse, primordial black holes originate from gravitational perturbations in the very early universe, within one second after the Big Bang. As a result, these objects can be extremely small compared to their counterparts made from stars.
“For normal black holes, you need to have at least a solar mass because it is created out of a star,” said Scholtz, who is a junior research fellow at the Institute for Particle Physics Phenomenology at Durham University. “These primordial black holes can be much lighter; for example, an Earth mass, or in fact, even lighter.”
Scholtz and Unwin started developing the paper after recognizing a surprising connection between the Planet Nine hypothesis and potential PBH observations captured by the Optical Gravitational Lensing Experiment (OGLE) project.
OGLE is an instrument that scans the skies for microlensing events, which occur when distant objects—typically thousands of light years beyond our own solar system—pass in front of even more remote stars. The gravitational fields of these small objects, which could be primordial black holes or free-ranging planets, bend light from background stellar sources, producing a lensing signature that OGLE can detect.
While it is difficult to distinguish what these objects actually are, the lensing events do enable scientists to estimate that they are about one half to 20 times as massive as Earth. This is about the same mass scale expected for Planet Nine, a coincidence that Scholtz and Unwin call “remarkable” in the paper.
Proponents of the Planet Nine hypothesis have noted that the expected location of this world, which is about 20 times as far from the Sun as Neptune on average, is a strange place to find a planet native to the solar system. One explanation is that Planet Nine was once a free-floating world, meaning a planet with no host star, that was ensnared by the Sun’s gravity.
But if a planet could be captured, why not a primordial black hole? Indeed, Scholtz and Unwin calculated that the probability of either class of object getting netted by the solar system are roughly the same. “It could equally be some sort of exotic object that got captured by the solar system at some point,” Unwin said.
So how do we find out if our solar system has had a secret black hole up its sleeve all these years? Theoretically, the authors said, a primordial black hole would produce “annihilation signals.”
These occur when the black hole’s dark matter particles are destroyed upon contact with their anti-particle counterparts. This process would create light radiation that could be potentially be picked up by instruments like the Fermi Gamma-ray Space Telescope or the Chandra X-ray Observatory.
“We actually expect [annihilation signals] to happen at quite a significant rate, so these things have the potential to just be glowing sources in the sky,” Unwin said. To that end, Scholtz and Unwin are planning to analyze public data from the Fermi telescope to look for signals that might correspond to a local black hole.
It bears mentioning that the hypothesis that Planet Nine might be a primordial black hole is extremely speculative, and it will take years to gather the data necessary to support the idea.
But let’s say, for fun, that it’s true that there is a freakin’ black hole in the outer solar system. It would not be an overstatement to call this one of the most consequential discoveries in the history of science, which would enable scientists to test fundamental theories.
“The first thing we could do is send something out there and do a lot of tests of general relativity,” Unwin said. “The hypothetical orbit of Planet Nine is far away, but it’s not an inaccessible distance.”
The opportunity to study an actual black hole up close would revolutionize our understanding of these exotic objects, and constrain theories about the origins and composition of dark matter. If it turns out to be a primordial black hole, scientists could also learn about the early universe and the formation of its elemental properties, such as electromagnetism and the weak nuclear force.
At this point, the idea remains a mind-boggling alternative to the Planet Nine hypothesis, which is itself fairly stupefying. But as scientists continue to probe the weird phenomena at the edge of our solar system, the identity of this enigmatic gravitational source will hopefully be revealed.
“We are only going to get better information,” Unwin said. “If you exhaust your conventional searches in visible light, this explanation that it could be a more exotic object really starts to gain traction.”