What If Planet Nine Is a Bowling-Ball-Sized Black Hole?

By Ryan F. Mandelbaum on at

Some of the most distant rocks in our solar system act in a way that suggests there’s some massive object out there we haven’t been able to see. A planet? Maybe. But why not a small black hole?

That’s a scenario a pair of scientists describe in a new paper. Of course, they recognise that a planet is more likely than an ancient black hole unlike any we’ve directly observed. But they simply want astronomers to think creatively while hunting for whatever this hypothetical object, often called Planet Nine, might be.

“By simply focussing on the concept of a planet, you restrict the experimental search that you’re undertaking,” James Unwin, one of the study’s authors and an assistant professor at the University of Illinois at Chicago, told Gizmodo. “Once you start thinking about more exotic objects, like primordial black holes, you think in different ways. We advocate that rather than just looking for it in visible light, maybe look for in gamma rays. Or cosmic rays.”

Past Neptune, the motion of rocky objects seems to be disturbed by something with a mass about five to 15 times that of Earth. Scientists have tentatively named this object Planet Nine and are hunting for it. But this isn’t not the only planet-mass gravitational anomaly in the galaxy. Scientists have detected short warps to incoming starlight, perhaps from planet-sized objects bending their gravity. Maybe they’re rogue planets – or maybe they’re tiny black holes.

“Primordial black holes” are a class of proposed objects that formed as a result of the chaotic early days of the universe. Like any other black hole, they would be incredibly dense regions where gravity warps space so much that light cannot escape. But these would weigh far less than stars, since they weren’t formed out of stars like the black holes we’ve actually observed – they would have formed out of places of leftover extra density in the rapidly expanding early universe. (And no, they wouldn’t contribute significantly to dark matter, the mysterious stuff that seems to comprise the lion’s share of the universe’s mass).

Unwin and his collaborator Jakub Scholtz, a junior research fellow at the Institute for Particle Physics Phenomenology at Durham University, proposed that perhaps a primordial black hole whizzed by, interacted with the solar system’s other members, and became trapped in an orbit. I asked Uniwn and Scholtz whether such an object would evaporate from tiny physical effects called Hawking radiation; they said that no, even a five-Earth-mass black hole would last for a very long time, far longer than the age of the universe.

If the planet really were a primordial black hole, rather than a planet-sized mass of regular matter, then it would be no use trying to find it with typical planet-searching means. A figure in the paper demonstrates that a five-Earth-mass black hole could fit in the palm of your hand (yes, this encounter would kill you), and a 10-Earth-mass black hole would be the size of a bowling ball. Finding it would require a dedicated telescope search looking for a source that present-day gamma ray telescopes aren’t used to seeing – a source of high-energy radiation moving quickly across the sky.

Planet Nine hunter Konstantin Batygin didn’t rule out the idea that it might actually be something more exotic. “Planet Nine could be a five-Earth-mass hamburger, and the math would still work out right. Of course, a hamburger has a comparable albedo,” or how much light it reflects, “to a planet, but a black hole the size of your wallet is a bit harder to find,” he told Gizmodo in an email. He he wrote that the scenario is a stretch but not entirely implausible, and a black hole becomes an interesting potential target if Planet Nine goes unfound by typical deep surveys and if the strange motion of the trans-Neptunian objects persists.

Something is causing unexpected behaviour in the distant solar system, whatever it may be. A primordial black hole is not the most obvious nor the most likely choice—but hey, science is about keeping an open mind and letting experiments disprove hypotheses. If there really is a massive object out there, even if it’s a planet, it would be “quite a shocker", Scholtz said.

Screenshot: Scholtz and Unwin (arXiv)