What If Planet 9 Is Actually a Tiny Black Hole?

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Scientists have had a hard time agreeing on how many planets there are in the solar system. Most experts will tell you there are eight that we know of, but some people like NASA’s Jim Bridenstine (and Jerry Smith) think Pluto should be among them. There’s increasing evidence of another planet out there at the extreme edge of the solar system, but a new analysis suggests that may not be a planet at all. Instead, it could be a tiny black hole

Most of the evidence for Planet 9 comes from the study of trans-Neptunian objects, or TNOs. These chunks of ice and rock like Pluto and Makemake have extremely long and often irregular orbits. As we learn more about the outer reaches of the solar system, it has become apparent that some of those orbits are a bit more irregular than they should be. The orbits of numerous TNOs like “The Goblin” and Sedna look like they’ve been nudged by a large, undiscovered object. That could be Planet 9, or perhaps it’s not a planet at all. 

Based on observations of TNOs, the mystery object should be between five and fifteen Earth masses. That’s by no means tiny, but no one has even identified a candidate object yet. Jakub Scholtz at Durham University in the UK and James Unwin at the University of Illinois have published a preprint paper that suggests Planet 9 is actually a primordial black hole. 

Black holes formed from dying stars are much more massive than the proposed object, but primordial black holes are different. We have no direct evidence these objects exist, but many cosmologists believe quantum fluctuations in the early universe may have compressed matter into tiny black holes that couldn’t form in the universe today. A primordial black hole of five Earth masses would be about five centimeters across and wouldn’t show up in visible light or infrared, which could explain why we have yet to spot a planet.

The paper includes an actual-size illustration of the proposed primordial black hole.

One of the prime hypotheses for Planet 9 is that it was a rogue planet captured by the sun. The study authors point out if that type of capture is possible, it’s also possible for the sun to capture a primordial black hole. Well, assuming they exist — the best we have right now is some analysis of gravitational lensing from the Optical Gravitational Lensing Experiment (OGLE) that suggests there are small, dense objects in the galaxy that could be primordial black holes. 

If Planet 9 is indeed a black hole, how does that change the search? The authors say current scanning methods won’t do any good. Such an object might have clumps of dark matter around it, and the interaction of dark matter with regular matter might produce a detectable gamma-ray source. The pair plan to comb through data from the Fermi Gamma Ray Space Telescope in the future in search of such a signal. For now, Planet 9 remains a mystery.

Top image: Simulated view of a black hole in front of the Large Magellanic Cloud/Alain r. (CC BY-SA 2.5)

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