Regardless, the new paper suggests the unusual orbits in the outer regions of our solar system could be the result of one of these primordial black holes. While the mysterious body's existence has yet to be officially proven, researchers strongly suspect that there is something massive lurking in space on the edge of our solar system.
As it's so small, it's unlikely the black hole poses any threat to Earth - for now.
It's widely accepted among astronomers that deep within the heart of the Milky Way galaxy lies a supermassive black hole.
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.
Astronomers Jakub Scholtz of Durham University and James Unwin of the University of IL at Chicago published a study on the arXiv pre-print server suggesting the mysterious planet could be a primordial black hole.
That may be true, but black holes aren't all the same, and researchers believe so-called primordial black holes (PBHs) that were created shortly after the Big Bang may still exist.
"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 IL at Chicago.
However, now two physicists are suggesting that the enigmatic "Planet X" isn't a planet at all, but a planet-mass black hole, sucking in matter from its surroundings. Indeed, Scholtz and Unwin calculated that the probability of either class of object getting netted by the solar system are roughly the same. Theoretically, the authors said, a primordial black hole would produce "annihilation signals".
According to them, the hypothetical world could have a mass about 10 times that of Earth and take up to 20,000 years to orbit the Sun. 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. 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. For now, Planet 9 remains a mystery.
Because primordial black holes formed so early on in the existence of the universe, they are much smaller than their modern counterparts as stars were smaller too.