Two physicists theorize that the hypothetical "Planet Nine" -- a mysterious gravitational effect on the outer reaches of our solar system past that could be a giant planet -- may actually be a black hole that's anywhere from the size of a baseball to a bowling ball.

In a paper published on arXiv, Jakub Scholtz and James Unwin theorize that a primordial black hole could explain why some Trans-Neptunian Objects such as asteroids in the Kuiper Belt have unique orbits.

The theory of Planet Nine started in 2015. According to NASA, CalTech researchers noted that some dwarf planets and other small, icy objects in the Kuiper Belt, beyond Neptune, tended to cluster together. They predicted there could be a large planet that we hadn't found yet with a mass about 10 times that of Earth, causing this gravitational anomaly.

But through their best efforts, astronomers have been unable to find this planet which theoretically lies so far away, it would take 10,000 to 20,000 years to orbit the sun. 

Which brings us to the black hole theory.

"The most mundane, or maybe the most sane explanation is that it's a planet. But as theoretical physicists, we know that early-universe cosmology can very readily introduce a range of very interesting new theoretical bodies — one of which… is primordial black holes," Scholtz told LiveScience.

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Just as with Planet Nine, this is only a theory at this point and has not been confirmed. But, other researchers who have reviewed the paper say it cannot be ruled out.

What makes a primordial black hole different from the black holes we normally hear about? While the massive black holes we know are formed from collapsing stars, primordial black holes date back to the birth of the universe after the Big Bang. Some hotter, denser areas of the early universe could have collapsed into these primordial black holes.

Scholtz is a physicist at Dunham University in the U.K. Unwin is a physicist at the University of Illinois at Chicago and the University of California.