Is Planet Nine Really Just an Old Black Hole?

New research argues Planet Nine is really just a black hole left over from the Big Bang.
Donna Fuscaldo
Planet NineCaltech

Planet Nine, the unconfirmed planet thought to be in the outer region of our Solar System, may not be a planet at all but rather a black hole, according to new research. 

Astronomers Jakub Scholtz of Durham University and James Unwin of the University of Illinois at Chicago published a study on the arXiv pre-print server suggesting the mysterious planet could be a primordial black hole.


Is Planet Nine really an old black hole?

Primordial black holes are old black holes that are small and were a result of the Big Bang. The primordial black holes that had the lowest mass are thought to have evaporated while those with bigger masses are still in existence. 

The astronomers argue the promodial black holes could be closer to Earth than previously thought and that the Planet Nine is one of those old black holes. "Capture of a free-floating planet is a leading explanation for the origin of Planet Nine, and we show that the probability of capturing a PBH instead is comparable," the astronomers wrote in the paper.

Researchers' theory may be tough to prove 

The researchers warned in the paper it may be tough to confirm its theory since a primordial black hole with a mass that is about five times the Earth and a radius of around five centimeters would have a temperature that is too cold for enough power to be radiated to detect its presence. To overcome that, the researchers suggested looking at the dark microhalo that surrounds Primordial black holes. 

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"The PBH hypothesis expands the required experimental program to search for the body responsible for TNO shepherding and motivates dedicated searches for moving sources in x-rays, gamma rays, and other high energy cosmic rays," wrote the researchers. "Conversely, if conventional searches fail to find Planet 9 and the evidence for TNO anomalies continues to grow, the PBH P9 hypothesis will become a compelling explanation."

Ever since Caltech researchers introduced the ninth planet in 2016, the space community has been divided with some arguing the astronomers didn't have enough evidence to claim there is actually a new planet. Since then, more research has been done to back up that theory. 

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