The scientists who predicted the planet's existence did so based on the odd, highly elliptical orbits of around 30 so-called Trans-Netpunian objects (TNOs.) These are clustered together in a way that is extremely unlikely to have occurred by chance. It's been hypothesized as an explanation for the unusual clustering of asteroids and comet orbits out beyond Neptune. Being so far away, we haven't been able to catch a glimpse of it yet. In the new study, scientists have proposed a way of tracking such a black hole.
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.
Science website Gizmodo is reporting the scientists behind the black hole proposal - Jakub Scholtz and James Unwin - still think a planet is the more likely explanation, but they made the black hole proposal because they wanted astronomers to be more creative in their search. "These primordial black holes can be much lighter; for example, an Earth mass, or in fact, even lighter".
At that size though, it may be impossible to spot.
The two scientists reached this hypothesis in a novel way, by observing that two different phenomena could be related: first, the observation of peculiar orbits of trans-Neptunian objects (meaning bodies orbiting our sun beyond Neptune), which might be explained by the presence of a distant planet with 5 to 15 times Earth's mass; and second, the number of "microlensing events", meaning telescope images that had been distorted by gravitational lensing of the kind caused by black holes or other massive, compact objects. Both scenarios would be equally exciting for astronomers.
They'll be looking in particular for groups of sporadic gamma ray flashes that would move slowly across the sky, as Planet Nine would be expected to do as seen from Earth.
This theory makes a lot of sense when considering the difficulty in locating Planet Nine.
Another theory suggests it could be a primordial black hole. The study's authors say that a black hole with a planet's mass would, with a high probability, have an about 1 billion kilometer black matter halo on each side, with dark matter and antimatter working together to release a gamma ray flash.
They aren't throwing the Planet 9 (also known as Planet X) hypothesis out the window; however, they hope that the possibility they are suggesting will take astronomers' creativity further, to broaden their scope of search.
Conspiracy nuts have claimed for years that a rogue planet called Nibiru may one day smash into Earth, destroying all life as we know it.