Astronomers have come up with a new theory explaining why they can't find the hypothesised 'Planet Nine' - it's not actually a planet, but a black hole.
There's evidence of a ninth planet beyond the orbit of Neptune - scientists can see its gravitational effects on the orbits of dwarf planets like Pluto and other objects in the distant Kuiper Belt.
NASA in 2017 said it almost definitely exists, and it's likely to be about 10 times bigger than Earth - but it probably only orbits the sun every 20,000 years, and is very, very far away, so no one's actually seen it yet.
In a new paper however, scientists from the US and UK say there could be a good reason for that - it's a minuscule black hole that's existed since the beginning of time itself.
"There is a growing body of observational anomalies connected to the orbits of trans-Neptunian Objects (TNOs)," authors Jakub Scholtz and James Unwin wrote in the new paper.
"These observations have been taken as evidence of a new ninth planet in our solar system, called Planet Nine."
But it could also be a primordial black hole, they say. Primordial black holes differ from other black holes in that they're not created in star collapses - they've existed since the first second after the Big Bang, created by gravitational perturbations in the fabric of spacetime, according to research by the late Stephen Hawking.
No one's ever found one, but Dr Hawking's calculations show they can be much smaller than normal black holes.
"These primordial black holes can be much lighter; for example, an Earth mass, or in fact, even lighter," Dr Scholtz told Vice.
If Planet Nine is actually a black hole, that would explain why no one's found it yet - not only might it be as small as a tennis ball, according to the paper, but the methods used for finding planets are different to those used to detect black holes.
"If conventional searches fail to find Planet Nine and the evidence for TNO anomalies continues to grow, the primordial black hole Planet Nine hypothesis will become a compelling explanation," the authors say.