NASA is now pretty sure there is in fact a ninth planet - and no, it's not Pluto.
The existence of the planet, believed to be about 10 times bigger than Earth and 20 times further away than Neptune, has been theorised for a number of years.
Now NASA says there would be more holes in its calculations if the planet didn't exist, than if it does.
"There are now five different lines of observational evidence pointing to the existence of Planet Nine," said Konstantin Batygin, a planetary astrophysicist at Caltech in Pasadena, California.
"If you were to remove this explanation and imagine Planet Nine does not exist, then you generate more problems than you solve. All of a sudden, you have five different puzzles, and you must come up with five different theories to explain them."
The biggest clue is that six known objects in the Kuiper Belt - a massive ring of icy rocks and comets beyond Neptune - have elliptical orbits all pointing in the same direction, and at a 30-degree angle compared to everything else.
Another big clue is that the solar system's planets orbit the sun on a six-degree angle away from the sun's equator.
"Over long periods of time, Planet Nine will make the entire solar-system plane precess or wobble, just like a top on a table," Prof Batygin said.
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There are also some objects in the Kuiper Belt which orbit the sun the "wrong" way.
"These things have been twisted out of the solar system plane with help from Planet Nine and then scattered inward by Neptune."
The problem, however, is that no one's actually seen Planet Nine yet. Astronomers are currently using a telescope in Hawaii to try and find it.
Pluto was considered the ninth planet in the solar system until around a decade ago, when it became clear there were plenty of other objects just as big floating around. It is now considered one of the solar system's five known dwarf planets.