Ideas about a possible ninth planet have been kicking around since shortly after we discovered the eighth in 1781. But so far, all that we’ve come up with is Pluto and a handful of other objects orbiting out in the Kuiper Belt. And these dwarf planets simply don’t have the mass to have a significant gravitational influence on our Solar System.

But our inability to find anything big beyond the known planets may just have been because we weren’t thinking radically enough. One of the people responsible for the discovery of a number of Kuiper Belt Objects noticed an odd alignment in their orbits. When running models of how that oddity could be produced, he and his team found that a large planet with an extreme orbit would work.

Calling it Planet 9, they suggested it could be over 10 times Earth’s mass and so far out it takes 20,000 years to complete one orbit. Planet 9, they speculated, has a lopsided orbit that’s tilted relative to the other planets and much closer to the Sun on one side.