For a hundred years scientists have been searching for a planet in the outer solar system beyond Neptune and Pluto (whose orbits cross over so they alternate being the further body.) This putative planet was called Planet X, both because it would be the 10th planet (from Latin numerals) and because X traditionally denotes the unknown.
Now boffins at CalTech think they have proved the existence of Planet X by looking at perturbations of smaller objects in the Kuiper belt.
Planet X: One of its years would be 10,000-20,000 Earth years
The irony is that while they were searching Pluto has been downgraded to a dwarf planet so the new world is only Planet IX.