The guy who demoted Pluto from major planet status to dwarf planet status believes that there is an actual Planet X with 10x the mass of Earth lurking in the outer reaches of the solar system.

Scientists Find Hints Of A Giant, Hidden Planet In Our Solar System • breaking news from NPR • 2016 Jan 20 • the two-way • NPR

The article refers to the hitherto undiscovered planet as “Planet 9” (since, I suppose, it would in fact be the 9th major planet in the solar system) but calling it Planet X wouldn’t be an error, since the “X” in Planet X stands for “unknown” just like in algebra, not the Roman numeral X.