This hypothesized ninth planet (not you, sorry Pluto) might explain the unusual commonalities of extreme trans-Neptunian objects (ETNOs) 100s of AU from the sun. These ETNOs (such as dwarf planets and sednoids) have remarkably aligned orbits, suggest the existence of an undiscovered celestial body, dubbed Planet Nine, influencing them gravitationally.
I thought Planet X was a hypothetical inner planet?
Nah, it’s the same thing. Just renamed to planet nine now.