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.
It can’t be the ninth planet. It literally can’t. It’s either not a planet, or its the tenth planet.
Everyone forgetting about my boy, Ceres.
Exactly! Fair enough if you want to define a planet as “anything big enough to be rounded by self-gravity, but too small to undergo fusion in its core.” But there is no definition of a planet, (or at least one without an arbitrary rule like "planets have to be X km in diameter,) that allows Pluto to be a planet without Ceres also being a planet.