The paper this article refers to actually refers to 'Planet X' as the theorized large planet in its section on orbital stability models. I think the planet X m

The paper this article refers to actually refers to 'Planet X' as the theorized large planet in its section on orbital stability models. I think the planet X moniker originally meant 'x' as unknown (it was coined during the search that found Pluto). Nowadays, it has been used to refer to several distinct theories including Planet IX. I dunno when exactly the X got re-interpreted. What I do know is WE'RE DOOOOMED! (according to the internet).