Welcome, Overlords of Y.
Hey what happened to X? Did they find it?
For that matter, Planet P.
The X was meant to show that an unnamed planet could be out there. That’s kinda been lost. OTOH, X is the Roman numeral for ten, which means, Planet 10 is being looked for. The demotion of Pluto was strictly political and should be ignored. Instead, it’s boilerplate.