"You know, I really like just plain "Planet Nine"."
No, no, no no,no!
It must be Planet Ten- else what of Planet X?
Also, no matter how large it is, it will probably be classed as a 'Dwarf, Elf or Gnome World.
Pluto is a bloody planet. Get over it. Planet #9.
s/Ninth/Tenth
grr!