3 ms·If it's the ninth planet, surely it should be "Planet IX"by troels 11y agoIf it's the ninth planet, surely it should be "Planet IX"Arnavion 11y agoThe X refers to the planet being mysterious / unknown, not the Roman numeral for 10.labster 11y agoAccording to Chinese President Eleven Jinping, yes.ceejayoz 11y agoThe term "Planet X" predates Pluto's demotion to dwarf planet.