Context: For anyone who grew up before 2006, Pluto was the ninth planet. Then the International Astronomical Union changed the definition of a planet and poor Pluto got demoted to dwarf planet status. Now it's just a nostalgic memory, and Grandpa Simpson says it best.
It wasn't, though. The planets were basically defined as "it's those 9 there".
People only started to push for a property-based definition when we started finding exoplanets. And only really moved into it when a 11th candidate was found and there were some 3 or 4 more possible ones.