I think it's fair to define planets based on an astronomical perspective; it's clear that Pluto, Eris and things like it, astronomically, belong to a different

I think it's fair to define planets based on an astronomical perspective; it's clear that Pluto, Eris and things like it, astronomically, belong to a different class than the other planets. Pluto is now classified as a dwarf planet. If we didn't "demote" it to a dwarf planet, then we would have to define a new term for the existing planets.