This is a must-read article for anyone who cares about whether Pluto is a planet or not. I didn’t know this history: Ceres went through the same progression as Pluto.
As the first body discovered in the now-named asteroid belt (and not coincidentally the largest), Ceres was originally classified as a planet. When science learned that Ceres was far from unique, it was demoted along with its asteroid peers.
Originally shared by Brian Koberlein
Many people lament the demotion of Pluto from a planet to a dwarf planet. I actually think the demotion was the right choice. It has also happened before, to a planet known as Ceres.