 The big two are Ceres and Aries. The realization that Ceres exists came first in the early 1800s. It was born out of the search for a planet in between Mars and Jupiter. Then there was the dwarf planet Aries. When Aries was found, that was the catalyst to declassify Pluto because all of a sudden you found an object that was more massive than Pluto, right? So either you also call that object a planet