Well at one time people “knew” that space was filled with Aether, and people knew that the universe was no more than 2 million years old.

But once you understand things more, some of the things you know turn out to be wrong.

In this case it’s more like “we called this thing a planet before we knew just how tiny it really is but it’s still cool so we now have categories for things just like it”

Which I think is neat.

You can be sad it’s not a planet, we can also be happy that it pulled one over on all of humanity for such a long time.