Pluto is a Planet again...?

Pluto is invited back into the "planets" gang! Why was he kicked out in the first place?

Fun Facts:
This is the first episode to feature all the planets of the Solar System, along with a dwarf planet at the same time. It is also the first time Neptune is seen, though as this is part of a dream sequence, this isn't considered the debut of the character.
While Earth explains to Pluto how the Earthlings promoted him to a planet during the dream sequence, Pluto is depicted as being larger than the Moon. In reality, Pluto is around 2/3rds the size of the Moon.