When Pluto was discovered, astronomers expected to find a planet similar to Earth in size. But as they made more observations, they realized that Pluto is actually much smaller than what they initially thought.

Since the early 20th century, the International Astronomical Union or IAU – an organization that brings together astronomers from around the world – defines the names used by astronomers to talk about objects in the Solar System. In August 2006, the IAU decided a planet is an object that