Pluto’s identity as a planet was called into question when discoveries were made in the far reaches of the solar system. Eris, Sedna, Makemake, and Haumea are just a few of over 1200 objects discovered beyond Neptune.
This forced astronomers to come up with an official definition of a planet. They argued until they came up with three requirements.
- It must be in orbit around the Sun.
- It must be big enough to be pulled into a sphere by its own gravity
- It must have cleared the neighbourhood around its orbit of other objects.
Put simply, Pluto didn’t meet the third criterion. This makes it a dwarf planet, along with Ceres, Haumea, Makemake, and Eris. Astronomers think there could be thousands of ‘dwarfs’ out there.
Short answer: Astronomers found so many ‘Plutos’ in recent years that they had to come up with an official classification system for planets and other objects.