In 2006, the International Astronomical Union created a classification to describe celestial bodies that aren’t planets but are too big to be other known space objects such as asteroids. The term dwarf planets refers to these bodies and the definition rests on several factors. It must:
- orbit the Sun
- be smaller than the planet Mercury
- be large enough for its own gravity to have rounded its shape substantially
- have not cleared it’s own orbit of smaller objects
Based on these criteria, Pluto was demoted from a planet to a dwarf, causing quite some controversy. Along with Pluto, the other dwarf planets are Ceres, Eris, Haumea, and Makemake.
Pluto was first identified as the ninth planet from the sun in 1930. After the definition of dwarf planets was identified, it was re-classified because there were several objects of similar size.
Eris is larger than Pluto and its discovery prompted the IAU to reconsider the definition of the term planet. Between Mars and Jupiter, Ceres is the smallest of the known dwarf planets. It’s so small that is can also be classified as an asteroid, albeit a large one.
The size of Makemake could also have stripped Pluto of its status as the ninth planet. It’s currently the second-brightest object in the Kuiper Belt— a very large region extending beyond the orbit of Neptune.
Haumea is smaller than Pluto but has a unique, elongated shape due to its fast rotational spin. Astronomers say further exploration of the Kuiper Belt could lead to the discovery of even more dwarf planets.