When the International Astronomical Union (IAU) demoted Pluto from a planet to a dwarf planet in 2006, it surprised a lot of people, including some scientists. Even many years later, some astronomers want to revise the definition of a planet to clarify the parameters that set planets apart from other celestial objects.
But why isn't Pluto considered a planet anymore? It starts with the definition of a planet — or lack thereof. Before 2006, there weren't strict criteria for a planet. Instead, planets were loosely regarded as objects larger than asteroids that orbited the sun. In the mid-1800s, for example, more than a dozen objects that we now regard as asteroids were considered to be planets.
