Why Was Pluto Reclassified? The Science and History Behind the Planet Definition
Pluto was reclassified in 2006 from the solar system's ninth planet to a dwarf planet after the discovery of numerous icy worlds in the Kuiper Belt. To resolve taxonomic ambiguity, the International Astronomical Union defined a planet as a body orbiting the Sun that maintains hydrostatic equilibrium and clears its orbital neighborhood—a criterion Pluto does not meet.