Pluto is not considered a planet because in 2006, the International Astronomical Union redefined the definition of a planet, and Pluto did not meet the new criteria. According to the new definition, a planet must orbit the sun, be spherical in shape, and have cleared its orbit of other debris. Pluto meets the first two criteria but fails the third one as it shares its orbit with other objects in the Kuiper Belt. Therefore, it is now classified as a dwarf planet.