Pluto is classified as a dwarf planet due to a redefinition of what constitutes a planet by the International Astronomical Union (IAU) in 2006. Prior to this reclassification, Pluto was considered the ninth planet in our solar system.
The IAU defined a planet as a celestial body that orbits the Sun, is spherical in shape, and has cleared its orbit of other debris. While Pluto meets the first two criteria, it fails to fulfill the third criterion. Pluto's orbit intersects with that of Neptune, and it shares its orbit with other objects in the Kuiper Belt, a region of the solar system beyond Neptune populated by numerous icy bodies.
As a result of this reclassification, Pluto was reclassified as a dwarf planet, a new category that includes other celestial bodies like Eris, Haumea, Makemake, and Ceres.