Pluto is not classified as a planet because it doesn't meet the International Astronomical Union's (IAU) 2006 criteria for a planet. According to the IAU, a planet must: 1. Orbit the Sun. 2. Be spherical or nearly so (in hydrostatic equilibrium). 3. Clear its orbit of other debris. Pluto satisfies the first two but fails the third—it resides in the Kuiper Belt, a region with many other objects, and hasn't cleared its orbit. Instead, it's classified as a **dwarf planet**, a body that meets the first two criteria but not the third. This reclassification reflects a refined understanding of our solar system's diversity, distinguishing planets from smaller bodies like Pluto, Eris, and Ceres.