Why is Pluto not a planet anymore?

In 2006 the International Astronomical Union adopted a formal definition requiring a planet to orbit the sun, to have enough mass that its own gravity pulls it into a nearly round shape, and to have cleared the neighborhood around its orbit of other comparable bodies. Pluto satisfies the first two and fails the third: it orbits within the Kuiper Belt among a large population of icy objects, and it is not gravitationally dominant there. It was reclassified as a dwarf planet, a category that also includes Eris, Ceres, Makemake, and Haumea.

The reclassification was driven by discovery rather than by opinion. Astronomers were finding increasing numbers of Kuiper Belt objects, and in 2005 Eris was found and initially estimated to be more massive than Pluto. Without a definition, the solar system was heading toward dozens of planets and counting. The choice was between expanding the list indefinitely and drawing a line, and the IAU drew a line.

The decision remains genuinely contested among scientists rather than settled. Critics including planetary scientists on the New Horizons mission argue that the clearing criterion is poorly defined, depends on orbital location rather than the body's own properties, and would technically disqualify Earth if it were placed in the Kuiper Belt. They favor a geophysical definition based on whether a body is round under its own gravity, which would restore Pluto and add many moons. The IAU definition remains the official one, and New Horizons' 2015 flyby revealed Pluto as a geologically complex world with mountains, glaciers of nitrogen ice, and possibly a subsurface ocean, which did not help the case for treating it as unremarkable.