Pluto's planetary status revoked 20 years ago: Why?

Twenty years ago, on August 24, 2006, the International Astronomical Union redefined what constitutes a planet, a decision that led to Pluto's reclassification. The IAU established three criteria for an object to be considered a planet in our solar system: it must orbit the Sun, possess enough mass to be nearly spherical, and have cleared its orbital neighborhood. Pluto meets the first two criteria, orbiting the Sun and maintaining a nearly spherical shape due to its gravity.

However, Pluto fails the third criterion. "Clearing the neighborhood" means being the gravitationally dominant body in its orbital zone, either by incorporating or expelling other objects. Pluto shares its orbital region with numerous bodies in the Kuiper Belt and lacks the mass to impose dominance. Its orbit does not define the structure of its environment; it is part of a larger population.

The discovery of Eris, a trans-Neptunian object comparable in size to Pluto, was a turning point. If Pluto were to remain a planet, Eris and potentially many other Kuiper Belt objects would also qualify, leading to a rapidly expanding planetary list. This prompted the IAU to establish a new category, "dwarf planet," to accommodate spherical objects that orbit the Sun but do not dominate their orbital regions. Pluto now resides in this category alongside Ceres and Eris.

This distinction between planets and dwarf planets helps astronomers differentiate between distinct dynamic systems within the solar system. The reclassification, made in Prague, provided a more organized framework for understanding the solar system's architecture.