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Why did the International Astronomical Union classify Pluto as a dwarf planet in 2006?

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Kuiper belt binary object — Kuiper belt An artist's rendition of a binary object in the Kuiper belt. The two objects depicted orbit each other at the edge of the solar system.

In 2006, the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet because it failed to meet the formal criteria established for a major planet.12

To be considered a planet in our solar system, a celestial body must satisfy three conditions:
* It must orbit the Sun.
* It must be massive enough for its own gravity to have pulled it into a spherical shape.
* It must have "cleared the neighborhood" around its orbit, meaning its gravity is sufficient to have removed other rocky and icy debris from its orbital path.23

While Pluto meets the first two requirements, it fails the third. Astronomers discovered that Pluto is a large member of the Kuiper belt—a vast region beyond Neptune filled with icy and rocky debris left over from the solar system's formation. Because Pluto shares its orbital environment with these other objects rather than dominating it, the IAU determined it did not qualify as a major planet.1423