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Why did Pluto fail the International Astronomical Union’s definition of a planet?

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Illustration showing the eight planets of the solar system in order from the Sun-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune-depicted to scale by size, with labeled diameters in miles and kilometers. The Sun is partially visible on the left for comparison. — Solar system The planets (in comparative size) in order of distance from the Sun.

In 2006, the International Astronomical Union (IAU) established a formal definition for a planet within our solar system. To qualify, a celestial body must meet three specific criteria:

It must orbit the Sun.
It must have sufficient mass to assume a nearly round shape through its own gravity.
It must have "cleared the neighbourhood" around its orbit, meaning its gravity has removed other rocky and icy debris from its orbital path.

Pluto meets the first two conditions but fails the third. Because it resides within the Kuiper belt—a region populated by numerous other icy bodies and debris—it lacks the gravitational dominance required to clear its orbital neighborhood. Consequently, the IAU reclassified Pluto as a dwarf planet, a category for objects that are spherical and orbit the Sun but have not cleared their orbital surroundings.123