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Why did the discovery of other Kuiper belt objects contribute to Pluto’s reclassification?

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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.

The discovery of numerous Kuiper belt objects (KBOs) beginning in the 1990s fundamentally changed the scientific understanding of Pluto, leading to its 2006 reclassification by the International Astronomical Union.12

Prior to these discoveries, Pluto was viewed as a unique, isolated entity at the edge of the solar system. However, the identification of many other icy bodies in the same region—some of which, like Eris, were comparable in size to Pluto—revealed that Pluto was not a singular planet but rather one of the largest members of a vast population of debris left over from the formation of the solar system.231

This realization prompted the IAU to establish a formal definition for a planet, which requires an object to have "cleared the neighbourhood" around its orbit. Because Pluto resides within the Kuiper belt and lacks the mass to exert sufficient gravitational dominance to clear its orbital path of other icy and rocky debris, it failed to meet this criterion. Consequently, Pluto was reclassified as a dwarf planet.432