The primary finding that led astronomers to doubt Pluto's status as a planet was the discovery of other similar-sized objects in the Kuiper Belt, a region beyond Neptune that contains many icy bodies. The realization that Pluto is part of a larger category of objects, known as trans-Neptunian objects, raised questions about what constitutes a planet.
Additionally, Pluto's orbit is quite eccentric and inclined compared to the other planets, which further contributed to the debate. Ultimately, in 2006, the International Astronomical Union (IAU) redefined the criteria for planet classification, and Pluto was reclassified as a "dwarf planet." This decision was based on the fact that Pluto does not clear its orbit of other debris, which is one of the criteria for defining a full planet.