“Pluto was removed from the list of planets because it is too small”

The idea that Pluto was demoted from planet status purely due to its diminutive size is a widespread misconception, though it touches on a kernel of truth. While Pluto is indeed smaller than all other planets, its size alone was not the sole or primary reason for its reclassification. This myth likely took hold because "too small" is an easily digestible explanation, especially when complex astronomical definitions are simplified for public consumption.

The scientific community, specifically the International Astronomical Union (IAU), formally redefined what constitutes a planet in 2006. This new definition established three criteria: a celestial body must orbit the Sun, be massive enough for its own gravity to pull it into a nearly round shape, and, crucially, have cleared its orbital neighborhood of other debris. Pluto meets the first two criteria, but it fails the third. Its orbit is shared with numerous other objects in the Kuiper Belt, including several that are quite substantial in size, such as Eris, which is comparable to Pluto.

People commonly believe the size myth because it offers an intuitive explanation for Pluto's reclassification. Before 2006, there wasn't a universally accepted, rigorous definition for a planet, and the discovery of other large, icy bodies beyond Neptune, some nearly as big as Pluto, forced astronomers to address this ambiguity. The realization that Pluto was just one of many similar objects in the Kuiper Belt, combined with its failure to dominate its orbital path, led to the creation of the "dwarf planet" category, into which Pluto was subsequently placed. Therefore, while its relative smallness contributed to the discussion, it was the specific orbital criterion that ultimately sealed its fate as a dwarf planet.