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[very fast rap]
In the summer of 2006, the International Astronomical Union (IAU) engaged in a heated discussion about Pluto’s status as a planet. The outcome was a new definition of a planet within our solar system that did not include Pluto, effectively demoting it. Concurrently, the IAU introduced the category of “dwarf planets,” which Pluto now belonged to. This decision was met with opposition from numerous scientists who found the criteria to be lacking in scientific rigor and called for a revision.
The IAU’s criteria for a body to be classified as a planet are threefold: it must orbit the Sun, assume a nearly spherical shape due to its own gravitational forces, and have cleared its orbital path of other debris. Pluto was disqualified based on the third criterion, as its orbit overlaps with that of the Kuiper belt, making it part of this region.
For an object to be considered a dwarf planet, it must satisfy the first two planetary criteria but fail the third; it should not have cleared its orbital zone and must not be a satellite of another planet. Under this definition, Pluto, the asteroid Ceres, and the Kuiper belt object Eris (discovered in 2005) are categorized as dwarf planets. However, Charon, despite being more than half the size of Pluto, is excluded from this classification because it is a moon of Pluto. It is anticipated that more celestial bodies will join the ranks of dwarf planets as they are discovered and found to fit these criteria.
Two years later, in 2008, the IAU further refined the classification by introducing “plutoids” as a subset of dwarf planets. Plutoids are distinguished by their orbits lying beyond Neptune, making them some of the largest members of the Kuiper belt. Among the dwarf planets, Pluto and Eris are recognized as plutoids, while Ceres is not, due to its position in the asteroid belt.
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