Pluto is not considered a planet because it does not meet all the criteria set by the International Astronomical Union (IAU) for defining a planet. The key reason is that Pluto has not "cleared the neighborhood" around its orbit. This means Pluto shares its orbital space with many other objects in the Kuiper Belt and is not gravitationally dominant in its orbit. Pluto does orbit the Sun and is spherical in shape, but lacking orbital dominance caused it to be reclassified as a dwarf planet in 2006. This decision was based on new planetary definitions established to distinguish planets from other celestial bodies more clearly.
Criteria for a Planet by IAU
The IAU defines a planet as a celestial body that:
- Orbits the Sun.
- Is massive enough for its gravity to make it nearly round in shape.
- Has cleared other objects from its orbital path.
Pluto meets the first two criteria but fails the third since it resides in the Kuiper Belt, a region filled with other objects of similar size and it isn't the dominant gravitational body there.
Historical Context
Pluto was classified as the ninth planet after its discovery in 1930. However, later discoveries of other similar-sized bodies in the Kuiper Belt cast doubt on its status. In 2006, the IAU formally redefined the term "planet" and reclassified Pluto as a dwarf planet.
Ongoing Debate
While the IAU's definition is widely accepted, some scientists and members of the public still argue for Pluto's planetary status based on its round shape and orbit around the Sun, as well as sentimental reasons. In summary, Pluto's reclassification arises from modern criteria emphasizing orbital dominance, which Pluto lacks, positioning it as a dwarf planet rather than a full planet in our Solar System.
