Pluto is no longer considered a planet because in 2006 the International Astronomical Union (IAU) redefined what constitutes a planet, and Pluto did not meet all the criteria. Specifically, Pluto meets two of the three criteria for a full-sized planet: it orbits the Sun, and it is massive enough to be nearly round in shape. However, it does not meet the third criterion: it has not "cleared its neighboring region of other objects," meaning it shares its orbital area with other objects in the Kuiper Belt and is not gravitationally dominant there. Because of this, Pluto was reclassified as a "dwarf planet" rather than a full planet.
Criteria for Planet Classification
The IAU defines a planet as a celestial body that:
- Orbits the Sun.
- Has sufficient mass to assume a nearly round shape (hydrostatic equilibrium).
- Has cleared the neighborhood around its orbit, meaning it is gravitationally dominant in its orbital zone.
Pluto fails the third test because it lives in a crowded space of other similar sized objects, such as those in the Kuiper Belt. Therefore, it cannot be classified as a planet under the current IAU definition.
Historical Context
Pluto was discovered in 1930 and was originally classified as the ninth planet of the Solar System. However, with the discovery in the 1990s and early 2000s of many other similar-sized objects beyond Neptune in the Kuiper Belt, scientists debated Pluto's status. This led to the IAU's 2006 decision to redefine the term "planet," resulting in Pluto's reclassification.
Pluto's New Status
Pluto is now officially classified as a dwarf planet, a category for objects that meet the first two criteria but not the third. This class also includes other objects in the Kuiper Belt like Eris and Haumea.
