Pluto is no longer classified as a full-fledged planet because, in 2006, the International Astronomical Union (IAU) redefined what it means to be a planet, and Pluto did not meet all the new criteria.
The Redefinition of a Planet
While NASA conducts extensive research and exploration of celestial bodies, the official classification of objects like planets falls under the purview of the International Astronomical Union (IAU). This global organization of professional astronomers is responsible for establishing definitions and naming conventions for astronomical objects.
The IAU's Three Criteria for a Planet
The IAU established three key criteria that a celestial body must meet to be considered a planet within our solar system:
- Criterion 1: It must orbit the Sun. This means the object must revolve directly around our star, rather than orbiting another planet. (Pluto fulfills this.)
- Criterion 2: It must have sufficient mass to assume hydrostatic equilibrium (a nearly round shape). This implies the object's own gravity is strong enough to pull it into a roughly spherical form. (Pluto fulfills this.)
- Criterion 3: It must have "cleared its neighborhood" around its orbit. This is the crucial criterion Pluto fails. It means the object must be gravitationally dominant enough to have swept up or ejected most of the other smaller objects in its orbital path.
Here's a summary of the criteria and Pluto's status:
| Criterion | Description | Pluto's Status |
|---|---|---|
| 1. Orbits the Sun | The object must revolve around our star. | Yes |
| 2. Sufficient Mass | It must be massive enough for its own gravity to pull it into a nearly round shape. | Yes |
| 3. Cleared its Neighborhood | It must have gravitationally cleared the area around its orbit of other objects. | No |
Why Pluto Fails the Third Criterion
Pluto's inability to clear its orbital path is the primary reason for its reclassification. Unlike the eight official planets, Pluto resides in the Kuiper Belt, a vast region of icy bodies and dwarf planets beyond Neptune. Its orbit is filled with numerous other objects of significant size, demonstrating that it has not become gravitationally dominant enough to sweep up or eject these neighboring debris.
This distinction led the IAU to create a new category: dwarf planet. Pluto, along with other celestial bodies like Eris and Ceres, now falls under this classification. The reclassification wasn't aimed at "demoting" Pluto but rather at refining our understanding and categorization of objects in the solar system, especially with the discovery of many other similar-sized bodies in the Kuiper Belt. For more details on Pluto's reclassification, you can refer to resources like the Library of Congress.
