Pluto is "left out" of the list of full-sized planets because, according to the International Astronomical Union (IAU), it does not meet all three established criteria for a planet, specifically failing to clear its orbit of other objects. Its reclassification to a dwarf planet in 2006 marked a significant step in our evolving understanding of the solar system.
The IAU's Definition of a Planet
In 2006, the International Astronomical Union (IAU), the global authority responsible for naming celestial objects and defining astronomical terms, established three specific criteria for an object to be classified as a full-sized planet within our solar system:
- It must orbit the Sun: The object must follow a path around our star, the Sun.
- It must be nearly round: The object must have sufficient mass for its own gravity to pull it into a nearly spherical shape (achieving hydrostatic equilibrium).
- It must have "cleared the neighborhood" around its orbit: This means the object has become gravitationally dominant in its orbital path, having either swept up or ejected most of the other smaller objects from its vicinity.
Why Pluto Doesn't Fit the Bill
Pluto successfully meets the first two criteria: it orbits the Sun, and its gravity has pulled it into a nearly round, albeit slightly flattened, shape. However, Pluto fails to meet the third and final criterion.
| Criterion | Full Planet (e.g., Earth) | Pluto |
|---|---|---|
| Orbits the Sun | Yes | Yes |
| Nearly Round Shape | Yes | Yes |
| Cleared its Orbit | Yes | No |
- Failure to Clear its Orbit: Pluto's orbit is located within the Kuiper Belt, a vast region beyond Neptune that is populated by thousands of icy bodies and other dwarf planets. Unlike Earth, Jupiter, or other full-sized planets, Pluto has not gravitationally cleared its orbital path of these numerous other objects. It shares its orbital region with many other significant bodies, indicating that it is not gravitationally dominant in its neighborhood.
This distinction is crucial because it differentiates full-sized planets, which have established clear paths, from smaller bodies that coexist within crowded regions. The reclassification of Pluto helped to bring consistency to the classification of celestial bodies within our solar system and paved the way for understanding the vast number of other objects beyond Neptune.
What is a Dwarf Planet?
A dwarf planet is a celestial body that orbits the Sun, is massive enough to be nearly round, but has not cleared the neighborhood around its orbit. Pluto is the most famous example, but others include Eris, Makemake, Haumea, and Ceres (located in the asteroid belt). This category allows for the classification of objects that are significant in their own right but do not fit the stricter definition of a planet. You can learn more about dwarf planets and their characteristics from reliable sources like NASA.
This reclassification reflects our evolving understanding of the solar system and the diverse objects within it, rather than diminishing Pluto's importance as a fascinating celestial body.
