1. It must be in orbit around the Sun. This means it is not a moon orbiting a planet, or a star with its own planetary system.
2. It must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. This basically means it's round, or nearly round, due to its own gravity.
3. It must have cleared the neighbourhood around its orbit. This means it is the dominant gravitational influence in its orbit and has "cleared out" other objects in its path.
These criteria were established after the discovery of Eris, a dwarf planet in the Kuiper belt, which is similar in size to Pluto. The debate over Pluto's planet status led to the IAU defining the criteria for planet status to avoid similar future arguments.
Here's a breakdown of the criteria:
* Orbiting the Sun: This is straightforward. A planet must be orbiting our star, the Sun.
* Hydrostatic equilibrium: This means the object's gravity is strong enough to pull it into a nearly round shape, overcoming its own internal rigidity.
* Clearing the neighborhood: This is the most controversial part. Planets are large enough to dominate their orbital path and "sweep up" or fling away other objects. Dwarf planets, like Pluto, haven't cleared their orbital paths.
So, while Pluto meets the first two criteria, it fails the third and is now classified as a dwarf planet.
It's important to remember that these are human-defined criteria. There is no scientific consensus on what makes a planet, and the debate continues. However, the IAU's definition is the most widely accepted at this time.
