1. Orbits the Sun: The object must be in orbit around the Sun, not another star or planet.
2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. This means that the object's own gravity is strong enough to pull it into a nearly round shape, rather than an irregular one.
3. Has cleared the neighborhood around its orbit. This means that the object has gravitationally dominated the region around its orbit and there are no other objects of comparable size.
Pluto was reclassified as a dwarf planet in 2006 because it does not meet the third criterion, as it shares its orbit with other objects in the Kuiper Belt.
In addition to these three main criteria, the IAU also specifies that a planet must not be a satellite of another planet.
There is still ongoing debate about the definition of a planet. Some scientists argue that the third criterion, "clearing the neighborhood around its orbit," is too strict and that other objects in the solar system should be considered planets. This debate is likely to continue as we learn more about the objects in our solar system and beyond.
