1. It is in orbit around the Sun. This means the object must be gravitationally bound to the Sun and not another star.
2. It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. This means the object must be large enough for its own gravity to pull it into a nearly spherical shape.
3. It has cleared the neighborhood around its orbit. This is the most controversial criterion. It means the object must be the dominant gravitational influence in its orbit, and it has cleared away any other objects in its path.
Objects that meet the first two criteria but not the third are classified as dwarf planets. For example, Pluto is a dwarf planet because it shares its orbit with other objects in the Kuiper Belt.
Here's a breakdown of the criteria:
* Orbiting the Sun: This is the most fundamental requirement. It ensures that the object is part of our solar system.
* Hydrostatic Equilibrium: This ensures that the object is large enough to have a nearly spherical shape.
* Clearing the Neighborhood: This criterion is the most debated and has led to the reclassification of Pluto as a dwarf planet. It is intended to distinguish planets from smaller objects that share their orbits.
The IAU's definition of a planet is still a subject of debate, and some scientists argue that the "clearing the neighborhood" criterion is too restrictive. However, the current definition is the one that is officially recognized by the scientific community.
