As of 2006, the International Astronomical Union defines a planet this way:[1]
- It must orbit a star (in our cosmic neighborhood, the Sun).
- It must be big enough to have enough gravity to force it into a spherical shape.
- It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.
Dwarf planet[]
A "dwarf planet" is a celestial body that[1]
- is in orbit around the Sun
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- has not cleared the neighbourhood around its orbit, and
- is not a satellite.
Other[]
- A celestial body that meets a planet requirement but is located outside the Solar System is called an exoplanet.
- Smaller planets that are not massive enough to be rounded by their own gravity are called minor planets.