A planet is a celestial body that is in orbit around a star, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, has cleared the neighbourhood around its orbit, and is neither a star nor a moon of a planet.
A pair of objects are considered a double planet system if they independently satisfy the definition of a planet, and the common center of gravity of the system (known as the barycenter) lies outside either body.
Plutons are planets with highly inclined orbits with large eccentricities and an orbital period of more than 200 years. The term "dwarf planet" is used to describe all planets smaller than the Sol system planet named Mercury. In centuries past, these planets would have simply been considered nothing more than spherical asteroids.