planethood

Planethood is the status of being a planet, a category of celestial bodies that orbit stars and have certain physical characteristics. The concept has varied across time; modern usage centers on the definition adopted by the International Astronomical Union (IAU) in 2006: a planet is a celestial body that orbits the Sun, has sufficient mass for hydrostatic equilibrium (is nearly round), and has cleared its orbital neighborhood of other debris. Objects satisfying the first two criteria but not the third are classed as dwarf planets, such as Pluto, Eris, and Ceres. Bodies that do not orbit the Sun, such as moons themselves or asteroids, are not planets under this definition. The requirement to orbit the Sun means the IAU definition currently excludes planets around other stars; such bodies are commonly called exoplanets.

The eight recognized planets of the Solar System under the IAU definition are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was reclassified as a dwarf planet in 2006, prompting public and scientific debate over planethood.

There are alternative or historical notions of planethood. Some scientists favor broader, geophysical criteria that would include any spherical body in orbit around a star, while others propose definitions that would apply to exoplanets. In practice, the term "planet" is often used flexibly in non-IAU contexts, especially for exoplanets, but IAU terminology remains the standard in astronomy.