IAU definition of planet

The International Astronomical Union’s 2006 definition classifies a Solar System body as a planet if it orbits the Sun, is nearly round, and has cleared its orbital neighborhood.

Hydrostatic equilibrium: A state in which a body's gravity balances internal pressure, producing an approximately spherical shape. This physical condition underlies the definition’s requirement that a planet be nearly round.
International Astronomical Union: An international organization of professional astronomers that coordinates astronomical nomenclature and standards. The IAU adopted the Solar System planet definition at its 2006 General Assembly.
Geophysical definition of planet: A classification that treats bodies as planets when they are massive enough for gravity to make them nearly round. Unlike the IAU definition, it does not require a body to clear its orbital neighborhood.
Resolution 5A: The 2006 IAU resolution that defined planets and dwarf planets in the Solar System. Its wording formalized the three criteria and placed Pluto outside the planet category.
Orbital clearing: The dynamical process by which a body becomes gravitationally dominant among objects sharing its orbital region. This criterion distinguishes planets from bodies such as Pluto that share their orbital zone.
2006 IAU General Assembly: The International Astronomical Union meeting in Prague where members adopted Resolution 5A on planets. Its vote established the definition and the separate dwarf-planet category.
Planetary nomenclature: The system of names and classification conventions used for planets and other bodies in planetary science. The IAU definition is one formal rule within the broader practice of naming and classifying worlds.
Solar System: The Sun and the objects gravitationally bound to it, including planets, dwarf planets, moons, and smaller bodies. The resolution applies its classification specifically to bodies orbiting the Sun.
Planetary-mass object: An astronomical body massive enough for gravity to shape it, regardless of whether it dominates its orbit. The IAU criteria narrow this broader class by adding an orbit and neighborhood requirement.
Eris (dwarf planet): A distant dwarf planet discovered in 2005 whose size sharpened debate over Pluto’s status. Eris’s discovery prompted astronomers to decide whether Pluto belonged in a larger class of planets.

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