On 24 August, the International Astronomical Union will vote on a proposed new definition of the term “planet” (see Three new planets may join solar system).
Here are the new categories it is proposing:
Planet: A round object orbiting a star. More precisely, according to the draft definition: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”
Pluton: A planet orbiting beyond Neptune, taking more than 200 Earth years to circle the Sun. So far, it would include Pluto; Pluto’s former moon, Charon; and Xena (2003 UB313).
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Satellite: Anything orbiting a planet, as long as the mutual centre of gravity does not fall outside the planet. Includes several bodies much larger than many planets, such as Jupiter’s moon Ganymede (diameter: 5262 kilometres).
Small solar system body: Anything orbiting the Sun that’s not a planet or a satellite. Most asteroids and comets would be SSSBs. Currently called minor planets.
Unofficial categories
Several unofficial, sub-categories of planets are also being discussed:
Dwarf planet: A planet smaller than Mercury (diameter: 4879 kilometres), which is the smallest uncontested planet. Would include the former asteroid Ceres; Pluto; Charon; and Xena.
Giant planet: Jupiter, Saturn, Uranus, Neptune.
Classical planet: The four giant planets plus the familiar four rocky, terrestrial planets: Mercury, Venus, Earth, Mars.
