Minor planet

Various visited minor planets and their diversity:

• Pluto and Ceres
• Vesta, Lutetia, and Arrokoth
• Itokawa, Bennu, and Dinkinesh and its satellite Selam

Sizes are not to scale.

According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet.^[a] Before 2006, the IAU officially used the term minor planet, but that year's meeting reclassified minor planets and comets into dwarf planets and small Solar System bodies (SSSBs).^[1] In contrast to the eight official planets of the Solar System, all minor planets fail to clear their orbital neighborhood.^[2][1]

Minor planets include asteroids (near-Earth objects, Earth trojans, Mars trojans, Mars-crossers, main-belt asteroids and Jupiter trojans), as well as distant minor planets (Uranus trojans, Neptune trojans, centaurs and trans-Neptunian objects), most of which reside in the Kuiper belt and the scattered disc. As of October 2024^[update], there are 1,392,085 known objects, divided into 740,000 numbered, with only one of them recognized as a dwarf planet (secured discoveries) and 652,085 unnumbered minor planets, with only five of those officially recognized as a dwarf planet.^[3]

The first minor planet to be discovered was Ceres in 1801, though it was called a 'planet' at the time and an 'asteroid' soon after; the term minor planet was not introduced until 1841, and was considered a subcategory of 'planet' until 1932.^[4] The term planetoid has also been used, especially for larger, planetary objects such as those the IAU has called dwarf planets since 2006.^[5][6] Historically, the terms asteroid, minor planet, and planetoid have been more or less synonymous.^[5][7] This terminology has become more complicated by the discovery of numerous minor planets beyond the orbit of Jupiter, especially trans-Neptunian objects that are generally not considered asteroids.^[7] A minor planet seen releasing gas may be dually classified as a comet.

Objects are called dwarf planets if their own gravity is sufficient to achieve hydrostatic equilibrium and form an ellipsoidal shape. All other minor planets and comets are called small Solar System bodies.^[1] The IAU stated that the term minor planet may still be used, but the term small Solar System body will be preferred.^[8] However, for purposes of numbering and naming, the traditional distinction between minor planet and comet is still used.

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