Minor planet
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]
"Planetoid" redirects here. For one specific type of planet-like astronomical objects, see dwarf planet. For other planetoids, see planetoid (disambiguation).
Minor planets include asteroids (near-Earth objects, Mars-crossers, main-belt asteroids and Jupiter trojans), as well as distant minor planets (centaurs and trans-Neptunian objects), most of which reside in the Kuiper belt and the scattered disc. As of June 2024, there are 1,364,189 known objects, divided into 699,991 numbered (secured discoveries) and 664,198 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.
Hundreds of thousands of minor planets have been discovered within the Solar System and thousands more are discovered each month. The Minor Planet Center has documented over 213 million observations and 794,832 minor planets, of which 541,128 have orbits known well enough to be assigned permanent official numbers.[9][10] Of these, 21,922 have official names.[9] As of 8 November 2021, the lowest-numbered unnamed minor planet is (4596) 1981 QB,[11] and the highest-numbered named minor planet is 594913 ꞌAylóꞌchaxnim.[12]
There are various broad minor-planet populations:
Physical properties of comets and minor planets[edit]
Commission 15[27] of the International Astronomical Union is dedicated to the Physical Study of Comets & Minor Planets.
Archival data on the physical properties of comets and minor planets are found in the PDS Asteroid/Dust Archive.[28] This includes standard asteroid physical characteristics such as the properties of binary systems, occultation timings and diameters, masses, densities, rotation periods, surface temperatures, albedoes, spin vectors, taxonomy, and absolute magnitudes and slopes. In addition, European Asteroid Research Node (E.A.R.N.), an association of asteroid research groups, maintains a Data Base of Physical and Dynamical Properties of Near Earth Asteroids.[29]