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).
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 May 2022[update], there are 1,131,201 known objects, divided into 611,678 numbered (secured discoveries) and 519,523 unnumbered minor planets, with only five of those officially recognized as a dwarf planet.
The first minor planet to be discovered was Ceres in 1801. The term minor planet has been used since the 19th century to describe these objects. The term planetoid has also been used, especially for larger, planetary objects such as those the IAU has called dwarf planets since 2006. Historically, the terms asteroid, minor planet, and planetoid have been more or less synonymous. 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. 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. The IAU stated that the term minor planet may still be used, but the term small Solar System body will be preferred. 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. Of these, 21,922 have official names. As of 8 November 2021[update], the lowest-numbered unnamed minor planet is (4596) 1981 QB, and the highest-numbered named minor planet is 594913 ꞌAylóꞌchaxnim.
A newly discovered minor planet is given a provisional designation. For example, the provisional designation 2002 AT4 consists of the year of discovery (2002) and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number, but dropping the parentheses is quite common. Informally, it is common to drop the number altogether or to drop it after the first mention when a name is repeated in running text.
Minor planets that have been given a number but not a name keep their provisional designation, e.g. (29075) 1950 DA. Because modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. The earliest discovered to be left unnamed was for a long time (3360) 1981 VA, now 3360 Syrinx. In November 2006 its position as the lowest-numbered unnamed asteroid passed to (3708) 1974 FV1 (now 3708 Socus), and in May 2021 to (4596) 1981 QB. On rare occasions, a small object's provisional designation may become used as a name in itself: the then-unnamed (15760) 1992 QB1 gave its "name" to a group of objects that became known as classical Kuiper belt objects ("cubewanos") before it was finally named 15760 Albion in January 2018.
Minor planets are awarded an official number once their orbits are confirmed. With the increasing rapidity of discovery, these are now six-figure numbers. The switch from five figures to six figures arrived with the publication of the Minor Planet Circular (MPC) of October 19, 2005, which saw the highest-numbered minor planet jump from 99947 to 118161.
Archival data on the physical properties of comets and minor planets are found in the PDS Asteroid/Dust Archive. 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.
Environmental characteristics have three aspects: space environment, surface environment and internal environment, including geological, optical, thermal and radiological environmental properties, etc., which are the basis for understanding the basic properties of minor planets, carrying out scientific research, and are also an important reference basis for designing the payload of exploration missions
Without the protection of an atmosphere and its own strong magnetic field, the minor planet's surface is directly exposed to the surrounding radiation environment. In the cosmic space where minor planets are located, the radiation on the surface of the planets can be divided into two categories according to their sources: one comes from the sun, including electromagnetic radiation from the sun, and ionizing radiation from the solar wind and solar energy particles; the other comes from the sun outside the solar system, that is, galactic cosmic rays, etc.
Usually during one rotation period of a minor planet, the albedo of a minor planet will change slightly due to its irregular shape and uneven distribution of material composition. This small change will be reflected in the periodic change of the planet's light curve, which can be observed by ground-based equipment, so as to obtain the planet's magnitude, rotation period, rotation axis orientation, shape, albedo distribution, and scattering properties. Generally speaking, the albedo of minor planets is usually low, and the overall statistical distribution is bimodal, corresponding to C-type (average 0.035) and S-type (average 0.15) minor planets. In the minor planet exploration mission, measuring the albedo and color changes of the planet surface is also the most basic method to directly know the difference in the material composition of the planet surface.
The geological environment on the surface of minor planets is similar to that of other unprotected celestial bodies, with the most widespread geomorphological feature present being impact craters: however, the fact that most minor planets are rubble pile structures, which are loose and porous, gives the impact action on the surface of minor planets its unique characteristics. On highly porous minor planets, small impact events produce spatter blankets similar to common impact events: whereas large impact events are dominated by compaction and spatter blankets are difficult to form, and the longer the planets receive such large impacts, the greater the overall density. In addition, statistical analysis of impact craters is an important means of obtaining information on the age of a planet surface. Although the Crater Size-Frequency Distribution (CSFD) method of dating commonly used on minor planet surfaces does not allow absolute ages to be obtained, it can be used to determine the relative ages of different geological bodies for comparison. In addition to impact, there are a variety of other rich geological effects on the surface of minor planets, such as mass wasting on slopes and impact crater walls, large-scale linear features associated with graben, and electrostatic transport of dust. By analysing the various geological processes on the surface of minor planets, it is possible to learn about the possible internal activity at this stage and some of the key evolutionary information about the long-term interaction with the external environment, which may lead to some indication of the nature of the parent body's origin. Many of the larger planets are often covered by a layer of soil (regolith) of unknown thickness. Compared to other atmosphere-free bodies in the solar system (e.g. the Moon), minor planets have weaker gravity fields and are less capable of retaining fine-grained material, resulting in a somewhat larger surface soil layer size. Soil layers are inevitably subject to intense space weathering that alters their physical and chemical properties due to direct exposure to the surrounding space environment. In silicate-rich soils, the outer layers of Fe are reduced to nano-phase Fe (np-Fe), which is the main product of space weathering. For some small planets, their surfaces are more exposed as boulders of varying sizes, up to 100 metres in diameter, due to their weaker gravitational pull. These boulders are of high scientific interest, as they may be either deeply buried material excavated by impact action or fragments of the planet's parent body that have survived. The rocks provide more direct and primitive information about the material inside the minor planet and the nature of its parent body than the soil layer, and the different colours and forms of the rocks indicate different sources of material on the surface of the minor planet or different evolutionary processes.
The Minor Planet Center (MPC) is the single worldwide location for receipt and distribution of positional measurements of minor planets, comets and outer irregular natural satellites of the major planets. The MPC is responsible for the identification, designation and orbit computation for all of these objects. This involves maintaining the master files of observations and orbits, keeping track of the discoverer of each object, and announcing discoveries to the rest of the world via electronic circulars and an extensive website. The MPC operates at the Smithsonian Astrophysical Observatory, under the auspices of Division F of the International Astronomical Union (IAU). 041b061a72