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According to the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(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. 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).Press release, IAU 2006 General Assembly: Result of the IAU Resolution votes
International Astronomical Union, August 24, 2006. Accessed May 5, 2008.
Minor planets include asteroids (
near-Earth object A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (Apsis, perihelion) is less than 1.3 astronomical unit ...
s, Mars-crossers, main-belt asteroids and Jupiter trojans), as well as distant minor planets (
centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
and trans-Neptunian objects), most of which reside in the Kuiper belt and the scattered disc. , there are 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.Planet, asteroid, minor planet: A case study in astronomical nomenclature, David W. Hughes, Brian G. Marsden, ''Journal of Astronomical History and Heritage'' 10, #1 (2007), pp. 21–30. 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 Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...
, especially trans-Neptunian objects that are generally not considered asteroids.Asteroid
, ''MSN Encarta'', Microsoft. Accessed May 5, 2008

2009-11-01.
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.


Populations

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. , the lowest-numbered unnamed minor planet is , and the highest-numbered named minor planet is 594913 ꞌAylóꞌchaxnim. There are various broad minor-planet populations: * Asteroids; traditionally, most have been bodies in the inner Solar System. **
Near-Earth asteroids A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun ( perihelion) is less than 1.3 astronomical units (AU ...
, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is used: ***
Apohele asteroid Atira asteroids or Apohele asteroids, also known as interior-Earth objects (IEOs), are asteroids whose orbits are entirely confined within Earth's orbit; that is, their orbit has an aphelion (farthest point from the Sun) smaller than Earth's peri ...
s orbit inside of Earth's perihelion distance and thus are contained entirely within the orbit of Earth. *** Aten asteroids, those that have semi-major axes of less than Earth's and aphelion (furthest distance from the Sun) greater than 0.983 AU. *** Apollo asteroids are those asteroids with a semimajor axis greater than Earth's while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers. *** Amor asteroids are those near-
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
asteroids that approach the orbit of Earth from beyond but do not cross it. Amor asteroids are further subdivided into four subgroups, depending on where their semimajor axis falls between Earth's orbit and the asteroid belt; **
Earth trojan An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points (leading 60°) or (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name ...
s, asteroids sharing Earth's orbit and gravitationally locked to it. As of 2022, two Earth trojans are known: 2010 TK7 and 2020 XL5. ** Mars trojans, asteroids sharing Mars's orbit and gravitationally locked to it. As of 2007, eight such asteroids are known. ** Asteroid belt, whose members follow roughly circular orbits between Mars and Jupiter. These are the original and best-known group of asteroids. ** Jupiter trojans, asteroids sharing Jupiter's orbit and gravitationally locked to it. Numerically they are estimated to equal the main-belt asteroids. * Distant minor planets; an umbrella term for minor planets in the outer Solar System. **
Centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
, bodies in the outer Solar System between Jupiter and Neptune. They have unstable orbits due to the gravitational influence of the giant planets, and therefore must have come from elsewhere, probably outside Neptune. ** Neptune trojans, bodies sharing Neptune's orbit and gravitationally locked to it. Although only a handful are known, there is evidence that Neptune trojans are more numerous than either the asteroids in the asteroid belt or the Jupiter trojans. ** Trans-Neptunian objects, bodies at or beyond the orbit of Neptune, the outermost planet. *** The Kuiper belt, objects inside an apparent population drop-off approximately 55 AU from the Sun. ****
Classical Kuiper belt object A classical Kuiper belt object, also called a cubewano ( "QB1-o"), is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major a ...
s like Makemake, also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune. **** Resonant Kuiper belt objects *****
Plutinos In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the l ...
, bodies like that are in a 2:3 resonance with Neptune. *** Scattered disc objects like Eris, with aphelia outside the Kuiper belt. These are thought to have been scattered by Neptune. **** Resonant scattered disc objects. ***
Detached object Detached objects are a dynamical class of minor planets in the outer reaches of the Solar System and belong to the broader family of trans-Neptunian objects (TNOs). These objects have orbits whose points of closest approach to the Sun ( perihel ...
s such as Sedna, with both aphelia and perihelia outside the Kuiper belt. **** Sednoids, detached objects with perihelia greater than 75 AU (Sedna, , and Leleākūhonua). *** The Oort cloud, a hypothetical population thought to be the source of long-period comets that may extend to 50,000 AU from the Sun.


Naming conventions

All
astronomical bodies An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...
in the Solar System need a distinct designation. The naming of minor planets runs through a three-step process. First, a provisional designation is given upon discovery—because the object still may turn out to be a false positive or become lost later on—called a ''provisionally designated minor planet''. After the observation arc is accurate enough to predict its future location, a minor planet is formally designated and receives a number. It is then a ''numbered minor planet''. Finally, in the third step, it may be named by its discoverers. However, only a small fraction of all minor planets have been named. The vast majority are either numbered or have still only a provisional designation. Example of the naming process: * – provisional designation upon discovery on 24 April 1932 * – formal designation, receives an official number * 1862 Apollo – named minor planet, receives a name, the alphanumeric code is dropped


Provisional designation

A newly discovered minor planet is given a
provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calc ...
. For example, the provisional designation 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 , provisional designation ', is a risk–listed asteroid, classified as a near-Earth object and potentially hazardous asteroid of the Apollo group, approximately in diameter. It once had the highest known probability of impacting Earth. In 200 ...
. 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 (now
3708 Socus 3708 Socus ( ''provisional designation'': ) is a large Jupiter trojan from the Trojan camp, approximately in diameter. It was discovered on 21 March 1974, by staff members of the Cerro El Roble Observatory owned and operated by the Department o ...
), and in May 2021 to . On rare occasions, a small object's provisional designation may become used as a name in itself: the then-unnamed gave its "name" to a group of objects that became known as
classical Kuiper belt object A classical Kuiper belt object, also called a cubewano ( "QB1-o"), is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major a ...
s ("cubewanos") before it was finally named 15760 Albion in January 2018. A few objects are cross-listed as both comets and asteroids, such as
4015 Wilson–Harrington 4015 Wilson–Harrington is an active asteroid known both as comet 107P/Wilson–Harrington and as asteroid 4015 Wilson–Harrington. It will pass from Earth on 20 July 2022 and then pass perihelion (closest approach to the Sun) on 24 August 20 ...
, which is also listed as
107P/Wilson–Harrington 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1. I ...
.


Numbering

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 The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function ...
(MPC) of October 19, 2005, which saw the highest-numbered minor planet jump from 99947 to 118161.


Naming

The first few asteroids were named after figures from Greek and
Roman mythology Roman mythology is the body of myths of ancient Rome as represented in the literature and visual arts of the Romans. One of a wide variety of genres of Roman folklore, ''Roman mythology'' may also refer to the modern study of these representa ...
, but as such names started to dwindle the names of famous people, literary characters, discoverers' spouses, children, colleagues, and even television characters were used.


Gender

: The first asteroid to be given a non-mythological name was 20 Massalia, named after the Greek name for the city of
Marseille Marseille ( , , ; also spelled in English as Marseilles; oc, Marselha ) is the prefecture of the French department of Bouches-du-Rhône and capital of the Provence-Alpes-Côte d'Azur region. Situated in the camargue region of southern Fra ...
. The first to be given an entirely non-Classical name was
45 Eugenia Eugenia (minor planet designation: 45 Eugenia) is a large asteroid of the asteroid belt. It is famed as one of the first asteroids to be found to have a moon orbiting it. It was also the second triple asteroid to be discovered, after 87 Sylvia. ...
, named after Empress Eugénie de Montijo, the wife of
Napoleon III Napoleon III (Charles Louis Napoléon Bonaparte; 20 April 18089 January 1873) was the first President of France (as Louis-Napoléon Bonaparte) from 1848 to 1852 and the last monarch of France as Emperor of the French from 1852 to 1870. A neph ...
. For some time only female (or feminized) names were used; Alexander von Humboldt was the first man to have an asteroid named after him, but his name was feminized to
54 Alexandra Alexandra (minor planet designation: 54 Alexandra) is a carbonaceous asteroid from the intermediate asteroid belt, approximately 155 kilometers in diameter. It was discovered by German-French astronomer Hermann Goldschmidt on 10 September 1858, a ...
. This unspoken tradition lasted until 334 Chicago was named; even then, female names showed up in the list for years after.


Eccentric

: As the number of asteroids began to run into the hundreds, and eventually, in the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were
482 Petrina Petrina (minor planet designation: 482 Petrina) is a minor planet orbiting the Sun. Attempts to produce a light curve for this object have yielded differing synodic rotation periods, perhaps in part because the period is close to half an Earth da ...
and
483 Seppina Seppina (minor planet designation: 483 Seppina) is a minor planet orbiting the Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in i ...
, named after the discoverer's pet dogs. However, there was little controversy about this until 1971, upon the naming of 2309 Mr. Spock (the name of the discoverer's cat). Although the IAU subsequently discouraged the use of pet names as sources, eccentric asteroid names are still being proposed and accepted, such as 4321 Zero,
6042 Cheshirecat 6042 Cheshirecat ( ''or'' ), provisional designation , is an eccentric, rare-type asteroid and large Mars-crosser from the outer regions of the asteroid belt, approximately 14 kilometers in diameter. It was discovered by Japanese astronomers Akir ...
, 9007 James Bond, 13579 Allodd and 24680 Alleven, and
26858 Misterrogers 26858 Misterrogers (), provisional designation , is a stony asteroid and sizable Mars-crosser on an eccentric orbit from the asteroid belt, approximately in diameter. It was discovered on March 21, 1993, by American astronomer Eleanor Helin at th ...
.


Discoverer's name

: A well-established rule is that, unlike comets, minor planets may not be named after their discoverer(s). One way to circumvent this rule has been for astronomers to exchange the courtesy of naming their discoveries after each other. An exception to this rule is 96747 Crespodasilva, which was named after its discoverer,
Lucy d'Escoffier Crespo da Silva This is a list of minor-planet discoverers credited by the Minor Planet Center with the discovery of one or several minor planets (such as near-Earth and main-belt asteroids, Jupiter trojans and distant objects). , the discovery of 612,011 nu ...
, because she died shortly after the discovery, at age 22.


Languages

: Names were adapted to various languages from the beginning.
1 Ceres Ceres (; minor-planet designation: 1 Ceres) is a dwarf planet in the asteroid belt between the orbits of Mars and Jupiter. It was the first asteroid discovered, on 1 January 1801, by Giuseppe Piazzi at Palermo Astronomical Observatory in Sici ...
, ''Ceres'' being its Anglo-Latin name, was actually named ''Cerere'', the Italian form of the name. German, French, Arabic, and Hindi use forms similar to the English, whereas Russian uses a form, ''Tserera'', similar to the Italian. In Greek, the name was translated to Δήμητρα ( Demeter), the Greek equivalent of the Roman goddess Ceres. In the early years, before it started causing conflicts, asteroids named after Roman figures were generally translated in Greek; other examples are Ἥρα ( Hera) for
3 Juno ) , mp_category=Main belt (Juno clump) , orbit_ref = , epoch= JD 2457000.5 (9 December 2014) , semimajor=2.67070 AU , perihelion=1.98847 AU , aphelion=3.35293 AU , eccentricity=0.25545 , period=4.36463 yr , inclination=12.9817° , asc ...
, Ἑστία ( Hestia) for 4 Vesta, Χλωρίς ( Chloris) for 8 Flora, and Πίστη ( Pistis) for 37 Fides. In Chinese, the names are not given the Chinese forms of the deities they are named after, but rather typically have a syllable or two for the character of the deity or person, followed by 神 'god(dess)' or 女 'woman' if just one syllable, plus 星 'star/planet', so that most asteroid names are written with three Chinese characters. Thus Ceres is 穀神星 'grain goddess planet', Pallas is 智神星 'wisdom goddess planet', etc.


Physical properties of comets and minor planets

Commission 15 of the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
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. 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. Most detailed information is available from : Minor planets visited by spacecraft and : Comets visited by spacecraft.


Environmental properties

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


Radiation environment

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 In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
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.


Optical environment

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.


Geological environment

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.


Magnetic environment

Usually in the interior of the planet, the convection of the conductive fluid will generate a large and strong magnetic field. However, the size of a minor planet is generally small and most of the minor planets have a "crushed stone pile" structure, and there is basically no "dynamo" structure inside, so it will not generate a self-generated dipole magnetic field like the Earth. But some minor planets do have magnetic fields, because——On the one hand, some minor planets have remanent Magnetism: if the parent body had a magnetic field or if the nearby planetary body has a strong magnetic field, the rocks on the parent body will be magnetised during the cooling process and the planet formed by the fission of the parent body will still retain remanence, which can also be detected in extraterrestrial meteorites from the minor planets; on the other hand, if the minor planets are composed of electrically conductive material and their internal conductivity is similar to that of carbon or iron-bearing meteorites, the interaction between the minor planets and the solar wind is likely to be unipolar induction, resulting in an external magnetic field for the minor planet. In addition, the magnetic fields of minor planets are not static; impact events, weathering in space and changes in the thermal environment can alter the existing magnetic fields of minor planets. At present, there are not many direct observations of minor planet magnetic fields, and the few existing planets detection projects generally carry magnetometers, with some targets such as Gaspra and Braille measured to have strong magnetic fields nearby, while others such as Lutetia have no magnetic field.


See also

* Groups of minor planets *
List of minor planets The following is a list of numbered minor planets in ascending numerical order. With the exception of comets, minor planets are all small bodies in the Solar System, including asteroids, distant objects and dwarf planets. The catalog consists ...
* Dwarf planet * Quasi-satellite *
Small Solar System body A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other obje ...
* Solar System


Notes


References


External links


Minor Planet CenterLogarithmic graph of asteroid discoveries from 1801-2015
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