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A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet.[a] Before 2006, the International Astronomical Union (IAU) officially used the term minor planet, but during that year's meeting it 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 June 2021[update], there are 1,086,655 known objects, divided into 567,132 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 ʼAylóʼchaxnim.
There are various broad minor-planet populations:
- Asteroids; traditionally, most have been bodies in the inner Solar System.
- Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is used:
- Apohele asteroids 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 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 trojans, asteroids sharing Earth's orbit and gravitationally locked to it. As of 2011, the only one known is 2010 TK7.
- 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.
- Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is used:
- Distant minor planets; an umbrella term for minor planets in the outer Solar System.
- Centaurs, 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.
- Scattered disc objects like Eris, with aphelia outside the Kuiper belt. These are thought to have been scattered by Neptune.
- Detached objects such as Sedna, with both aphelia and perihelia outside the Kuiper belt.
- 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.
All astronomical bodies 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:
- 1932 HA – provisional designation upon discovery on 24 April 1932
- (1862) 1932 HA – formal designation, receives an official number
- 1862 Apollo – named minor planet, receives a name, the alphanumeric code is dropped
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.
The first few asteroids were named after figures from Greek and Roman mythology, but as such names started to dwindle the names of famous people, literary characters, discoverers' spouses, children, colleagues, and even television characters were used.
- The first asteroid to be given a non-mythological name was 20 Massalia, named after the Greek name for the city of Marseille. The first to be given an entirely non-Classical name was 45 Eugenia, named after Empress Eugénie de Montijo, the wife of Napoleon III. 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. This unspoken tradition lasted until 334 Chicago was named; even then, female names showed up in the list for years after.
- 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 and 483 Seppina, 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 banned pet names as sources, eccentric asteroid names are still being proposed and accepted, such as 4321 Zero, 6042 Cheshirecat, 9007 James Bond, 13579 Allodd and 24680 Alleven, and 26858 Misterrogers.
- 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, because she died shortly after the discovery, at age 22.
- Names were adapted to various languages from the beginning. 1 Ceres, 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, 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
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.
- Groups of minor planets
- List of minor planets
- Dwarf planet
- Small Solar System body
- Solar System
- Objects (generally centaurs) that were originally discovered and classified as minor planets, but later discovered to be comets are listed both as minor planets and comets. Objects that are first discovered as comets are not dually classified.
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- 谷 'valley' being a common abbreviation of 穀 'grain' that would be formally adopted with simplified Chinese characters.
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