Minor planet


According to the International Astronomical Union, 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 small Solar System bodies and dwarf planets, including into the respective categories comets and Pluto, which were generally not considered minor planets. In contrast to the eight official planets of the Solar System, all minor planets fail to clear their orbital neighborhood.
Minor planets include asteroids, as well as distant minor planets, most of which reside in the Kuiper belt and the scattered disc., there are known objects, divided into 875,150 numbered, with only one of them recognized as a dwarf planet and 597,816 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, 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. 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.

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.
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 a semimajor axis of less than Earth's and an aphelion 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 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, 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 objects like Makemake, also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune.
  • *** Resonant Kuiper belt objects.
  • **** Plutinos, 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 objects such as Sedna, with both an aphelion and a perihelion outside the Kuiper belt.
  • *** Sednoids, detached objects with a perihelion greater than 75 AU.
  • ** The Oort cloud, a hypothetical population thought to be the source of long-period comets and that may extend to 50,000 AU from the Sun.

    Naming conventions

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:
  • – 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. For example, the provisional designation consists of the year of discovery 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. 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. 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 1981 VA, now 3360 Syrinx. In November 2006 its position as the lowest-numbered unnamed asteroid passed to , 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 objects before it was finally named Albion in January 2018.
A few objects are cross-listed as both comets and asteroids, such as 4015 Wilson–Harrington, which is also listed as 107P/Wilson–Harrington.

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 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, 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

Eccentric

Discoverer's name

Languages

Physical properties of comets and minor planets

Commission 15 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. 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, an association of asteroid research groups, maintains a Data Base of Physical and Dynamical Properties of Near Earth Asteroids.

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