Minor planet

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Euler diagram showing the types of bodies in the Solar System Euler diagram of solar system bodies.svg
Euler diagram showing the types of bodies in the Solar System

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. [lower-alpha 1] 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). [1]


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, 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. [2]

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. [3] The term planetoid has also been used, especially for larger, planetary objects such as those the IAU has called dwarf planets since 2006. [4] [5] Historically, the terms asteroid, minor planet, and planetoid have been more or less synonymous. [4] [6] 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. [6] 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. [7] 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. [8] [9] Of these, 21,922 have official names. [8] As of 8 November 2021, the lowest-numbered unnamed minor planet is (4596) 1981 QB , [10] and the highest-numbered named minor planet is 594913 ʼAylóʼchaxnim. [11]

There are various broad minor-planet populations:

Naming conventions

Out of a total of more than 700,000 discovered minor planets, 66% have been numbered (green) and 34% remain unnumbered (red). Only a small fraction of 20,071 minor planets (3%) have been named (purple). Minor planet count.svg
Out of a total of more than 700,000 discovered minor planets, 66% have been numbered (green) and 34% remain unnumbered (red). Only a small fraction of 20,071 minor planets (3%) have been named (purple).

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

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. [18]

A few objects are cross-listed as both comets and asteroids, such as 4015 Wilson–Harrington, which is also listed as 107P/Wilson–Harrington.


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. [8]


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. [19] 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, [20] 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.

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, because she died shortly after the discovery, at age 22. [21] [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', [23] Pallas is 智神星 'wisdom goddess planet', etc.[ citation needed ]

Physical properties of comets and minor planets

Commission 15 [24] 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. [25] 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. [26]

Most detailed information is available from Category: Minor planets visited by spacecraft and Category: Comets visited by spacecraft.

See also


  1. 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.

Related Research Articles

Asteroid Minor planet that is not a comet

An asteroid is a minor planet of the inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt. Thus the term "asteroid" now generally refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter. Larger asteroids are often called planetoids.

Kuiper belt Area of the Solar System beyond the planets, comprising small bodies

The Kuiper belt is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia, and water. The Kuiper belt is home to three objects identified as dwarf planets by the IAU: Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region.

Solar System The Sun, its planets and their moons

The Solar System is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, the dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the natural satellites—two are larger than the smallest planet, Mercury, and one more almost equals it in size.

Jupiter trojan Asteroid sharing the orbit of Jupiter

The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange points: either L4, existing 60° ahead of the planet in its orbit, or L5, 60° behind. Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis of about 5.2 AU.

Asteroid belt Circumstellar disk (accumulation of matter) in an orbit between those of Mars and Jupiter

The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes but much smaller than planets, called asteroids or minor planets. This asteroid belt is also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids.

Timeline of Solar System astronomy Timeline of the history of Solar System astronomy

The following is a timeline of Solar System astronomy.

Centaur (small Solar System body)

In planetary astronomy, a centaur is a small Solar System body with either a perihelion or a semi-major axis between those of the outer planets. Centaurs generally have unstable orbits because they cross or have crossed the orbits of one or more of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻawela, which may be in a stable orbit. Centaurs typically exhibit the characteristics of both asteroids and comets. They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000.

15760 Albion Trans-Neptunian object, prototype of cubewanos

15760 Albion, provisional designation 1992 QB1, was the first trans-Neptunian object to be discovered after Pluto and Charon. Measuring about 108–167 kilometres in diameter, it was discovered in 1992 by David C. Jewitt and Jane X. Luu at the Mauna Kea Observatory, Hawaii. After the discovery, they dubbed the object "Smiley" and it was shortly hailed as the tenth planet by the press. It is a "cold" classical Kuiper belt object and gave rise to the name cubewano for this kind of object, after the QB1 portion of its designation. Decoding its provisional designation, "QB1" reveals that it was the 27th object found in the second half of August of that year. As of January 2018, around 2,400 further objects have been found beyond Neptune, a majority of which are classical Kuiper belt objects. It was named after Albion from William Blake's mythology.

In ancient times, only the Sun and Moon, a few stars, and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year. Astronomers need to be able to assign systematic designations to unambiguously identify all of these objects, and at the same time give names to the most interesting objects, and where relevant, features of those objects.

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 calculated. Approximately 47 % of the more than 1,100,000 known minor planets remain provisionally designated, as hundreds of thousands have been discovered in the last two decades.

Definition of <i>planet</i> History of the word "planet" and its definition

The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai, "wandering stars", for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids.

<span class="nowrap">1998 WW<sub>31</sub></span>

1998 WW31, is a non-resonant trans-Neptunian object and binary system from the Kuiper belt located in the outermost region of the Solar System, approximately 148 kilometers (92 miles) in diameter. It was first observed on 18 November 1998, by American astronomer Marc Buie and Robert Millis at the Kitt Peak National Observatory in Arizona, United States. In December 2000, a minor-planet moon, designated S/2000 (1998 WW31) 1 with a diameter of 123 kilometers (76 miles), was discovered in its orbit. After Charon in 1978, it was the first of nearly 100 satellites since discovered in the outer Solar System.

Dwarf planet Planetary-mass object

A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that, being possibly differentiated and geologically active bodies, they may display planetary geology, an expectation borne out by the Dawn mission to Ceres and the New Horizons mission to Pluto in 2015.

IAU definition of <i>planet</i> Formal definition of a planet in the context of the Solar System as ratified by the IAU in 2006

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body that:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium, and
  3. has "cleared the neighbourhood" around its orbit.
Small Solar System body Object in the Solar System

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 objects, except satellites, orbiting the Sun shall be referred to collectively as 'Small Solar System Bodies' ".

Discovery and exploration of the Solar System Observation, visitation and increase in knowledge and understanding of Earths cosmic neighborhood

Discovery and exploration of the Solar System is observation, visitation, and increase in knowledge and understanding of Earth's "cosmic neighborhood". This includes the Sun, Earth and the Moon, the major planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, their satellites, as well as smaller bodies including comets, asteroids, and dust.

The Solar System — our Sun's system of planets, moons, and smaller debris — is humankind's cosmic backyard. Small by factors of millions compared to interstellar distances, the spaces between the planets are daunting, but technologically surmountable.


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