Minor planet

"Planetoid" redirects here. For other uses, see Planetoid (disambiguation).

A minor planet is an astronomical object in direct orbit around the Sun that is neither a dominant planet nor a comet. The first minor planet discovered was Ceres in 1801. Since then, more than 200,000 minor planets have been discovered, most of them lying in the asteroid belt.

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 objects. Historically, the terms asteroid, minor planet, and planetoid have been more or less synonymous, but the issue has been complicated by the discovery of numerous minor planets beyond the orbit of Jupiter and especially Neptune that are not universally considered asteroids.

Before 2006, the International Astronomical Union had officially used the term minor planet. During its 2006 meeting, it reclassified minor planets and comets into dwarf planets and small solar system bodies. Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, that is, an ellipsoidal shape, with all other minor planets and comets called "small solar system bodies". The IAU states: "the term 'minor planet' may still be used, but generally 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 followed.

The Saturnian moon Mimas is the smallest body known to be in hydrostatic equilibrium (though not eligible to be a dwarf planet since it does not orbit the Sun), while the asteroid Pallas may be the largest that is not. The IAU has so far officially classified five objects as dwarf planets. In order both of discovery and distance from the Sun, they are Ceres, Pluto, Haumea, Makemake, and Eris.

Populations

Main article: Groups of minor planets

Hundreds of thousands of minor planets have been discovered within the Solar System, with the rate of discovery currently running at around 5,000 per month. Of the more than 400,000 registered minor planets, 210,454 have orbits known well enough to be assigned permanent official numbers. Of these, some 15,000 have official names. As of March 2009, the lowest-numbered unnamed minor planet is (3708) 1974 FV1; but there are also some named minor planets above number 200,000.

These fall into several broad populations:

• Asteroids; traditionally, most have been bodies in the inner solar system.

• Main belt asteroids, those following roughly circular orbits between Mars and Jupiter. These are the original and best-known group of asteroids or minor planets.
• Near-earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is traditional.

• Aten asteroids, those that have semi-major axes of less than one Earth orbit. Those Aten asteroids that have their aphelion within Earth's orbit are known as Apohele asteroids.
• Amor asteroids are those near-Earth asteroids that approach the orbit of the 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 main asteroid belt.
• Apollo asteroids are those asteroids with a semimajor axis greater than that of the earth while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers.

• Jupiter Trojans, asteroids sharing Jupiter's orbit and gravitationally locked to it. Numerically they are estimated to equal the main-belt asteroids.

• Centaurs, bodies in the outer solar system between Jupiter and Neptune. They have unstable orbits due to the influence of the giant planets' gravity, and therefore must have come from elsewhere, probably outside Neptune.
• Trans-Neptunian objects, bodies at or beyond the orbit of Neptune, the outermost planet.

• Neptune Trojans, bodies sharing Neptune's orbit and gravitationally locked to it. Although only a handful are known, they are thought to outnumber main-belt asteroids by an order of magnitude.
• The Kuiper belt, objects inside an apparent population drop-off approximately 55 AU from the Sun.

• Classical Kuiper belt objects, also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune.
• Plutinos, bodies like Pluto that are in a 2:3 resonance with Neptune, and other resonant trans-Neptunian objects. This is a rather large group.

• Scattered disc objects, 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, is a hypothetical population thought to be the source of long period comets that may extend out to 50,000 AU from the Sun.

Naming

See also: Pronunciation of asteroid names and Centaur and TNO pronunciation

A newly discovered minor planet is given a provisional designation (such as 2002 AT4) consisting 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 (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. As modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. The first to be left unnamed was for a long time (3360) 1981 VA, now 3360 Syrinx; as of September 2008, this distinction is held by (3708) 1974 FV1. On rare occasions, a small object's provisional designation may become used as a name in itself: the still unnamed (15760) 1992 QB1 gave its name to a group of Kuiper belt objects which became known as cubewanos.

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 with 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.

Sources for names

Main article: Meanings of minor planet names

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, discoverer's wives, children, 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 Marseilles. The first to be given an entirely non-Classical name was 45 Eugenia, named after Empress Eugénie de Montijo. 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, oddly feminised names show up in the list for years afterward.

As the number of asteroids began to run into the hundreds, and eventually 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.

Special naming rules

Minor planet naming is not always a free-for-all: there are some populations for which rules have developed about the sources of names. For instance, centaurs (orbiting between Saturn and Neptune) are all named after mythological centaurs; Jupiter trojans after heroes from the Trojan War; resonant trans-Neptunian objects after underworld spirits; and non-resonant TNOs after creation deities.

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, E.A.R.N. maintains a Data Base of Physical and Dynamical Properties of Near Earth Asteroids.

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

See also

• List of minor planets
• Solar System
• Groups of minor planets
• Small Solar System body

External links

• Minor Planet Center