Names of Solar System objects and features

by Wm. Robert Johnston
updated 24 August 2006

While thousands of Solar System objects and features have been named so far by the astronomical community, most of these names are unknown to the general public. Nonetheless, astronomers take the naming process very seriously. As astronomy developed scientifically from the 18th century to the 1900s, different practices were used--and a few became standard. In 1919 the International Astronomical Union (IAU) was founded as a coordinating body with the mission "to promote and safeguard the science of astronomy in all its aspects through international cooperation." One of the IAU‘s roles is to oversee names and designations of planets, satellites, asteroids, comets, and surface features on these bodies. The scientific community has granted the IAU authority regarding procedures for submitting names and final approval of those names. The IAU has specified procedures for assigning routine designations to asteroids and comets. It has also assigned the Minor Planet Center (MPC) at the Smithsonian Astrophysical Observatory the role of implementing some of these procedures for asteroids and comets. The IAU's Working Group for Planetary System Nomenclature coordinates names of surface features.

Planets and Satellites

The major planets are named for Roman mythological deities. The five planets visible to the naked eye were recognized and named in ancient times by many different cultures. The Roman names are now standard: Mercury, Venus, Mars, Jupiter, and Saturn. The Earth, Moon, and Sun are the accepted names for those bodies and are used as such routinely in the scientific community. (Note that they are capitalized: this is the IAU's recommended convention, as is capitalization of the Solar System.) The remaining two major planets, discovered in modern times, are also named for Roman mythological deities: Uranus and Neptune. It may be noted that other names were initially considered: it took years for the name "Uranus" to prevail over alternatives such as "Herschel" (name of its discoverer) or "Georgium Sidus" (the Georgian Star, referring to King George III of England).

From its discovery in 1930, Pluto was classified as a planet (the name "Pluto" winning out over alternatives such as "Lowell", an astronomer who had predicted a ninth planet, and "Minerva"). Beginning in 1992, a new class of solar system objects were discovered beyond Neptune and designated trans-Neptunian objects (TNOs). It was recognized that Pluto was one of the largest members of this extended population of objects, and not a unique object. Considerable debate in the astronomical community ensued for years over the appropriate classification of Pluto. In August 2006, the IAU decided to apply the term "planet" only to the eight larger planets, and place Pluto in a new intermediate category "dwarf planet". This new category includes Pluto, 2003 UB313 (a currently unnamed Pluto-sized TNO), and (1) Ceres, the largest asteroid. More objects will likely be designated "dwarf planets", in addition to their status as either asteroids or TNOs. Interestingly, this parallels the situation in the mid-1800s for asteroids: the first asteroids discovered were classified with planets, until it became apparent that they were but the first of a large population of minor bodies.

Satellites of the planets are generally named for characters from Roman mythology, though not necessarily deities. Some names are drawn from Greek mythology. In some cases the characters are associated with the mythological deity for which the associated planet is named. For example, Jupiter's satellites Io, Europa, and Callisto are named for victims of Jupiter's unfaithfulness to his wife in the Roman myths. John Herschel, whose father discovered Uranus, began the practice of naming satellites of Uranus for characters from the plays of William Shakespeare. When Voyager 2 discovered ten new satellites of Uranus by early 1986, shortly after loss of the space shuttle Challenger, some suggested naming seven for the Challenger astronauts. Given that this would have been the first occasion to name planetary satellites for real people, this was perceived as a slight by the Soviets--who had previously lost four cosmonauts in space flight accidents. In the end, asteroids were named for the Challenger astronauts (see below).

Confirmed planetary satellites are permanently numbered by Roman numeral. New discoveries are assigned provisional designations including "S/" for satellite, the year of discovery, the first letter of the planet name, and a number indicating sequential discoveries that year; e.g. S/1980 S1 or S/2000 S10, provisional satellites of Saturn. Some provisionally designated objects are found not to be real; others may be the same as an object previously given a designation. Once a satellite's existence and orbit are well confirmed, it is assigned a permanent Roman numeral and a name. For example, S/1966 S2 and S/1980 S1 were found to be the same object, which is now known as Janus or Saturn X or S X. Once a name is assigned, it is the most common way to refer to a satellite.


Comet designations have varied considerably over the years. Comets were known in ancient times but only recognized as Solar System objects in the 1600s. As transient objects, little thought was given to naming them in the 1600s and 1700s. Several particularly outstanding comets are still known as they were then: Great Comet of 1770, for example. Edmund Halley first recognized that several historical comets were the same object, which is now known as Halley's Comet or Comet Halley.

The convention developed of naming comets for their discoverers (surname only) during the 1800s. While specific procedures are now applied, basically the discoverer is the person who first observes and reports a new comet to the astronomical community. Simultaneous reports are possible, in which case the surnames are hyphenated: Comet Shoemaker-Levy, for example. Comets discovered as a result of spacecraft detections or team survey programs are usually named for the spacecraft (e.g. Comet SOHO) or program (e.g. Comet Spacewatch).

From around 1950 to the end of 1994, as discovered a comet would be initially designated by the year of discovery (or recovery, in the case of returning comets) and a lower-case letter indicating discovery sequence in that year (1982i was the 9th comet discovered in 1982). The permanent designation would include the year in which the comet passed perihelion (closest approach to the Sun) and a Roman numeral indicating sequence of passage in that year. Thus 1982i as the third comet known to pass perihelion in 1986 was also 1986 III. Both types of designations were considered valid, although once the permanent designation was assigned it was preferred. Both designations were specific to a given approach to the Sun; thus, returning comets would receive multiple designations. So to illustrate: Comet Halley is also known by 1986 III = 1983i, 1910 II = 1909c, 1835 III, 1759 I, 1066, and 25 other designations.

Short period comets--those with orbital periods of less than 200 years--are also assigned a permanent number the second time they are observed. Short period comets with the same name (generally having been discovered by the same individual) were sometimes distinguished by sequential numbers, a practice which developed in the late 1800s and early 1900s. For example, Comet Shoemaker-Levy 9 was the ninth short-period comet co-discovered by Eugene Shoemaker and David Levy. Long-period comets with common names were distinguished by designation rather than name. (Some non-standard designations still occur: the 250-plus comets discovered by the SOHO spacecraft are numbered by the SOHO team.)

This designation system had become cumbersome by the early 1990s, so the IAU revised the system beginning in 1995. Currently a single designation is assigned when a comet is discovered or recovered. The designation incorporates a letter for type of comet (P/ for periodic or short-period, C/ for non-periodic, D/ for those which no longer exist, and X/ for those where no orbit can be determined), the year of observation, an upper-case letter for the half-month of discovery, and a number for discovery sequence. This designation is preceded by or replaced by the permanent number when applicable. This designation system has been generated retroactively for all past comets to provide a consistent designation system. Comet Halley can now be referred to as 1P/1982 U1, or simply 1P. Comet Shoemaker-Levy 9 is D/1993 F2 ("D" referring to its demise when its fragments hit Jupiter in 1994).

In late 2000 the IAU decided to delete numbers from the names of comets, such as Shoemaker-Levy 9. The designation system adopted in 1995 would then be necessary to distinguish comets with common names. The IAU currently recommends that these designations be given preference (although it may still review the issue).


The first asteroid was discovered in 1801, and in the next 6 years three more objects were identified orbiting between Mars and Jupiter. The four asteroids were named for mythological characters: Ceres, Pallas, Juno, and Vesta. More asteroids were discovered starting in 1845, and soon other names were drawn upon. Latin and Greek geographical names (such as Parthenope and Lutetia) were first used in the 1850s. The first asteroid explicitly named for a person was Eugenia, named for the wife of Napoleon. (Earlier asteroid names did make implied reference to a human namesake, as when Victoria was named by a subject of Queen Victoria.) In 1852 it was proposed to sequentially number asteroids. This practice was later formalized. This permanent number along with the name (if yet assigned) is the conventional manner of referring to an asteroid. Thus we have (1) Ceres, (2) Pallas, (11) Parthenope, (12) Victoria, (21) Lutetia, (45) Eugenia, (3352) McAuliffe, (5065) Johnstone, etc.

Several different systems of provisional designations were used beginning in 1892. Since 1925 most designations have been determined by the system described below. Several asteroid surveys since 1960 have produced unique identifiers which are still used.

Currently, when an asteroid is discovered it receives a designation including the year of discovery and letters/numbers indicating time of year and sequence of discovery. Asteroids will receive multiple such designations. When an asteroid is well observed, allowing good determination of its orbit (at which time it will have at least two provisional designations) it receives a permanent number. The permanent numbers are sequentially assigned by the Minor Planet Center (as are the provisional designations). An asteroid’s discoverer has preference for proposing a name, within certain guidelines.

In the 1980s the IAU adopted some guidelines regarding asteroid names. These stipulated that they should be no more than 16 characters long, should (preferably) be one word (prompting names like (11943) Davidhartley), and should not be too similar to the name of another asteroid or planetary moon. The last constraint should eliminate duplications like (38) Leda, (52) Europa and (85) Io, which duplicate names of satellites of Jupiter, or (106) Dione, which duplicates the name of a satellite of Saturn. Names of persons or events of primarily military or political significance cannot be used until 100 years after event or the person’s death. A final restriction was prompted by the naming of (2309) Mr. Spock in the 1980s. The discoverer named this asteroid, according to his citation, in reference to his pet cat and not for the character of Star Trek fame. The resultant uproar among astronomers prompted the IAU to decree "Names of pet animals are discouraged."

By the 1990s several automated sky surveys were discovering asteroids by the thousands, with the process of naming them falling embarrassingly far behind. One result was some pressure in September 2001 to name an asteroid for each victim of the 11 September terrorist attacks in the United States. The IAU nomenclature committee ended up approving the naming of three asteroids in October: (8990) Compassion, (8991) Solidarity, and (8992) Magnanimity. Later in October, 80 asteroids were named to honor 40 middle school science contest winners and their 40 teachers, illustrating one response to the "naming crisis".

Certain groups of asteroids are currently being named by certain guidelines. Asteroids orbiting in Jupiter's L4 and L5 positions are named for heroes of the Trojan War (Greek names for L4 asteroids and Trojan names for L5 asteroids). Asteroids orbiting between the orbits of Jupiter and Neptune are named for centaurs--which is also the name of this class of asteroids. Trans-Neptunian objects are to be named with mythological names associated with the underworld. Earth approaching asteroids are to receive mythological names.

In connection to these guidelines, generally any new class of asteroidal body is named for the first recognized member of that class. Apollo asteroids, those which cross the Earth's orbit but have an average distance from the Sun greater than that for the Earth, derive that name from (1862) Apollo. One group of Trans-Neptunian objects is called Cubewanos, with the first of this class the as-yet unnamed 1992 QB1.

The first satellite of an asteroid was discovered in 1993. The provisional designation system resembles that for planets: S/2000 (762) 1 is the satellite of (762) Pulcova, discovered in 2000. Six asteroid satellites were named by 2006: Dactyl, satellite of (243) Ida; Petit-Prince, satellite of (45) Eugenia; Linus, satellite of (22) Kalliope; Remus and Romulus, satellites of (87) Sylvia; and Menoetius, satellite of (617) Patroclus. The discoverers of Petit-Prince had proposed that asteroid satellites be named for children or comparable associates of the namesake of the primary asteroid (this is the case for Dactyl and Petit-Prince as well).

Surface Features

The invention of the telescope allowed the observation of surface features on the Moon and Mars. Automated spacecraft have observed surface features on three planets, 28 planetary satellites, four asteroids, and one asteroid satellite. The Hubble Space Telescope and ground-based adaptive optics telescopes have also identified surface features on one more planet, one more moon, and two more asteroids.

In some cases the history of observations of an object are reflected in some feature names. Large-scale features on the Moon and Mars, observed by telescope, have Latin names. Many craters on the Moon's near side are named for astronomers. Some of the largest features on the far side of the Moon are named for Russians, since the first photographs of the Moon’s far side were taken by the Soviet space probe Luna 1 in 1959.

The IAU's Working Group for Planetary System Nomenclature has established parameters for the types of names to be used for each type of feature on each planet, satellite, or asteroid. Various features on Mars and its satellites are named for scientists who studied Mars or for mythological names associated with Mars. Features on Venus are named for female mythological characters or for famous women. Features on Jupiter's volcanically-active satellite Io are named for mythological characters associated with fire, sun, thunder, volcanoes, etc. (given Hawaiian names a good representation) and names from Dante's Inferno. Features on Enceladus, a satellite of Saturn, are named from Burton’s Arabian Nights. Craters on asteroid (243) Ida are named for caverns and grottos of the world. The IAU Group maintains an elaborate database to ensure "equitable" representation of various countries, ethnic groups, and cultures in such names.

For further reading:

At the International Astronomical Union web site:
   The official answer to the issue of selling star names. (Yes, astronomers have a sense of humor.)

The IAU's Committee on Small Body Nomenclature assigns comet and asteroid names:
   Current guidelines for asteroid names.

The IAU‘s Working Group for Planetary System Nomenclature assigns names to surface features on planets, satellites, and asteroids.

The proposal for the name of (45) Eugenia's moon is an example of a citation for a name submitted to the IAU.

At the Minor Planet Center web site:
   Up-to-date alphabetical listing of asteroid names

The National Space Science Data Center web site includes up-to-date information on planets and satellites.
NASA's Jet Propulsion Laboratories also has up-to-date planetary data.

The USGS Astrogeology section includes the following:
USGS page on names of planets and satellites.

William Graves Hoyt in Planets X and Pluto, 1980 (Tucson, AZ: Univ. of Arizona Press) discusses the debate over naming Uranus, Neptune and Pluto.

James L. Hilton's article When did the asteroids become minor planets? describes early designation conventions, among other topics.

© 2001-2005, 2006 by Wm. Robert Johnston.
Last modified 24 August 2006.
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