Naming of Astronomical Objects
Celestial nomenclature has long been a controversial topic. At its inaugural meeting in 1922 in Rome, the IAU standardized the constellation names and abbreviations. More recently IAU Committees or Working Groups have certified the names of astronomical objects and features. In the following links you can find further information on how different objects and features are named.
Spelling of Names
Questions have been asked about the proper English spelling of names of astronomical objects, especially as regards capitalization of such names.
The IAU formally recommends that the initial letters of the names of individual astronomical objects should be printed as capitals (see the IAU Style Manual, Trans. Int. Astron. Union, volume 20B, 1989; Chapter 8, page S30 – PDF file); e.g., Earth, Sun, Moon, etc. "The Earth's equator" and "Earth is a planet in the Solar System" are examples of correct spelling according to these rules.
It is emphasized, however, that language conventions are the responsibility of individual nations or groups of nations. While the IAU is willing to help to achieve a minimum degree of orthographic consistency as regards astronomical terms, it cannot undertake to do so for all languages, nor is it in the power of the IAU to enforce the application of any such conventions.
If a name is difficult to spell or pronounce, it may not be the best choice for use on maps and in presentations. Sometimes multi-word names are discouraged for this reason. The purpose of nomenclature is to provide simple, clear, unambiguous names for features.
Naming of Solar System Objects and Features
The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.
Major Planets and the Moon
The eight major planets in our Solar System and Earth's satellite have official IAU names. The names of the major planets were already in common use when the IAU formed in 1919 (e.g. scientifically, in professional and amateur astronomy literature, in nautical almanacs, etc.). However, the names of the planets have been included in wording for IAU resolutions multiple times since the IAU's founding and these names can be considered formally adopted by the IAU membership. While there are cultural names for the planets and Earth's satellite in other languages, there are classic names for the major planets and Moon which appear in English language IAU resolutions and the IAU Style Manual (which was approved by an IAU resolution in 1988).
What follows is a partial list of instances of use of these planet names, but it is by no means exhaustive. This compilation demonstrates, however, that the names of the planets and Moon have appeared in IAU resolutions (or in wording of documents approved by IAU resolution) approved by IAU General Assemblies multiple times, and continue to be in ubiquitous use.
1976: The names of the then major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) and the Earth's satellite (Moon) appear in IAU Resolution No. 10, which was approved by the XVIth General Assembly of the IAU in Grenoble, France in 1976.
1988: The IAU XXth General Assembly — which met in Baltimore in 1988 — approved IAU Resolution A3 on the Improvement of Publications, which recognized "the importance of identifying astronomical objects by clear and unambiguous designations" and recommended "that the authors and editors of the astronomical literature adopt the recommendations in the IAU Style Manual". The "IAU Style Manual (1989): The Preparation of Astronomical Papers and Reports" by George A. Wilkins (President of IAU Commission 5) was published in December 1988, and reprinted as Chapter VIII ("IAU Style Book") in the "Transactions of the International Astronomical Union Vol. XXB: Proceedings of the Twentieth General Assembly Baltimore 1988" (1990; ed. Derek McNally; Kluwer Academic Publishers; Dordrecht). The IAU Style Manual Sec. 5.25 lists the names of the "principal planets" as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
2006: IAU Resolution B5 (Definition of a Planet in the Solar System) explicitly lists the eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. While Resolution B5 defined the category of dwarf planet, Resolution B6 explicitly lists Pluto as an example.
One also sees these planet names are used ubiquitously by modern IAU working groups (e.g. IAU Working Group for Planetary System Nomenclature, the IAU Working Group on Cartographic Coordinates and Rotational Elements in their recent report by Archinal et al. 2011 Celestial Mechanics and Dynamical Astronomy, Vol. 109, Issue 2, pp. 101-135, etc.).
So the IAU does recognize official names for the major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and Earth's satellite (Moon).
Planetary nomenclature, like terrestrial nomenclature, is used to uniquely identify a feature on the surface of a planet or satellite so that the feature can be easily located, described, and discussed. The procedure is as follows:
- When the first images of the surface of a planet or satellite are obtained, themes for naming features are chosen and names of a few important features are proposed, usually by members of the appropriate IAU task group.
- As higher resolution images and maps become available, names for additional features may be requested by investigators mapping or describing specific surfaces or geological formations.
- At this point, anyone may suggest that a specific name be considered by a Task Group, but there is no guarantee that the name will be accepted. Please submit name requests via this form.
- Names successfully reviewed by a task group are submitted by the task group chair to the Working Group for Planetary System Nomenclature (WGPSN).
- Upon successful review by vote of the members of the WGPSN, names are considered approved as official IAU nomenclature, and can be used on maps and in publications. Approved names are immediately entered into the Gazetteer of Planetary Nomenclature, and posted on its web site. Any objections to these names based on significant substantive problems or inconsistent application of normal IAU naming convention must be forwarded in writing or email to the IAU General-Secretary within three months from the time the name was placed on the web site. The General-Secretary will make a recommendation to the WGPSN Chair as to whether or not the approved name(s) should be reconsidered. The General-Secretary, in concurrence with the IAU President, may seek the advice of external consultants.
- Approved names are also listed in the transactions of the IAU.
- The categories of the planetary features are listed here.
Dwarf planets are planetary-mass objects orbiting the Sun that are massive enough to be rounded by their own gravity, but are not planets or satellites. Unlike planets, these bodies have not cleared the neighbourhood around their orbits, and their paths sometimes cross with other, often similar, objects.
There are currently five identified dwarf planets in our Solar System, each named after a God from Greek, Polynesian, or Roman mythologies. These five bodies are Ceres, Pluto, Haumea, Makemake, and Eris. In addition all of these but Ceres are also classified as plutoids, meaning that they are dwarf planets that orbit beyond Neptune and have an absolute magnitude H greater than 1.
There are several stages before a proposed name is accepted:
- When a body is initially sighted it is given a provisional name, which is later superseded by a permanent numerical designation once its orbit has been well determined.
- The discovery team suggests a suitable name to the two relevant IAU groups — the working groups for Small Body Nomenclature (CSBN) and Planetary System Nomenclature (WGPSN) — who together are responsible for naming dwarf planets. The name is intended to reflect the characteristics of the body itself, and be an appropriate moniker derived from mythology. Objects, including dwarf planets, far beyond the orbit of Neptune are expected to be given the name of a deity or figure related to creation; for example Makemake, the Polynesian creator of humanity and god of fertility, and Haumea, the Hawaiian goddess of fertility and childbirth.
- The IAU finally decides on the assignment of the name, priority given to the ones proposed by the discoverers.
- Dwarf planets may not share a name with any other small Solar System bodies.
The names of features on the bodies in the Pluto system are related to mythology and the literature and history of exploration:
- Names for the Underworld from the world's mythologies.
- Gods, goddesses, and dwarfs associated with the Underworld.
- Heroes and other explorers of the Underworld.
- Writers associated with Pluto and the Kuiper Belt.
- Scientists and engineers associated with Pluto and the Kuiper Belt.
- Destinations and milestones of fictional space and other exploration.
- Fictional and mythological vessels of space and other exploration.
- Fictional and mythological voyagers, travellers and explorers.
- Dogs from literature, mythology and history.
- Legendary serpents and dragons.
Satellites of Planets in the Solar System
The WGPSN is responsible for naming of satellites of planets. With the agreement of the WGPSN, the CSBN will assume responsibility for the naming of satellites of minor planets. The WGPSN is responsible for naming of satellites of planets.
Modern technology has made it possible to discover satellites down to 1 km in size or even smaller. The greatly increased discovery rate of satellites has made it necessary to extend the existing name categories for the satellites of Jupiter and Saturn whose names are drawn from the Greco-Roman mythology. The Jovian satellites have previously been named for Zeus/Jupiter's lovers and favorites but now Zeus' descendants are also included as an allowable source of names. The satellites of Saturn have so far been named for the Greco-Roman Titans, descendants of the Titans, Giants and the Roman god of the beginning. In order to internationalize the names, we now also allow names of giants and monsters in other mythologies (so far Gallic, Inuit and Norse).
The process of naming newly discovered natural satellites is as follows:
- When reported to the IAU Central Bureau for Astronomical Telegrams, the object is assigned a provisional name, consisting on the letter S followed by the year of discovery and a number indicating the order of discovery within that year.
- When the satellite is confirmed, the discoverer suggests a final name. Expanding on past practice, satellites of minor planets will, where possible and appropriate, receive names of mythological characters closely related to the name of the primary and suggesting the relative sizes. For example, binary transneptunian objects of comparable size should receive the names of twins or siblings, consistent with the current principle of using names of gods of creation or the underworld. As another example, satellites that share Pluto's orbital rhythm should take the name of underworld deities, as Pluto itself is named after the Roman god of the underworld who was able to render himself invisible.
- The IAU finally decides on the assignment of the name, priority given to the ones proposed by the discoverers.
The assignment of a particular name to a particular minor planet is the end of a long process that can take many decades:
- It begins with the discovery of a Minor Planet that cannot be identified with any already-known object. Such Minor Planets are given a provisional designation. The provisional designations are based on the date of discovery and are assigned by the Minor Planet Center (MPC) according to a well defined formula that involves the year of discovery, two letters and, if need be, further digits (for example 1989 AC or 2002 LM60).
- When the orbit of a Minor Planet becomes well enough determined that the position can be reliably predicted far into the future (typically this means after the Minor Planet has been observed at four or more oppositions), the Minor Planet receives a permanent designation - number issued sequentially by the Minor Planet Center, for example (433), (4179) or (50000).
- When a Minor Planet receives a permanent number, the discoverer of the Minor Planet is invited to suggest a name for it. The discoverer has this privilege for a period of ten years following the numbering of the object. The discoverer writes a short citation explaining the reasons for assigning the name according to the guidelines of the IAU.
- All proposed names are judged by the fifteen-person Working Group for Small Body Nomenclature (CSBN) of the IAU, comprised of professional astronomers with research interests connected with Minor Planets and/or comets from around the world.
Proposed names should be:
- 16 characters or less in length
- preferably one word
- pronounceable (in some language)
- not too similar to an existing name of a Minor Planet or natural Planetary satellite.
The names of individuals or events principally known for political or military activities are unsuitable until 100 years after the death of the individual or the occurrence of the event.
- names of pet animals are discouraged
- names of a purely or principally commercial nature are not allowed.
There are more detailed guidelines for unusual Minor Planets in certain dynamical groups, for example:
- Trojan asteroids (those that librate in 1:1 resonance with Jupiter) are named for heroes of the Trojan War (Greeks at L4 and Trojans at L5).
- Trans-Jovian Planets crossing or approaching the orbit of a giant Planet but not in a stabilizing resonance (so called Centaurs) are named for centaurs.
- Objects crossing or approaching the orbit of Neptune and in stabilizing resonances other than 1:1 (notably the Plutinos at the 2:3 resonance) are given mythological names associated with the underworld.
- Objects sufficiently outside Neptune's orbit that orbital stability is reasonably assured for a substantial fraction of the lifetime of the solar system (so called Cubewanos or "classical" TNOs) are given mythological names associated with creation.
- Objects that approach or cross Earth's orbit (so called Near Earth Asteroids) are generally given mythological names.
Accepted names become official when they are published, along with their accompanying citations, in the Minor Planet Circulars, issued monthly by the Minor Planet Center.
The CSBN recognizes the need to limit the numbers of Minor Planets named, and it requests individual discoverers and teams to propose no more than two names each two months.
Contrary to some recent media reports it is not possible to buy a name for a minor planet. If you have a name you would like to apply to a minor planet, the best advice is "Go out and discover one!".
The alphabetic list of all names is available at the Minor Planet Center including the discovery circumstances.
A comet is a body made of rock and ice, typically a few kilometres in diameter, which orbits the Sun. Comets may pass by the Sun only once or go through the Solar System periodically. A comet’s tail is formed when the Sun’s heat warms the coma or nucleus, which releases vapours into space.
During the 19th century, comets were only given names after their second apparition, while those that had only appeared once were designated by a combination of year of discovery, numbers (both Arabic and Roman) and letters. Sometimes, the name of the discoverer was referred to in parentheses. It was not until the 20th century that comets were routinely named after their discoverers.
Today, the IAU’s Committee on Small Body Nomenclature (CSBN) is the responsible body for strategic matters related to comet naming. When a comet is discovered and confirmed, the Central Bureau for Astronomical Telegrams (CBAT) announces it on behalf of the IAU. It is then given a designation according to the following pattern (see Resolution C.5 approved by the IAU in 1995 on p.32 of this PDF):
- A prefix, alluding to the type of comet, which can be any of the following:
- P/ for a periodic comet.
- C/ for a comet that is not periodic.
- X/ for a comet for which a meaningful orbit cannot be computed.
- D/ for a periodic comet that no longer exists or is deemed to have disappeared.
- The year of discovery.
- An uppercase letter identifying the half-month of observation during that year (A for first half of January, B for second half and so on).
- A number representing the order of discovery within that half month.
As an example, the third comet discovered in the second half of January 2013, and classified as periodic, would be designated as P/2013 B3. The precise method, including exceptions and special cases, is described in the Cometary Designation System IAU resolution.
When a periodic comet is observed after its second apparition, the IAU’s Minor Planet Center (MPC) gives it a permanent number indicating the order of the discovery.
To complete the designation, a comet is given the name of its two first discoverers (last name for an individual or one word or acronym for a team of astronomers). The names appear in chronological order and separated by a hyphen. In very rare cases the title can consist of three discoverers, or can even be generic.
Examples of complete titles for comets (whether provisional or final) are 119P/Parker-Hartley, C/1995 O1 (Hale-Bopp) or 146P/Shoemaker-LINEAR.
More detailed guidelines explaining the process of assigning the names of the discoverers to a comet can be found in this IAU document. Also read this essay on the web pages of the International Comet Quarterly.
Naming Objects Outside the Solar System
See the dedicated page "Naming Stars".
Nebulae, Galaxies, and Other Objects
The designation of astronomical objects beyond the Solar System should consist of at least two parts — a leading acronym and a sequence value.
- An acronym is a code specifying the catalogue or collection of sources, conforming to the following rules, among others:
- It should consist of at least three characters (letters and/or numerals, avoiding special characters).
- The acronym must be unique.
- Acronyms should not be excessively long.
- Sequence: a string of usually alpha-numerical characters that uniquely identify the source within the catalogue. Common values for the sequence are:
- Running number.
- Based on the coordinates of the object. Equatorial Coordinates shall always be preceded by J if they are for the standard equinox of J2000.0.
Complete specifications concerning designations for astronomical radiation sources outside the solar system are published by the Working Group on Astronomical Designations in IAU Commission B2. See http://cds.u-strasbg.fr/vizier/Dic/iau-spec.htx for more details.
See the dedicated page "Naming of exoplanets".