Naming 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.
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.
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 contact Jennifer S. Blue.
- 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 the members of the WGPSN, names are considered approved 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 must be forwarded in writing or email to the IAU Division F President within three months from the time the name was placed on the web site. Approved names are also listed in the transactions of the IAU.
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.
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
If you are reading this because you want to buy a star name, check the "Layman's Guide to Naming Stars". Otherwise, read the text below, adapted from SEDS:
The earliest naming system which is still popular was introduced by Johann Bayer in his Uranometria star catalog of 1603. As many predecessors and successors, he used constellations to identify stars within them. To distinguish the stars in each constellation, he labelled them with Greek letters, and approximately in the order of their (apparent) brightness, so that the brightest star was labelled Alpha, the second brightest Beta, and so on.
For example, the brightest star in Cygnus (the Swan) is Alpha Cygni (note the use of the genitive of the Latin constellation name) which is also called Deneb, or the brightest star in Leo (the Lion) is Alpha Leonis, also called Regulus.
Misestimates and other irregularities are the reasons why this is only an approximate scheme: for example, the brightest star in Gemini (the Twins) is Beta Geminorum (Pollux) while Alpha Geminorum (Castor) is only the second brightest star of the constellation.
Unfortunately, the Greek alphabet has only 24 letters, and many constellations contain many more stars, even if the naming is restricted to those visible to the naked eye. Johann Bayer then employed low case letters from "a" to "z" and then upper case letters from "A" to "Z" for the stars number 25 to 50 and 51 to 76 in each constellation, respectively.
Another popular naming scheme is the use of the so-called Flamsteed numbers, which were introduced in the catalog Historia Coelestis Britannica which had been compiled by John Flamsteed (1646-1719), and was published unauthorised in 1712 after editing by Edmond Halley (1656-1742).
Flamsteed's own corrected publication of this work did not contain the numbers, by the way. In this scheme, stars of each constellation are numbered in the order of their Right Ascension (for example: 61 Cygni). Because the numbers were taken from the preliminary, error-rich version of the catalog, there are many deviations from the desired order in the numbers.
Other schemes have been introduced, e.g. one by Gould which is occasionally referenced as, e.g., 38G Puppis. But these are no more very popular now.
Fainter stars are normally identified by their numbers in some catalog, such as the Bonner Durchmusterung (BD), the Henry Draper Catalog (HD) or the General Catalog (GC) of Boss, for example, BD +75 deg 752 (star number 752 in the Declination zone +75 deg) = HD 197433 = GC 28804. BD is supplemented by the Cordoba Durchmusterung (CD) and the Cape Durchmusterung for southern stars. Other lists commonly used are the Smithsonian Astrophysical Observatory Catalog (SAO), the Bright Star Catalog (Harvard Revised Photometry, HR), or the Positions and Proper Motions Catalog (PPM).
An example with a lot of names is Betelgeuse = Alpha Orionis = HR 2061 = BD +7 1055 = HD 39801 = SAO 113271 = PPM 149643, whose coordinates in the sky are RA 05:55:10.306, Dec +07:24:25.35 (2000.0), the bright red supergiant in Orion.
Binary and multiple systems
Components of binary or multiple stars are usually labelled by capital roman letters, following the designation of the star, may this be a common name, Bayer or Flamsteed designation, or a catalog number. For example, the brightest star in the sky, Sirius, has a white dwarf companion which is identified by each of the following designations: Sirius B, Alpha Canis Majoris B, or e.g. HD 48915 B.
A notable nomenclature scheme has been developed for variable stars, by Friedrich Wilhelm Argelander in 1862. He proposed to reserve the letters "R" to "Z" for naming the variable stars in each constellation, as Bayer's naming scheme had left these over in each case (and some constellations had a star "Q" in this scheme, e.g. Centaurus, Puppis and Vela).
At that time, the 9 possible names per constellation seemed more than enough, as the number of known variables was small. However, it turned out that it was completely insufficient, and thus the scheme was first extended to two-letter designations, then even to numbers.
Eventually, variables are named as follows: the designation of variables consists of one or two letters and the constellation name (such as U Sagittarii or RR Lyrae) or a number preceded by "V" and the constellation name (e.g., V 1500 Cygni). In each constellation, the first variable discovered is assigned the letter "R" and the genitive of the constellation name, e.g., "R Andromedae" (a long period variable), the second one is "S" (e.g., "S Andromedae" is the supernova which occured in the Andromeda galaxy, M31), and so on up to "Z" for number 9; then the tenth variable is assigned "RR", followed by "RS" etc up to "RZ", "SS" (not "RS"), etc. up to "SZ", and so on up to "YY", "YZ", "ZZ", and then "AA", "AB", etc to "AZ", "BB" to "BZ", up to "QQ" to "QZ" (where the letter "J" is not used to avoid confusion with the letter "I").
Counting, this scheme provides 334 designations for each constellation, and variables starting from number 335 are designated "V 335", "V 336" etc. Those already assigned a Bayer designation are not given a new name according to this scheme (such as Delta Cephei, Beta Lyrae, Beta Persei, or Omicron Ceti).
Variable stars are classified by types which are then named after one typical representative, e.g., "Mira stars", "RR Lyrae stars", or "Delta Cephei stars" (often called "Cepheids" in deviation from the usual scheme).
Novae and supernovae
Special names are assigned for new novae and supernovae. Novae are named according to their constellation together with the year of their occurrance (e.g., "Nova Cygni 1975"), and later given a variable star designation ("Nova Cygni 1975" is also "V 1500 Cygni"). Supernovae are named for their year of occurrance and an uppercase letter, e.g., "SN 1987A". If the alphabet is exhausted, double lower case naming is used: [Year] aa .. az, ba .. bz, etc; e.g., "SN 1997bs".
A summary of guidelines for naming stars and other astronomical objects has been brought out in 1983 by the IAU: First Dictionary of the Nomenclature of Celestial Objects.
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 5. See http://cds.u-strasbg.fr/vizier/Dic/iau-spec.htx for more details.
See the dedicated page "Naming of exoplanets".