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. Until now, however, the IAU has never named a planet, and it has been unclear whether there are potential planets to be named.

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.

Quick links:


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 planets and planetary features in the Solar System

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. Later, 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. Anyone may suggest that a specific name be considered by a task group, but there is no guarantee that the name will be accepted. 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 III 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.


More information on planetary nomenclature can be found on the USGS FAQ section

Definition of a Planet

We invite you to consult the IAU Resolutions 5 and 6 (PDF file, 92KB) adopted on August 2006, at our XXVIth General Assembly in Prague.

Satellites of Planets in the Solar System

The number of planetary satellites now (April 2008) stands at 165. The CCD technology has made it possible to discover satellites down to 1 km in size.  At some time in the future it may be advisable to stop naming very small satellites.  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).

More information:

The latest updates on nomenclature of features on the surface of satellites (craters, mountains, valleys, etc) can be found on the Planetary Surface Feature News page

Minor Planets

The discoverer of a particular object has the privilege of suggesting a name to a special Committee of the IAU that judges its suitability. Contrary to some recent media reports it is not possible to buy 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!".

A more detailed description of how Minor Planets are named:

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. More detailed info about the whole process from discovery to numbering of newly discovered Minor Planets can be found here or here. 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)
  • non-offensive
  • 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.

In addition,

  • 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.

The alphabetic list of all names is available at the Minor Planet Center including the discovery circumstances.

Naming Objects outside the Solar System

Naming stars

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, an 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.

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 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 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).

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.


  • Michael E. Bakich, The Cambridge Guide to the Constellations. 1995 Cambridge University Press, ISBN 0-521-44921-9. Contains a short chapter on star designations and a long list of 850 common star names (including variants).


  • Common Names:


    Naming nebulae, galaxies, and other objects

    The following text was published by the Task Group on Astronomical Designations from IAU Commission 5:

    International Year of Astronomy

    Beyond 2009

    Strategic Plan