International Astronomical Union

The International Astronomical Union or IAU
is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy.

The IAU is a member of the International Council for Science (ICSU). Its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU maintains friendly relations with organizations that include amateur astronomers in their membership. The IAU has its head office on the second floor of the Institut d’Astrophysique de Paris. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams. The Minor Planet Center (MPC), a clearinghouse for all non-planetary or non-moon bodies in the solar system, also operates under the IAU.


The IAU was founded in 1919, as a merger of various international projects including the Carte du Ciel, the Solar Union and the International Time Bureau (Bureau International de l’Heure). The first appointed President was Benjamin BaillaudPieter Johannes van Rhijn served as president from 1932 to 1958. In the IAU Information Bulletin No. 100, twelve of the fourteen past General Secretaries since 1964, each one in office for the three years between General Assemblies, recall the IAU history with its difficulties, e.g. with Soviet bloc officials, with the Greek military junta, and the reasons behind the unpopular decision to hold an additional Extraordinary General Assembly in Poland on the occasion of Nicolaus Copernicus‘ 500th birthday in February 1973, shortly after the regular GA in Australia.





IAU member states

The IAU has 10,871 individual members, all of whom are professional astronomers and most of whom hold PhDs. There are also 73 nationalmembers who represent countries affiliated with the IAU. 86% of individual members are male, while 14% are female, among them the union’s former president, astronomer Catherine J. Cesarsky.

The sovereign body of the IAU is its General Assembly, which comprises all members. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union (and amendments proposed thereto) and elects various committees.

The right to vote on matters brought before the Assembly varies according to the type of business under discussion. The Statutes consider such business to be divided into two categories:

  • issues of a “primarily scientific nature” (as determined by the Executive Committee), upon which voting is restricted to individual members, and
  • all other matters (such as Statute revision and procedural questions), upon which voting is restricted to the representatives of national members.

On budget matters (which fall into the second category), votes are weighted according to the relative subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid. An absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union.


General Assemblies

Since 1922, the IAU General Assembly meets every three years, with the exception of the period between 1938 to 1948, due to World War II. After a Polish request in 1967, and by a controversial decision of the then President of the IAU, an Extraordinary IAU General Assembly was held in February 1973 in Warsaw, Poland, to commemorate the 500th anniversary of the birth of Nicolaus Copernicus, soon after the regular 1973 GA had been held in Australia.

Meeting Year Venue
Ist IAU General Assembly (1st) 1922 Rome, Italy
IInd IAU General Assembly (2nd) 1925 Cambridge, England, United Kingdom
IIIrd IAU General Assembly (3rd) 1928 Leiden, Netherlands
IVth IAU General Assembly (4th) 1932 CambridgeMassachusetts, United States
Vth IAU General Assembly (5th) 1935 Paris, France
VIth IAU General Assembly (6th) 1938 Stockholm, Sweden
VIIth IAU General Assembly (7th) 1948 Zürich, Switzerland
VIIIth IAU General Assembly (8th) 1952 Rome, Italy
IXth IAU General Assembly (9th) 1955 DublinIreland
Xth IAU General Assembly (10th) 1958 Moscow, Soviet Union
XIth IAU General Assembly (11th) 1961 BerkeleyCalifornia, United States
XIIth IAU General Assembly (12th) 1964 Hamburg, West Germany
XIIIth IAU General Assembly (13th) 1967 PragueCzechoslovakia
XIVth IAU General Assembly (14th) 1970 Brighton, England, United Kingdom
XVth IAU General Assembly (15th) 1973 Sydney, New South Wales, Australia
XVIth IAU General Assembly (16th) 1976 Grenoble, France
XVIIth IAU General Assembly (17th) 1979 MontrealQuebec, Canada
XVIIIth IAU General Assembly (18th) 1982 Patras, Greece
XIXth IAU General Assembly (19th) 1985 New Delhi, India
XXth IAU General Assembly (20th) 1988 BaltimoreMaryland, United States
XXIst IAU General Assembly (21st) 1991 Buenos AiresArgentina
XXIInd IAU General Assembly (22nd) 1994 The Hague, Netherlands
XXIIIrd IAU General Assembly (23rd) 1997 Kyoto, Japan
XXIVth IAU General Assembly (24th) 2000 Manchester, England, United Kingdom
XXVth IAU General Assembly (25th) 2003 Sydney, New South Wales, Australia
XXVIth IAU General Assembly (26th) 2006 Prague, Czech Republic
XXVIIth IAU General Assembly (27th) 2009 Rio de JaneiroBrazil
XXVIIIth IAU General Assembly (28th) 2012 Beijing, China
XXIXth IAU General Assembly (29th) 2015 Honolulu, Hawaii, United States
XXXth IAU General Assembly (30th) 2018 Vienna, Austria
XXXIst IAU General Assembly (31st) 2021 Busan, South Korea (POSTPONED TO AUGUST 2022)
XXXIst IAU General Assembly (32nd) 2024 Cape Town South Africa

The XXVIth General Assembly and the definition of a planet

The XXVIth General Assembly of the International Astronomical Union was held from 14 to 25 August 2006 in Prague, Czech Republic. On 15 August the Assembly decided to restore to individual members the right to vote on scientific matters, which had been removed from them at the XXVth Assembly in 2003. Among the business before the Assembly was a proposal to adopt a formal definition of planet. During the General Assembly the text of the definition evolved from the initial proposal that would have created 12 known planets in the solar system (adding the asteroid that was briefly considered a planet in 1801, CeresPluto‘s present moon CharonEris, and would retain Pluto as a planet) to the final definition of a planet resolution that was passed on 24 August by the Assembly, which classified Ceres, Eris and Pluto as dwarf planets, and reduced the number of planets in the solar system to 8. The voting procedure followed IAU’s Statutes and Working Rules. The General Assembly lasted 12 days and had 2412 participants, most of them for only part of the duration of the Assembly. 424 of the 9785 individual IAU members attended the Closing Ceremony on 24 August 2006. Following the Closing Ceremony, parts of the scientific community did not agree with this ruling, especially the specific wording of the resolution. In the ensuing public debate, a number of laypersons expressed (at times strong) disagreement with the vote. Another, less vocal, fraction of the scientific community backs the resolution, including the discoverer of the dwarf planet ErisMike Brown.

A final decision was made, announced 11 June 2008, for acceptance of the term Plutoid and its official IAU definition:

Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves.

The Commission 46: Education in astronomy

Commission 46 is a Committee of the Executive Committee of the IAU. As a prestigious international scientific union, the IAU plays a special role in the discussion of astronomy development with governments and scientific academies and in interceding about such matters at the highest levels. The IAU is affiliated with the International Council of Scientific Unions (ICSU), a non-governmental organization representing a global membership that includes both national scientific bodies and international scientific unions. When appropriate, the President and officers of the IAU are proactive in persuading the authorities of the importance of astronomy for development and education and in encouraging countries to become members of the IAU. A strategic plan for the period 2010-2020 has been published.

The Commission seeks to further the development and improvement of astronomical education at all levels throughout the world, through various projects initiated, maintained, and to be developed by the Commission and by disseminating information concerning astronomy education at all levels. Part of Commission 46, the Teaching Astronomy for Development (TAD) program is intended to help enhance astronomy education significantly in countries where there is currently very little on offer. TAD operates on the basis of a proposal from a professional astronomy organization or a contract between the IAU and an academic institution, usually a university.

The IAU has launched in 2009 the Galileo Teacher Training Program (GTTP), a Cornerstone project of the International Year of Astronomy 2009, among which Hands-On Universe is a major partner. Hands-On Universe is now officially included in the Astronomy for the Developing World Strategic Plan 2010-20 of IAU, under Section 3.4.2 Astronomy for Children and Schools. During the next decade the IAU will concentrate more resources on education activities for children and schools designed to advance sustainable global development.

The GTTP is concerned with the effective use and transfer of astronomy education tools and resources into classroom science curricula. By training a worldwide network of “Galileo Ambassadors” who will train new “Galileo Teachers” the effect of the program will be multiplied. The GTTP is closely affiliated with the Global Hands-on Universe Program.

Outreach to teachers will involve the provision of training courses, development and translation of materials and harnessing global technological resources in the service of primary and secondary education. A specific goal will be to provide expertise for at least one teacher training course in each region every year, to be organized together with the regional coordinators.

Related programs (leader name): Hands-On Universe (Dr Roger Ferlet), and Universe Awareness (Dr Carolina Ödman).

Information resource: IAU


Fragmentation of data in hundreds of catalogs worldwide, has led to the need to create international catalog of heavenly bodies for the comprehensive development of information on astronomical bodies for centralized collection, storage and tracking of changes.

ICHB has become a controlling entity that registers names to astronomical bodies. For this purpose ICHB has developed a Unified Heavenly Bodies Catalog synchronising all major data from other registrators.



Information resource:  ICHB



A special project with a limited period of validity, which provides access to the free naming of exoplanets for the general public, among valid members of the IAU.

The ExoWorlds list of exoplanets and their host stars being available for public naming at the initiative of the IAU is a list compiled from several exoplanet databases, included in ICHB. The ICHB supports this initiative to attract the general public to the naming of space bodies.

This list includes well-studied exoplanets discovered over twenty years, up to 31 December 2008. A period of at least five years since the discovery has been considered as a simple and satisfactory criterion to include exoplanets which can be considered as confirmed. All the discoveries were made using various methods, including radial velocities, transits, microlensing and direct imagery.

For these exoplanets, the scientific nomenclature follows the nomenclature rules widely adopted by the scientific community, which are drawn from the rules for naming binary stars. For each planet, the name of the host star (around which planets are orbiting) is followed by a lower-case letter: b for the first discovered exoplanet, c for the second, etc. (The letters are capitalized in the case of binaries: the “primary” star name is followed by “A”, and its companion stars are labelled by the same name followed by “B”, “C”, etc.).

In the ExoWorlds list, five stars already have common names: Fomalhaut (alpha Piscis Austrini) is one of the four “royal stars” of ancient Persia, with Aldebaran, Antares, and Regulus; Pollux (beta Geminorum) is the twin brother of Castor, son of Zeus (Jupiter) and Leda, from the ancient Greek and Roman mythologies — the constellation Gemini is named after them (Gemini means twins in latin). Three other stars also have common names: gamma Cephei (Errai, Arabic for shepherd), epsilon Tauri (Ain, Arabic for the bull’s eye) and iota Draconis (Edasich, Arabic also). These stars have common names as well in other cultures.

Consequently these five stars cannot be considered for public naming.

Project site:  NameExoWorlds (At this moment the project is over. We’ll let you know if it opens again.)


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International Astronomical Union