iau0804 — News Release

Plutoid chosen as name for Solar System objects like Pluto

11 June 2008, Paris

The International Astronomical Union has decided on the term plutoid as a name for dwarf planets like Pluto at a meeting of its Executive Committee in Oslo.

Almost two years after the International Astronomical Union (IAU) General Assembly introduced the category of dwarf planets, the IAU, as promised, has decided on a name for transneptunian dwarf planets similar to Pluto. The name plutoid was proposed by the members of the IAU Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III, by the IAU Working Group for Planetary System Nomenclature (WGPSN) and approved by the IAU Executive Committee at its recent meeting in Oslo, Norway.

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, even if they are massive enough that their shape is dictated by self-gravity. The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made.

The dwarf planet Ceres is not a plutoid as it is located in the asteroid belt between Mars and Jupiter. Current scientific knowledge lends credence to the belief that Ceres is the only object of its kind. Therefore, a separate category of Ceres-like dwarf planets will not be proposed at this time.

The IAU has been responsible for naming planetary bodies and their satellites since the early 1900s. The IAU CSBN, who originally proposed the term plutoid, is responsible for naming small bodies (except satellites of the major planets) in the Solar System. The CSBN will be working with the IAU WGPSN to determine the names of new plutoids to ensure that no dwarf planet shares the name of another small Solar System body. The WGPSN oversees the assignment of names to surface features on bodies in the Solar System. These two committees have previously worked together to accept the names of dwarf planet Eris and its satellite Dysnomia.

In Oslo, members of the IAU also discussed the timing involved with the naming of new plutoids. Again, following the advice of the Division III Board and the two Working Groups, it was decided that, for naming purposes, any Solar System body having (a) a semimajor axis greater than that of Neptune, and (b) an absolute magnitude brighter than H = +1 (see Notes) will, for the purpose of naming, be considered to be a plutoid, and be named by the WGPSN and the CSBN. Name(s) proposed by the discovery team(s) will be given deference. If further investigations show that the object is not massive enough and does not qualify as a plutoid, it will keep its name but change category.

In French plutoid is plutoïde, in Spanish plutoide and in Japanese 冥王星型天体.


The IAU is the international astronomical organisation that brings together almost 10 000 distinguished astronomers from all nations of the world. Its mission is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU also serves as the internationally recognised authority for assigning designations to celestial bodies and the surface features on them. Founded in 1919, the IAU is the world's largest professional body for astronomers.

 The absolute magnitude H for planets, dwarf planets, comets and asteroids allows astronomers to compare the brightness of objects as if they all sat one astronomical unit from the Sun and the Earth and at a phase angle of zero degrees. In this scale, increasing brightness corresponds to a decreasing magnitude. Apparently bright objects can have negative magnitudes while positive magnitudes indicate to dim objects.


For more information, please contact:

Dr. Edward L.G. Bowell

IAU Division III President

Lowell Observatory, USA

Tel: +1-928-774-3358



Dr. Catherine J. Cesarsky

IAU President

CEA-Saclay DSM / IRFU, France

Phone: +33-1-3024-5090



Dr. Karel A. van der Hucht

IAU General Secretary

International Astronomical Union, France

Tel: +31-30-2535729/5600

Cell: +31-6-53-960237



Mr. Lars Lindberg Christensen

IAU Press Officer

ESA/Hubble, Garching, Germany

Tel: +49-89-32-00-63-06

Cellular: +49-173-3872-621




IAU website:

Information about Pluto and the other dwarf planets:





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