Renaming Pluto

In this artist’s concept, Pluto and its moon Charon are seen from the surface of one of Pluto’s other satellites.
Credit: David A. Aguilar of the Harvard-Smithsonian CfA

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 distance 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 neighborhood around their orbit. 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.

Artist’s concept of Eris (the object formerly known as Xena) and its satellite, Dysnomia. The sun and planets appear in the distance.
Credit: R. Hurt, IPAC

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.

The orbits of the outer planets and Pluto in the solar system, with the blue and purple dots representing the location of Kuiper Belt objects. Plutoids are dwarf planets like Pluto that orbit the sun at a distance greater than Neptune.
Credit: NASA/JPL-Caltech

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 magnitude will 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.

Plutoids and other objects beyond Neptune are of great interest to astrobiologists. Objects in this distant region are thought to be relics from the early Solar System at a time when Earth was forming. Studying these objects can provide information about the history and evolution of Earth and the other planets, and can help us understand how Earth became habitable. The distant reaches of the solar system are also thought to be an important source of comets, which can impact the Earth if they enter into the inner solar system. These comets may have delivered water and organics to the early Earth, aiding in the origin of life. Currently, NASA’s New Horizon mission is en route to Pluto and the Kuiper Belt. It will be the first mission to visit Pluto, and will provide important information about this relatively unknown region of space.

Artist’s conception of eight dwarf planets. No clear and detailed photographs have yet been made of these objects.

Related Web Sites

Astrobiology Roadmap Goal 1: Habitable Planets
Astrobiology Roadmap Goal 3: Origins of Life
Questioning Dwarfs
New Horizons Launches to Pluto
Tenth Planet
Eris Outweighs Pluto
Astrobiology Top 10: New Horizons
The Solar System’s Ice Machine