Solar System Exploration: Planets: Dwarf Planets

Dwarf Planets:

What Defines a Planet, Dwarf Planet or Plutoid?




	Ceres' round shape suggests that its interior is layered like those of terrestrial planets such as Earth.

What constitutes a planet? The International Astronomical Union (IAU) developed some definitions in 2001, modified them again in 2003, and as of August 24, 2006, the IAU has come up with another definition. The IAU said in a statement that the definition for a planet is now officially known as "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has cleared the neighborhood around its orbit."

Dwarf Planets

A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

Plutoids

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 trans-neptunian 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 semi-major 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 neighborhood 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 three known and named plutoids are Pluto, Eris and as of July 2008, MakeMake. It is expected that more plutoids will be named as science progresses and new discoveries are made.

While all plutoids are dwarf planets, it is interesting to note that not all dwarf planets are plutoids, as is the case with Ceres.

What about Pluto? Once known as the smallest, coldest, and most distant planet from the Sun, Pluto has a dual identity, not to mention being enshrouded in controversy since its discovery in 1930. On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet to a dwarf planet. According to the new rules a planet meets three criteria: it must orbit the Sun, it must be big enough for gravity to squash it into a round ball, and it must have cleared other things out of the way in its orbital neighborhood. The latter measure knocks out Pluto and 2003UB313 (Eris), which orbit among the icy wrecks of the Kuiper Belt, and Ceres, which is in the asteroid belt. In 2008, Pluto, Eris and most recently, MakeMake have been classified as "plutoids," while presently there are no plans to reclassify Ceres. Discovered by American astronomer Clyde Tombaugh in 1930, Pluto takes 248 years to orbit the Sun. Pluto's most recent close approach to the Sun was in 1989. Between 1979 and 1999, Pluto's highly elliptical orbit brought it closer to the Sun than Neptune, providing rare opportunities to study this small, cold, distant world and its companion moon, Charon. Most of what we know about Pluto we have learned since the late 1970s from Earth-based observations, the Infrared Astronomical Satellite (IRAS), and the Hubble Space Telescope. Many of the key questions about Pluto, Charon, and the outer fringes of our solar system await close-up observations by a robotic space flight mission. No spacecraft have yet visited Pluto. However, NASA launched a mission called New Horizons that is en route to this icy world and will explore both Pluto and the Kuiper Belt region. All other objects except satellites orbiting the sun shall be referred to collectively as "Small Solar-System Bodies." According to the IAU, more dwarf planets as well as plutoids are expected to be announced in the coming months and years. For example, in July, 2008, a new plutoid dwarf planet has been officially named by the IAU. The name Makemake has been approved for the Transneptunian Dwarf Planet (136472) 2005 FY9. Makemake is named after a creator god of Rapa Nui (Easter Island). Currently, more than a dozen candidate dwarf planets are listed on IAU's dwarf planet watch list, which keeps changing as new objects are found and the physics of the existing candidates becomes better-known.