A dwarf planet is a planetary-mass object that is neither a planet nor a satellite. More explicitly, the International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravitational rather than mechanical forces (that is, it has sufficient mass to overcome its internal compressive strength and achieve hydrostatic equilibrium, and is thus an ellipsoid in shape), but that unlike a planet has not cleared its orbital region of other objects. The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown, who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern, who had coined the term dwarf planet in 1990.
The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1) are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.
It is suspected that another hundred or so known objects in the Solar System are dwarf planets. Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered. Individual astronomers have accepted several additional bodies as dwarf planets, and Mike Brown published in August 2011 a list of 390 candidate objects, organized in categories from "nearly certainly" to "possibly" meeting the IAU's criteria, along with his classification methodology. Brown identifies nine known objects – the five accepted by the IAU plus 2007 OR10, Sedna, Quaoar, and Orcus – as "virtually certain", with another two dozen highly likely, and there are probably a hundred or so such objects in total.
The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.
Other articles related to "planet, dwarf planet, dwarf planets, planets":
... for a celestial body to be considered a planet in the Solar System ... three criteria adopted by the International Astronomical Union (IAU) in its 2006 definition of planet ... In the end stages of planet formation, a planet will have "cleared the neighbourhood" of its own orbital zone, meaning it has become gravitationally dominant, and there are no other bodies of comparable ...
... When the definition of "dwarf planet" was instated at the IAU General Assembly of 2006, Ceres, Pluto and Eris were identified by name as the initial ... an official procedure for calculating the lower bound of size to be a "dwarf planet", no further bodies could be formally recognized as either dwarf planets or plutoids ... of Makemake and Haumea, and their formal recognition as plutoids and dwarf planets, bringing the number of IAU-accepted plutoids to four ...
... In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution ... Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight ... Alan Stern, the director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than ...
... This is the defining limit of a dwarf planet ... The upper and lower size and mass limits of dwarf planets have not been specified by the IAU ... than Mercury that has not "cleared the neighbourhood around its orbit" would be classified as a dwarf planet ...
... The phrase refers to an orbiting body (a planet or protoplanet) "sweeping out" its orbital region over time, by gravitationally interacting with smaller ... the IAU planets), and the unterplanets are the rest (i.e ... the IAU dwarf planets) ...
Famous quotes containing the words planet and/or dwarf:
“Until they saw, over the mists
of Venus, two fish creatures stop
on spangled legs and crawl
from the belly of the sea.
And from the planet park
they heard the new fruit drop.”
—Anne Sexton (19281974)
“A dwarf who brings a standard along with him to measure his own sizetake my word, is a dwarf in more articles than one.”
—Laurence Sterne (17131768)