::Dwarf planet


Planets::dwarf    Planet::align    Title::pluto    Objects::first    Brown::system    Author::style


Ceres, the only dwarf planet in the asteroid belt imaged by Dawn Pluto seen by New Horizons on 13 July 2015
|-Haumea with its two moons, as seen by KeckMakemake imaged by the Hubble Telescope in 2006      
|-Eris and its moon seen from Hubble Sedna seen through Hubble
The first five recognized dwarf planets (from top left):

(see full list of likeliest possible dwarf planets)

A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. That is, it is in direct orbit of the Sun, and is massive enough for its shape to be in hydrostatic equilibrium under its own gravity, but has not cleared the neighborhood around its orbit.<ref name="finalresolution" /><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,<ref name="finalresolution"/> brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris.<ref name="Brown Schaller 2007">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> 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 astronomer Mike Brown,<ref>{{#invoke:citation/CS1|citation |CitationClass=news }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=news }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=news }}</ref> who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern,<ref name="Stern">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Stern2009">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> who had coined the term dwarf planet in 1990.<ref>S. Alan Stern, "On the number of planets in the outer solar system: Evidence of a substantial population of 1000-km bodies", Icarus 90:2, April 1991</ref>

The International Astronomical Union (IAU) currently recognizes five dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris.<ref name="WGPSN"/> Brown criticizes this official recognition: "A reasonable person might think that this means that there are five known objects in the solar system which fit the IAU definition of dwarf planet, but this reasonable person would be nowhere close to correct."<ref name=free/>

It is suspected that another hundred or so known objects in the Solar System are dwarf planets.<ref name="BrownList"/> 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.<ref name="Stern2012"/> Individual astronomers recognize several of these,<ref name="BrownList"/> and in August 2011 Mike Brown published a list of 390 candidate objects, ranging from "nearly certain" to "possible" dwarf planets.<ref name=free>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Brown currently identifies eleven known objects – the five accepted by the IAU plus 2007 OR10, Quaoar, Sedna, Orcus, (307261) 2002 MS4 and Salacia – as "virtually certain", with another dozen highly likely.<ref name="BrownList">Mike Brown, 'How many dwarf planets are there in the outer solar system?' Accessed 2013-11-15</ref> Stern states that there are more than a dozen known dwarf planets.<ref name=Stern2012>Alan Stern, The PI's Perspective, August 24, 2012</ref>

However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They 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)<ref name="bruton">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> are to be named under the assumption that they are dwarf planets.<ref name=IAU2008/> The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets. The question of whether other likely objects are dwarf planets has never been addressed by the IAU.

The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.<ref name="Draft Resolution 5"/>

Dwarf planet sections
Intro   History of the concept    Name    Characteristics    Dwarf planets and possible dwarf planets   Exploration   Contention    Planetary-mass moons   Gallery   See also    Notes    References    External links   

PREVIOUS: IntroNEXT: History of the concept