Dwarf Planet

A **dwarf planet** is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to planetary geologists is that they may be geologically active bodies, an expectation that was borne out in 2015 by the *Dawn* mission to Ceres and the *New Horizons* mission to Pluto.

Astronomers are in general agreement that at least the nine largest candidates are dwarf planets: Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. Of these nine plus the tenth-largest candidate Salacia, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine the nature of these worlds. Only one, Sedna, has neither been visited nor has any known moons, making an accurate estimate of mass difficult. Some astronomers include many smaller bodies as well, but there is no consensus that these are likely to be dwarf planets.

The term *dwarf planet* was coined by planetary scientist Alan Stern as part of a three-way categorization of planetary-mass objects in the Solar System: classical planets, dwarf planets, and satellite planets. Dwarf planets were thus conceived of as a category of planet. In 2006, however, the concept was adopted by the International Astronomical Union (IAU) as a category of *sub*–planetary objects, part of a three-way recategorization of bodies orbiting the Sun: planets, dwarf planets and small Solar System bodies. Thus Stern and other planetary geologists consider dwarf planets and large satellites to be planets, but since 2006, the IAU and perhaps the majority of astronomers have excluded them from the roster of planets.

Names for large subplanetary bodies include *dwarf planet*, *planetoid* (more general term), *meso-planet* (narrowly used for sizes between Mercury and Ceres), *quasi-planet* and (in the transneptunian region) *plutoid*. *Dwarf planet*, however, was originally coined as a term for the smallest planets, not the largest sub-planets, and is still used that way by many planetary astronomers.

Alan Stern coined the term *dwarf planet*, analogous to the term *dwarf star*, as part of a three-fold classification of planets, and he and many of his colleagues continue to classify dwarf planets as a class of planets. The IAU decided that dwarf planets are not to be considered planets, but kept Stern's term for them. Other terms for the IAU definition of the largest subplanetary bodies that do not have such conflicting connotations or usage include *quasi-planet* and the older term *planetoid* ("having the form of a planet"). Michael E. Brown stated that *planetoid* is "a perfectly good word" that has been used for these bodies for years, and that the use of the term *dwarf planet* for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution. Indeed, the draft of Resolution 5A had called these median bodies *planetoids,* but the plenary session voted unanimously to change the name to *dwarf planet.* The second resolution, 5B, defined *dwarf planets* as a subtype of *planet*, as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight *classical planets* and four *dwarf planets*. Resolution 5B was defeated in the same session that 5A was passed. Because of the semantic inconsistency of a *dwarf planet* not being a planet due to the failure of Resolution 5B, alternative terms such as *nanoplanet* and *subplanet* were discussed, but there was no consensus among the CSBN to change it.

In most languages equivalent terms have been created by translating *dwarf planet* more-or-less literally: French *planète naine*, Spanish *planeta enano*, German *Zwergplanet*, Russian *karlikovaya planeta* (карликовая планета), Arabic *kaukab qazm* (كوكب قزم), Chinese *ǎixíngxīng* (矮行星), Korean *waesohangseong* (왜소행성 / 矮小行星) or *waehangseong* (왜행성 / 矮行星), but in Japanese they are called *junwakusei* (準惑星), meaning "quasi-planets" or "peneplanets" (pene- meaning "almost").

IAU Resolution 6a of 2006 recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as *plutons* and *plutonian objects* but neither name was carried forward, perhaps due to objections from geologists that this would create confusion with their *pluton*.

On June 11, 2008, the IAU Executive Committee announced a new term, *plutoid*, and a definition: all trans-Neptunian dwarf planets are plutoids. Other departments of the IAU have rejected the term:

> ...in part because of an email miscommunication, the WG-PSN [Working Group for Planetary System Nomenclature] was not involved in choosing the word plutoid. ... In fact, a vote taken by the WG-PSN subsequent to the Executive Committee meeting has rejected the use of that specific term..."
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The category of 'plutoid' captured an earlier distinction between the 'terrestrial dwarf' Ceres and the 'ice dwarfs' of the outer Solar system, part of a conception of a threefold division of the Solar System into inner *terrestrial planets*, central *gas giants* and outer *ice dwarfs*, of which Pluto was the principal member. 'Ice dwarf' also saw some use as an umbrella term for all trans-Neptunian minor planets, or for the ice asteroids of the outer Solar System; one attempted definition was that an ice dwarf "is larger than the nucleus of a normal comet and icier than a typical asteroid."

Since the *Dawn* mission, it has been recognized that Ceres is an icy body more similar to the icy moons of the outer planets and to TNOs such as Pluto than it is to the terrestrial planets, blurring the distinction, and Ceres has since been called an ice dwarf as well.