Dwarf planet

{{Main|Geophysical definition of planet|IAU definition of planet}}

File:Pluto and charon near true color.png and its moon Charon. Separation to scale]]

File:Vesta in natural color (cropped).jpg, an asteroid that was once a dwarf planet{{Cite web|url=https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/4-vesta/in-depth|title=In Depth | 4 Vesta|website=NASA Solar System Exploration|date=November 10, 2017 |access-date=February 29, 2020|archive-date=February 29, 2020|archive-url=https://web.archive.org/web/20200229050949/https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/4-vesta/in-depth/|url-status=live}}]]

Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter that for decades were considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid (from Greek, meaning 'star-like' or 'star-shaped') for the smaller bodies and began to distinguish them as minor planets rather than major planets.{{cite journal |url=http://philsci-archive.pitt.edu/3418/ |author=Mauro Murzi |title=Changes in a scientific concept: what is a planet? |date=2007 |access-date=April 6, 2013 |journal=Preprints in Philosophy of Science |type=Preprint |publisher=University of Pittsburgh |archive-date=June 11, 2019 |archive-url=https://web.archive.org/web/20190611023948/http://philsci-archive.pitt.edu/3418/ |url-status=live }}

With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine major planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years, Pluto was thought to be larger than Mercury,{{cite web |title=Pluto Revealed |url=http://www.discoveryofpluto.com/pluto06.html |first=Brad |last=Mager |access-date=January 26, 2008 |publisher=discoveryofpluto.com |archive-url=https://web.archive.org/web/20110722004610/http://www.discoveryofpluto.com/pluto06.html |archive-date=July 22, 2011 |url-status=dead }}{{cite web |title=Is Pluto a planet? |url=http://curious.astro.cornell.edu/question.php?number=624 |date=September 14, 2007 |author1=Cuk, Matija |author2=Masters, Karen |publisher=Cornell University, Astronomy Department |access-date=January 26, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20071012225232/http://curious.astro.cornell.edu/question.php?number=624 |archive-date=October 12, 2007 }} but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates.{{cite journal |title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2 |journal=The Astronomical Journal |date=2006 |issue=1 |pages=290–298 |doi=10.1086/504422 |first=Marc W. |last=Buie |author2=Grundy, William M. |author3=Young, Eliot F. |author4=Young, Leslie A. |author5=Stern, S. Alan |volume=132 |bibcode=2006AJ....132..290B |arxiv=astro-ph/0512491|s2cid=119386667 }} It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had only one-fifth the mass of Earth's Moon.{{cite book |first=David |last=Jewitt |author2=Delsanti, Audrey |title=The Solar System Beyond The Planets in Solar System Update : Topical and Timely Reviews in Solar System Sciences |publisher=Springer |doi=10.1007/3-540-37683-6 |isbn=978-3-540-37683-5 |date=2006 |url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |access-date=February 10, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20060525051103/http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |archive-date=May 25, 2006 }} Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets.{{cite book |title=Is Pluto a Planet? A Historical Journey through the Solar System |first=David A. |last=Weintraub |date=2006 |pages=[https://archive.org/details/isplutoplanethis00wein/page/1 1–272] |isbn=978-0-691-12348-6 |publisher=Princeton Univ. Press |location=Princeton, N.J. |url-access=registration |url=https://archive.org/details/isplutoplanethis00wein/page/1 }}

In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away.{{cite web |title=Much Ado about Pluto |publisher=PlutoPetition.com |first=Tony |last=Phillips |author2=Phillips, Amelia |date=September 4, 2006 |url=http://www.plutopetition.com/unplanet.php |access-date=January 26, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080125225956/http://www.plutopetition.com/unplanet.php |archive-date=January 25, 2008 }}

Many of these shared several of Pluto's key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects, the plutinos. It became clear that either the larger of these bodies would also have to be classified as planets, or Pluto would have to be reclassified, much as Ceres had been reclassified after the discovery of additional asteroids.{{cite web |title=What is the definition of a planet? |date=2004 |url=http://web.gps.caltech.edu/~mbrown/sedna/index.html#planets |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |access-date=January 26, 2008 |archive-url=https://web.archive.org/web/20110719160217/http://web.gps.caltech.edu/~mbrown/sedna/index.html#planets |archive-date=July 19, 2011 |url-status=dead }}

This led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets.{{cite web |last1=Eicher |first1=David J. |author1-link=David J. Eicher |title=Should Pluto Be Considered a Planet? |url=https://astronomy.com/magazine/greatest-mysteries/2007/07/32-should-pluto-be-considered-a-planet |website=Astronomy |access-date=November 28, 2022 |language=en |date=July 21, 2007 |archive-date=November 28, 2022 |archive-url=https://web.archive.org/web/20221128115033/https://astronomy.com/magazine/greatest-mysteries/2007/07/32-should-pluto-be-considered-a-planet |url-status=live }}{{cite news |url=https://hubblesite.org/contents/news-releases/2004/news-2004-14.html/ |date=April 14, 2004 |title=Hubble Observes Planetoid Sedna, Mystery Deepens |publisher=NASA's Hubble Space Telescope home site |access-date=January 26, 2008 |archive-url=https://web.archive.org/web/20210113110507/https://hubblesite.org/contents/news-releases/2004/news-2004-14.html |archive-date=January 13, 2021 |url-status=live}}

Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain classified as a planet.{{cite news |title=War of the Worlds |work=The New York Times |author=Brown, Mike |date=August 16, 2006 |url=https://www.nytimes.com/2006/08/16/opinion/16brown.html |access-date=February 20, 2008 |archive-date=February 13, 2017 |archive-url=https://web.archive.org/web/20170213193022/http://www.nytimes.com/2006/08/16/opinion/16brown.html |url-status=live }}

Eris (then known as {{mp|2003 UB|313}}), a trans-Neptunian object, was discovered in January 2005;{{Cite web |url=http://web.gps.caltech.edu/~mbrown/planetlila/ |title=California Institute of Technology, Retrieved 4-12-2015 |access-date=April 12, 2015 |archive-date=May 17, 2012 |archive-url=https://web.archive.org/web/20120517130215/http://web.gps.caltech.edu/~mbrown/planetlila/ |url-status=live }} it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet.{{cite news |title=Astronomers Measure Mass of Largest Dwarf Planet |publisher=NASA's Hubble Space Telescope home site |url=http://hubblesite.org/newscenter/archive/releases/2007/24/full/ |date=June 14, 2007 |access-date=January 26, 2008 |archive-date=August 7, 2011 |archive-url=https://web.archive.org/web/20110807132150/http://hubblesite.org/newscenter/archive/releases/2007/24/full/ |url-status=live }} As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006.{{cite web |title=What makes a planet? |url=http://www.gps.caltech.edu/~mbrown/whatsaplanet/ |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |access-date=January 26, 2008 |archive-date=May 16, 2012 |archive-url=https://web.archive.org/web/20120516023200/http://www.gps.caltech.edu/~mbrown/whatsaplanet/ |url-status=live }} The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi: They proposed an intermediate category for objects large enough to be round but that had not cleared their orbits of planetesimals. Beside dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits.{{cite web |date=August 19, 2006 |first=Robert Roy |last=Britt |title=Details Emerge on Plan to Demote Pluto |publisher=Space.com |access-date=August 18, 2006 |url=http://www.space.com/scienceastronomy/060819_new_proposal.html |archive-date=June 28, 2011 |archive-url=https://web.archive.org/web/20110628232939/http://www.space.com/scienceastronomy/060819_new_proposal.html |url-status=live }}

Although concerns were raised about the classification of planets orbiting other stars,{{cite web |title=The IAU draft definition of "planet" and "plutons" |url=http://www.iau.org/news/pressreleases/detail/iau0601/ |publisher=International Astronomical Union |date=August 16, 2006 |access-date=May 17, 2008 |archive-date=April 29, 2014 |archive-url=https://web.archive.org/web/20140429050716/http://www.iau.org/news/pressreleases/detail/iau0601/ |url-status=live }} the issue was not resolved; it was proposed instead to decide this only when dwarf-planet-size objects start to be observed.

In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution. Campaigns included car bumper stickers and T-shirts.{{cite news |url=https://www.usatoday.com/tech/science/space/2006-08-25-pluto-memorabilia_x.htm |title=Online merchants see green in Pluto news |first=Alicia |last=Chang |agency=Associated Press |publisher=USA Today |date=August 25, 2006 |access-date=January 25, 2008 |archive-date=May 11, 2008 |archive-url=https://web.archive.org/web/20080511172619/http://www.usatoday.com/tech/science/space/2006-08-25-pluto-memorabilia_x.htm |url-status=live }} Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.{{cite web |title=The Eight Planets |first=Michael E. |last=Brown |url=http://web.gps.caltech.edu/~mbrown/eightplanets/ |publisher=California Institute of Technology, Department of Geological Sciences |access-date=January 26, 2008 |archive-url=https://web.archive.org/web/20110719162512/http://web.gps.caltech.edu/~mbrown/eightplanets/ |archive-date=July 19, 2011 |url-status=dead }}

NASA announced in 2006 that it would use the new guidelines established by the IAU.{{cite press release|title= {{Sic|hide=y|Hotly|-}}Debated Solar System Object Gets a Name|publisher=NASA|url= http://www.nasa.gov/vision/universe/solarsystem/erisf-20060914.html|date= September 14, 2006|access-date= January 26, 2008|archive-date= June 29, 2011|archive-url= https://web.archive.org/web/20110629155731/http://www.nasa.gov/vision/universe/solarsystem/erisf-20060914.html|url-status= dead}} 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 a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets.{{cite web |title=Unabashedly Onward to the Ninth Planet |first=Alan |last=Stern |date=September 6, 2006 |publisher=New Horizons Web Site |url=http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_09_06_2006.php |access-date=January 26, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20131207024954/http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_09_06_2006.php |archive-date=December 7, 2013 }} Thus, in 2011, he still referred to Pluto as a planet,{{cite web |url=http://www.space.com/12710-pluto-defender-alan-stern-dwarf-planet-interview.html |first=Mike |last=Wall |publisher=Space.com |title=Pluto's Planet Title Defender: Q & A With Planetary Scientist Alan Stern |date=August 24, 2011 |access-date=December 3, 2012 |archive-date=August 14, 2012 |archive-url=https://web.archive.org/web/20120814212232/http://www.space.com/12710-pluto-defender-alan-stern-dwarf-planet-interview.html |url-status=live }} and accepted other likely dwarf planets such as Ceres and Eris, as well as the larger moons, as additional planets. Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets".

File:Euler diagram of solar system bodies.svg showing the IAU Executive Committee conception of the types of bodies in the Solar System (except the Sun)]]

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 in 1990 or 1991,{{Cite book |last1=Stern |first1=Alan |author-link1=Alan Stern |title=Chasing new horizons: inside the epic first mission to Pluto |title-link=Chasing New Horizons |last2=Grinspoon |first2=David |author-link2=David Grinspoon |date=2018 |publisher=Picador |isbn=978-1-250-09896-2 |location=New York | pages=136 | quote=a term Alan coined in a 1991 research paper}}{{dubious|date=April 2025|Verifiability issues on Stern's coining of "dwarf planet"}} as an analogy to the term dwarf star for small stars.{{cn|date=April 2025}} Since the passing of the IAU definition, he and many of his colleagues continued 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{{cite news |first=Tom |last=Service |title=Sounds of the solar system: probing Pluto's predicted score |newspaper=The Guardian |date=July 15, 2015 |url=https://www.theguardian.com/music/tomserviceblog/2015/jul/15/sounds-pluto-new-horizons-colin-matthews-holst |access-date=December 26, 2019 |archive-date=December 26, 2019 |archive-url=https://web.archive.org/web/20191226002852/https://www.theguardian.com/music/tomserviceblog/2015/jul/15/sounds-pluto-new-horizons-colin-matthews-holst |url-status=live }} and the older term planetoid ("having the form of a planet").{{cite book |editor=Karttunen |display-editors=etal |title=Fundamental Astronomy |publisher=Springer |date=2007 |edition=5 |title-link=Fundamental Astronomy (book)}} 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.{{cite book |first=Mike |last=Brown |title=How I Killed Pluto and Why It Had It Coming |page=[https://archive.org/details/howikilledplutow00mike/page/223 223] |publisher=Spiegel & Grau |date=2010 |title-link=How I Killed Pluto and Why It Had It Coming}} Indeed, the draft of Resolution 5A had called these median bodies planetoids,{{cite web |title=Comments & discussions on Resolution 5: The definition of a planet – Planets Galore |first=Mark E. |last=Bailey |url=http://astro.cas.cz/nuncius/appendix.html |work=Dissertatio cum Nuncio Sidereo, Series Tertia – official newspaper of the IAU General Assembly 2006 |publisher=Astronomical Institute Prague |access-date=February 9, 2008 |archive-date=July 20, 2011 |archive-url=https://web.archive.org/web/20110720030946/http://astro.cas.cz/nuncius/appendix.html |url-status=live }}{{cite web |title=Dos uruguayos, Julio Fernández y Gonzalo Tancredi en la historia de la astronomía:reducen la cantidad de planetas de 9 a 8 ...&Anotaciones de Tancredi |language=es |publisher=Science and Research Institute, Mercedes, Uruguay |url=http://www.ici.edu.uy/perfilASTRO.htm |access-date=February 11, 2008 |archive-url=https://web.archive.org/web/20071220063342/http://www.ici.edu.uy/perfilASTRO.htm |archive-date=December 20, 2007}} but the plenary session voted unanimously to change the name to dwarf planet.{{cite news |url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |title=Definition of a Planet in the Solar System: Resolutions 5 and 6 |date=August 24, 2006 |work=IAU 2006 General Assembly |publisher=International Astronomical Union |access-date=January 26, 2008 |author=IAU |archive-date=June 20, 2009 |archive-url=https://web.archive.org/web/20090620102000/http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |url-status=live }} 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 {{lang|fr|planète naine}}, Spanish {{lang|es|planeta enano}}, German {{lang|de|Zwergplanet}}, Russian karlikovaya planeta ({{lang|ru|карликовая планета}}), Arabic kaukab qazm ({{lang|ar|كوكب قزم}}), Chinese ǎixíngxīng ({{lang|cmn|矮行星}}), Korean waesohangseong ({{lang|ko|왜소행성 / 矮小行星}}) or waehangseong ({{lang|ko|왜행성 / 矮行星}}), but in Japanese they are called junwakusei ({{lang|ja|準惑星}}), meaning "quasi-planets" or "peneplanets" (pene- meaning "almost").

IAU Resolution 6a of 2006{{Cite web|date=August 24, 2006|url=https://www.iau.org/news/pressreleases/detail/iau0603/|title=International Astronomical Union 2006 General Assembly: Result of the IAU Resolution votes|website=IAU|access-date=August 10, 2021|archive-date=April 29, 2014|archive-url=https://web.archive.org/web/20140429212224/https://www.iau.org/news/pressreleases/detail/iau0603/|url-status=live}} 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.{{Cite web |date=June 11, 2008|url=https://www.iau.org/news/pressreleases/detail/iau0804/|title=Plutoid chosen as name for Solar System objects like Pluto|website=IAU|access-date=August 10, 2021|archive-date=November 23, 2020|archive-url=https://web.archive.org/web/20201123162642/https://www.iau.org/news/pressreleases/detail/iau0804/|url-status=live|location=Paris}} Other departments of the IAU have rejected the term: {{blockquote|...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..."{{cite journal |last1=Bowell |first1=Edward L.G. |author-link1=Edward L. G. Bowell |last2=Meech |first2=Karen J. |author-link2=Karen Jean Meech |last3=Williams |first3=Iwan P. |author-link3=:fr:Iwan Williams |last4=Boss |first4=Alan P. |author-link4=Alan Boss |last5=Consolmagno |first5=Guy J. |author-link5=Guy Consolmagno |last6=Courtin |first6=Régis |last7=Fernández |first7=Julio A. |author-link7=Julio Ángel Fernández |last8=Gustafson |first8=Bo Å. S. |last9=Huebner |first9=Walter F. |author-link9=Walter F. Huebner |last10=Levasseur-Regourd |first10=Anny-Chantal |author-link10=Anny-Chantal Levasseur-Regourd |last11=Marov |first11=Mikhail Ya. |author-link11=Mikhail Marov |last12=Mayor |first12=Michel |author-link12=Michel Mayor |last13=Schulz |first13=Rita M. |author-link13=:de:Rita Schulz |last14=Spurný |first14=Pavel |author-link14=:cs:Pavel Spurný |last15=Valsecchi |first15=Giovanni B. |last16=Watanabe |first16=Jun-ichi |author-link16=:ja:渡部潤一 |last17=Witt |first17=Adolf N. |display-authors=3 |date=2008-12-01 |title=Division III: Planetary Systems Sciences |journal=Proceedings of the International Astronomical Union |volume=4 |issue=T27A |pages=149–153 |publisher=Cambridge University Press |doi=10.1017/S1743921308025398 |doi-access=free}}}}

The category of 'plutoid' captured an earlier distinction between the 'terrestrial dwarf' Ceres and the 'ice dwarfs' of the outer Solar system,{{cite book |last=Carson |first=Mary Kay |date=2011 |title=Far-Out Guide to the Icy Dwarf Planets |url=https://archive.org/details/faroutguidetoicy0000cars/mode/2up |url-access=registration |publisher=Enslow Publishers |isbn=9780766031876 |oclc=441945398 |via=Internet Archive}} part of a conception of a threefold division of the Solar System into inner terrestrial planets, central giant planets, and outer ice dwarfs, of which Pluto was the principal member.{{cite book |last=Lew |first=Kristi |date=2010 |title=Space! The Dwarf Planet Pluto |url=https://archive.org/details/dwarfplanetpluto0000lewk |url-access=registration |publisher=Marshall Cavendish Benchmark |place=New York |page=10 |isbn=9780761445531 |oclc=562529871 |via=Internet Archive}} '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."{{cite web

|title=Ice dwarf |website=Encyclopedia of Astrobiology, Astronomy and Spaceflight |editor-last=Darling |editor-first=David |editor-link=David J. Darling |url=http://www.daviddarling.info/encyclopedia/I/icedwarf.html |access-date=June 22, 2008 |url-status =dead |archive-url=https://web.archive.org/web/20080706143614/http://www.daviddarling.info/encyclopedia/I/icedwarf.html |archive-date=July 6, 2008

}}

Since the Dawn mission, it has been recognized that Ceres is a geologically icy body that may have originated from the outer Solar System.{{Cite web|url = https://www.space.com/33934-dwarf-planet-ceres-ice-volcano-discoveries.html|title = Ice Volcanoes and More: Dwarf Planet Ceres Continues to Surprise|website = Space.com|date = September 2016|access-date = December 19, 2019|archive-date = October 12, 2019|archive-url = https://web.archive.org/web/20191012004925/https://www.space.com/33934-dwarf-planet-ceres-ice-volcano-discoveries.html|url-status = live}}{{cite web|title=Dawn at Ceres: What Have we Learned?|display-authors=etal|first1=J. C.|last1=Castillo-Rogez|first2=C. A.|last2=Raymond|first3=C. T.|last3=Russell|url=http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183286.pdf|publisher=Committee on Astrobiology and Planetary Science|date=September 12, 2017|access-date=October 12, 2019|archive-date=October 8, 2018|archive-url=https://web.archive.org/web/20181008123813/http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_183286.pdf|url-status=live}}

Ceres has since been called an ice dwarf as well.{{Cite book|chapter=Ceres: The First Known Ice Dwarf Planet |title=Ice Worlds of the Solar System: Their Tortured Landscapes and Biological Potential |first=Michael |last=Carroll|author-link=Michael Carroll (space artist) |date=October 23, 2019 |doi=10.1007/978-3-030-28120-5 |publisher=Springer Cham |isbn=978-3-030-28120-5}}

|}

The category dwarf planet arose from a conflict between dynamical and geophysical ideas of what a useful conception of a planet would be. In terms of the dynamics of the Solar System, the major distinction is between bodies that gravitationally dominate their neighbourhood (Mercury through Neptune) and those that do not (such as the asteroids and Kuiper belt objects). A celestial body may have a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight, which results in the body acquiring a round shape. Because this requires a much lower mass than gravitationally dominating the region of space near their orbit, there are a population of objects that are massive enough to have a world-like appearance and planetary geology, but not massive enough to clear their neighborhood. Examples are Ceres in the asteroid belt and Pluto in the Kuiper belt.

Dynamicists usually prefer using gravitational dominance as the threshold for planethood, because from their perspective smaller bodies are better grouped with their neighbours, e.g. Ceres as simply a large asteroid and Pluto as a large Kuiper belt object.{{cite web |first=Mike |last=Brown |title=The eight planets |website=gps.caltech.edu |publisher=Caltech |url=http://web.gps.caltech.edu/~mbrown/eightplanets/ |access-date=January 26, 2008 |archive-date=July 19, 2011 |archive-url=https://web.archive.org/web/20110719162512/http://web.gps.caltech.edu/~mbrown/eightplanets/ |url-status=live }}{{cite web |first=David |last=Jewitt |title=Classification of Pluto |website=ess.ucla.edu |publisher=UCLA |url=http://www2.ess.ucla.edu/~jewitt/kb/pluto_classification.html |access-date=August 19, 2021 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819074719/http://www2.ess.ucla.edu/~jewitt/kb/pluto_classification.html |url-status=live }} Geoscientists usually prefer roundness as the threshold, because from their perspective the internally driven geology of a body like Ceres makes it more similar to a classical planet like Mars, than to a small asteroid that lacks internally driven geology. This necessitated the creation of the category of dwarf planets to describe this intermediate class.{{cite web |first1=Emily |last1=Lakdawalla |author-link1=Emily Lakdawalla|display-authors=etal |date=April 21, 2020 |title=What is a planet? |website=planetary.org |publisher=The Planetary Society |url=https://www.planetary.org/worlds/what-is-a-planet |access-date=August 19, 2021 |archive-date=January 22, 2022 |archive-url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |url-status=live }}

{{Main|Clearing the neighbourhood}}

Alan Stern and Harold F. Levison introduced a parameter {{math|Λ}} (upper case lambda) in 2000, expressing the likelihood of an encounter resulting in a given deflection of orbit.{{cite conference |last1=Stern |first1=S.A. |author1-link=Alan Stern |last2=Levison |first2=H.F. |author2-link=Harold F. Levison |date=August 7–18, 2000 |publication-date=2002 |title=Regarding the criteria for planethood and proposed planetary classification schemes |journal=Highlights of Astronomy |volume=12 |pages=205–213 |conference=XXIVth General Assembly of the IAU – 2000 |place=Manchester, UK |bibcode=2002HiA....12..205S |doi=10.1017/S1539299600013289 |doi-access=free |url=http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf |access-date=January 26, 2008 |archive-date=September 23, 2015 |archive-url=https://web.archive.org/web/20150923220319/http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf |url-status=live }} The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. This value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where {{math|  Λ > 1  }} will eventually clear it. A gap of five orders of magnitude in {{math|Λ}} was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects.

Using this parameter, Steven Soter and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), whereas dwarf planets lack the mass to do so. Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol {{mvar|µ}} (mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where {{mvar|µ}} is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where {{nobr| {{math|  µ > 100  }} }} is deemed to be cleared.{{cite journal |first=S. |last=Soter |author-link=Steven Soter |date=August 16, 2006 |title=What is a Planet? |journal=The Astronomical Journal |volume=132 |issue=6 |pages=2513–2519 |doi=10.1086/508861 |bibcode=2006AJ....132.2513S |s2cid=14676169 |arxiv=astro-ph/0608359 }}

Jean-Luc Margot refined Stern and Levison's concept to produce a similar parameter {{math|Π}} (upper case Pi). It is based on theory, avoiding the empirical data used by {{math|Λ .}} {{nobr| {{math|  Π > 1  }} }} indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets.

There are several other schemes that try to differentiate between planets and dwarf planets,{{cite news |first=Paul |last=Rincon |date=August 25, 2006 |title=Pluto vote 'hijacked' in revolt |agency=BBC News |publisher=British Broadcasting Corporation |url=http://news.bbc.co.uk/2/hi/science/nature/5283956.stm |access-date=January 26, 2008 |archive-date=July 23, 2011 |archive-url=https://web.archive.org/web/20110723033823/http://news.bbc.co.uk/2/hi/science/nature/5283956.stm |url-status=live }} but the 2006 definition uses this concept.

{{Main|Hydrostatic equilibrium}}

{{Image frame

|content={{Graph:Chart

| width=200

| height=200

| type=rect

| x = Eris, Pluto, Haumea, Makemake, Gonggong, Charon, Quaoar, Ceres, Orcus, Salacia

| y1 = 16.6, 13.03, 4.01, 3.1, 1.75, 1.586, 1.4, 0.938, 0.61, 0.492

| showValues=offset:1,format:.2

| xAxisAngle = 45

}}

|width=240

|caption=Comparative masses of the likeliest dwarf planets, with Charon for comparison. The unit of mass is ~{{10^|21}} kg. Eris and Pluto dominate. Unmeasured Sedna is excluded, but is likely on the order of Ceres. The Moon in contrast is 73.5×{{10^|21}}, over four times more massive than Eris.

|border=no

}}

File:Pie chart of relative masses of dwarf planets.png

Enough internal pressure, caused by the body's gravitation, will turn a body plastic, and enough plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to have an irregular shape and may be rubble piles. Larger objects, where gravity is significant but not dominant, are potato-shaped; the more massive the body, the higher its internal pressure, the more solid it is and the more rounded its shape, until the pressure is enough to overcome its compressive strength and it achieves hydrostatic equilibrium. Then, a body is as round as it is possible to be, given its rotation and tidal effects, and is an ellipsoid in shape. This is the defining limit of a dwarf planet.{{cite conference |first1=Charles H. |last1=Lineweaver |first2=Marc |last2=Norman |date=September 28–30, 2009 |publication-date=2010 |title=The potato radius: A lower minimum size for dwarf planets |editor-last1=Short |editor-first1=W. |editor-last2=Cairns |editor-first2=I. |conference=9th Australian Space Science Conference |book-title=Proceedings of 2009 Australian Space Science Conference |pages=67–78 |publisher=National Space Society of Australia |arxiv=1004.1091 |isbn=9780977574032 |url=https://www.mso.anu.edu.au/~charley/papers/Potato%20Radiusv8.pdf |access-date=August 11, 2023 |archive-date=March 10, 2023 |archive-url=https://web.archive.org/web/20230310103945/https://www.mso.anu.edu.au/~charley/papers/Potato%20Radiusv8.pdf |url-status=live }}

If an object is in hydrostatic equilibrium, a global layer of liquid on its surface would form a surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. The body will have a spherical shape if it does not rotate and an ellipsoidal one if it does. The faster it rotates, the more oblate or even scalene it becomes. If such a rotating body were heated until it melts, its shape would not change. The extreme example of a body that may be scalene due to rapid rotation is {{dp|Haumea}}, which is twice as long on its major axis as it is at the poles. If the body has a massive nearby companion, then tidal forces gradually slow its rotation until it is tidally locked; that is, it always presents the same face to its companion. Tidally locked bodies are also scalene, though sometimes only slightly so. Earth's Moon is tidally locked, as are all the rounded satellites of the gas giants. Pluto and Charon are tidally locked to each other, as are Eris and Dysnomia, and probably also Orcus and Vanth.

There are no specific size or mass limits of dwarf planets, as those are not defining features. There is no clear upper limit: an object very far out in the Solar System that is more massive than Mercury might not have had time to clear its neighbourhood, and such a body would fit the definition of dwarf planet rather than planet. Indeed, Mike Brown set out to find such an object.{{cite AV media |people=Julia Sweeney (interviewer & host), M.E. Brown (interviewed astronomer) |date=June 28, 2007 |title=Julia Sweeney and Michael E. Brown |series=Hammer Conversations |publisher=KCET |medium=podcast |quote=Actress and comedienne Julia Sweeney (God Said Ha!) discusses the discovery that dwarfed Pluto with Caltech astronomer Michael E. Brown. |url=http://www.pluggd.tv/audio/channels/kcet_podcast__hammer_conversations/episodes/2h10l |access-date=June 28, 2008 |archive-url=https://web.archive.org/web/20080626220548/http://www.pluggd.tv/audio/channels/kcet_podcast__hammer_conversations/episodes/2h10l |archive-date=June 26, 2008}} The lower limit is determined by the requirements of achieving and retaining hydrostatic equilibrium, but the size or mass at which an object attains and retains equilibrium depends on its composition and thermal history, not simply its mass. An IAU 2006 press release question-and-answer section estimated that objects with mass above {{val|0.5|e=21|u=kg}} and radius greater than 400 km would "normally" be in hydrostatic equilibrium (the shape ... would normally be determined by self-gravity), but that all borderline cases would need to be determined by observation.{{cite press release |title='Planet definition' questions & answers sheet |publisher=International Astronomical Union |date=August 24, 2006 |url=https://www.iau.org/static/archives/releases/doc/iau0601_q_answers.doc |access-date=October 16, 2021 |archive-date=May 7, 2021 |archive-url=https://web.archive.org/web/20210507125920/https://www.iau.org/static/archives/releases/doc/iau0601_q_answers.doc |url-status=live }} This is close to what as of 2019 is believed to be roughly the limit for objects beyond Neptune that are fully compact, solid bodies, with {{dp|Salacia}} {{nobr|( {{mvar|r}} {{=}} {{val|423|11|u=km}} ,}} {{nobr| {{mvar|m}} {{=}} {{val|0.492|0.007|e=21|u=kg}} )}} being a borderline case both for the 2006 Q&A expectations and in more recent evaluations, and with {{dp|Orcus}} being just above the expected limit. No other body with a measured mass is close to the expected mass limit, though several without a measured mass approach the expected size limit.

{{clear}}

{{Main|List of possible dwarf planets}}

File:Trans-Neptunians_Size_Albedo_Color.svg

Though the definition of a dwarf planet is clear, evidence about whether a given trans-Neptunian object is large and malleable enough to be shaped by its own gravitational field is often inconclusive. There are also outstanding questions relating to the interpretation of the IAU criterion in certain instances. Consequently the number of currently conformed TNOs which meet the hydrostatic equilibrium criterion is uncertain.

The three objects under consideration during the debates leading up to the 2006 IAU acceptance of the category of dwarf planet – Ceres, Pluto and Eris – are generally accepted as dwarf planets, including by those astronomers who continue to classify dwarf planets as planets. Only one of them – Pluto – has been observed in enough detail to verify that its current shape fits what would be expected from hydrostatic equilibrium.

{{cite journal |last1=Nimmo |first1=Francis |display-authors=etal |title=Mean radius and shape of Pluto and Charon from New Horizons images|journal=Icarus |date=2017 |volume=287 |pages=12–29 |doi=10.1016/j.icarus.2016.06.027|bibcode=2017Icar..287...12N |arxiv=1603.00821|s2cid=44935431 }} Ceres is close to equilibrium, but some gravitational anomalies remain unexplained.{{cite book|chapter-url=https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf|display-authors=4|author1=Raymond, C.|author2=Castillo-Rogez, J.C.|author3=Park, R.S.|author4=Ermakov, A.|author5=Bland, M.T.|author6=Marchi, S.|author7=Prettyman, T.|author8=Ammannito, E.|author9=De Sanctis, M.C.|author10=Russell, C.T.|date=September 2018|chapter=Dawn Data Reveal Ceres' Complex Crustal Evolution|title=European Planetary Science Congress|volume=12|access-date=July 19, 2020|archive-date=January 30, 2020|archive-url=https://web.archive.org/web/20200130111631/https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf|url-status=live}} Eris is generally assumed to be a dwarf planet because it is more massive than Pluto.

In order of discovery, these three bodies are:

1. {{dp|Ceres}} – discovered January 1, 1801, and announced January 24, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006.
2. {{dp|Pluto}} – discovered February 18, 1930, and announced March 13. Considered a planet for 76 years. Explicitly reclassified as a dwarf planet by the IAU with Resolution 6A on August 24, 2006.'Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects' Five known moons.
3. {{dp|Eris}} ({{mp|2003 UB|313}}) – discovered January 5, 2005, and announced July 29. Called the "tenth planet" in media reports. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006, and named by the IAU dwarf-planet naming committee on September 13 of that year. One known moon.

The IAU only established guidelines for which committee would oversee the naming of likely dwarf planets: any unnamed trans-Neptunian object with an absolute magnitude brighter than +1 (and hence a minimum diameter of 838 km at the maximum geometric albedo of 1){{cite web |title=Conversion of Absolute Magnitude to Diameter for Minor Planets |publisher=Department of Physics & Astronomy (Stephen F. Austin State University) |author=Dan Bruton |url=http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html |access-date=June 13, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20100323180835/http://www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html |archive-date=March 23, 2010 }} was to be named by a joint committee consisting of the Minor Planet Center and the planetary working group of the IAU. At the time (and still as of 2023), the only bodies to meet this threshold were {{dp|Haumea}} and {{dp|Makemake}}. These bodies are generally assumed to be dwarf planets, although they have not yet been demonstrated to be in hydrostatic equilibrium, and there is some disagreement for Haumea:{{cite journal

| display-authors = etal

| last1 = Ortiz

| first1 = J. L.

| last2 = Santos-Sanz

| first2 = P.

| last3 = Sicardy

| first3 = B.

| last4 = Benedetti-Rossi

| first4 = G.

| last5 = Bérard

| first5 = D.

| last6 = Morales

| first6 = N.

| title = The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation

| url = http://www.astrosurf.com/sogorb/occultations/nature24051.pdf

| journal = Nature

| volume = 550

| issue = 7675

| year = 2017

| pages = 219–223

| doi = 10.1038/nature24051

| pmid = 29022593

| bibcode = 2017Natur.550..219O

| arxiv = 2006.03113

| hdl = 10045/70230

| s2cid = 205260767

| hdl-access = free

| access-date = January 14, 2022

| archive-date = November 7, 2020

| archive-url = https://web.archive.org/web/20201107052958/http://www.astrosurf.com/sogorb/occultations/nature24051.pdf

| url-status = live

}}

{{cite journal

| title = Haumea's Shape, Composition, and Internal Structure

| author1 = Dunham, E. T.

| author2 = Desch, S. J.

| author3 = Probst, L.

| date = April 2019

| journal = The Astrophysical Journal

| volume = 877

| issue = 1

| pages = 11

| doi = 10.3847/1538-4357/ab13b3

| bibcode = 2019ApJ...877...41D

| arxiv = 1904.00522

| s2cid = 90262114

| doi-access = free

}}

{{ordered list|start=4

| {{dp|Haumea}} ({{mp|2003 EL|61}}) – discovered by Brown et al. December 28, 2004, and announced by Ortiz et al. on July 27, 2005. Named by the IAU dwarf-planet naming committee on September 17, 2008. Two known moons and one known ring.

| {{dp|Makemake}} ({{mp|2005 FY|9}}) – discovered March 31, 2005, and announced July 29. Named by the IAU dwarf-planet naming committee on July 11, 2008. One known moon.

}}

These five bodies – the three under consideration in 2006 (Pluto, Ceres and Eris) plus the two named in 2008 (Haumea and Makemake) – are commonly presented as the dwarf planets of the Solar System, though the limiting factor (albedo) is not what defines an object as a dwarf planet.{{cite web |date=July 11, 2008 |title=Dwarf Planets and their Systems |publisher=Working Group for Planetary System Nomenclature (WGPSN) |url=http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets |access-date=September 12, 2019 |archive-date=July 14, 2007 |archive-url=https://web.archive.org/web/20070714225312/http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets |url-status=live }}

The astronomical community commonly refers to other larger TNOs as dwarf planets as well.{{cite book |last1=Pinilla-Alonso |first1= Noemi |last2=Stansberry |first2= John A. |last3= Holler |first3= Bryan J. |date=November 22, 2019 |chapter=Surface properties of large TNOs: Expanding the study to longer wavelengths with the James Webb Space Telescope |arxiv=1905.12320 |title= The Transneptunian Solar System |editor= Dina Prialnik |editor2=Maria Antonietta Barucci |editor3=Leslie Young |publisher= Elsevier}} At least four additional bodies meet the preliminary criteria of Brown, of Tancredi et al., of Grundy et al., and of Emery et al. for identifying dwarf planets, and are generally called dwarf planets by astronomers as well:

{{ordered list|start=6

| {{dp|Quaoar}} ({{mp|2002 LM|60}}) – discovered June 5, 2002, and announced October 7 of that year. One known moon and two known rings.

| {{dp|Sedna}} ({{mp|2003 VB|12}}) – discovered November 14, 2003, and announced March 15, 2004.

| {{dp|Orcus}} ({{mp|2004 DW}}) – discovered February 17, 2004, and announced two days later. One known moon.

| {{dp|Gonggong}} ({{mp|2007 OR|10}}) – discovered July 17, 2007, and announced January 2009. One known moon.

}}

For instance, JPL/NASA called Gonggong, a dwarf planet after observations in 2016,{{cite web |url=https://www.jpl.nasa.gov/news/news.php?feature=6509 |title=2007 OR10: Largest Unnamed World in the Solar System |website=Jet Propulsion Laboratory |first=Preston |last=Dyches |date=May 11, 2016 |access-date=September 12, 2019 |archive-date=November 23, 2020 |archive-url=https://web.archive.org/web/20201123155324/https://www.jpl.nasa.gov/news/news.php?feature=6509 |url-status=live }} and Simon Porter of the Southwest Research Institute spoke of "the big eight [TNO] dwarf planets" in 2018, referring to Pluto, Eris, Haumea, Makemake, Gonggong, {{dp|Quaoar}}, {{dp|Sedna}} and {{dp|Orcus}}.{{cite web

| first = Simon

| last = Porter

| url = https://twitter.com/AscendingNode/status/978592143626375169

| title = #TNO2018

| publisher = Twitter

| date = March 27, 2018

| access-date = March 27, 2018

| archive-date = October 2, 2018

| archive-url = https://web.archive.org/web/20181002133130/https://twitter.com/AscendingNode/status/978592143626375169

| url-status = live

}} The IAU itself has called Quaoar a dwarf planet in a 2022–2023 annual report.{{cite web |url=https://www.iau.org/static/science/scientific_bodies/divisions/f/division-f-annual-report-2022-2023.pdf |title=Report of Division F "Planetary Systems and Astrobiology": Annual Report 2022-23 |last= |first= |date=2022–2023 |website= |publisher=International Astronomical Union |access-date=December 8, 2023 |quote= |archive-date=December 8, 2023 |archive-url=https://web.archive.org/web/20231208112824/https://www.iau.org/static/science/scientific_bodies/divisions/f/division-f-annual-report-2022-2023.pdf |url-status=live }}

More bodies have been proposed, such as {{dp|Salacia}} and {{mpl|(307261) 2002 MS|4}} by Brown; {{dp|Varuna}} and {{dp|Ixion}} by Tancredi et al., and {{mpl|(532037) 2013 FY|27}} by Sheppard et al.{{cite journal |last1=Sheppard |first1=Scott S. |last2=Fernandez |first2=Yanga R. |first3=Arielle |last3=Moullet |date=November 16, 2018 |title=The Albedos, Sizes, Colors, and Satellites of Dwarf Planets Compared with Newly Measured Dwarf Planet 2013 FY27 |journal=The Astronomical Journal |volume=156 |issue=6 |page=270 |doi=10.3847/1538-3881/aae92a |arxiv=1809.02184 |bibcode=2018AJ....156..270S |s2cid=119522310 |doi-access=free }} Most of the larger bodies have moons, which enables a determination of their mass and thus their density, which inform estimates of whether they could be dwarf planets. The largest TNOs that are not known to have moons are Sedna, {{mpl|(307261) 2002 MS|4}}, {{mpl|(55565) 2002 AW|197}} and Ixion. In particular, Salacia has a known mass and diameter, putting it as a borderline case by the IAU's 2006 Q&A.

{{ordered list|start=10

| {{dp|Salacia}} ({{mp|2004 SB|60}}) – discovered September 22, 2004. One known moon.

}}

At the time Makemake and Haumea were named, it was thought that trans-Neptunian objects (TNOs) with icy cores would require a diameter of only about 400 km (250 mi), or 3% the size of Earth{{snd}}the size of the moons Mimas, the smallest moon that is round, and Proteus, the largest that is not{{snd}}to relax into gravitational equilibrium. Researchers thought that the number of such bodies could prove to be around 200 in the Kuiper belt, with thousands more beyond.{{cite web |author-link=Michael E. Brown |title=The Dwarf Planets |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |url=http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |access-date=January 26, 2008 |archive-date=July 19, 2011 |archive-url=https://web.archive.org/web/20110719164447/http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |url-status=live }}{{cite web |last=Brown |first=Mike |author-link=Michael E. Brown |title=How many dwarf planets are there in the outer solar system? |url=http://www.gps.caltech.edu/~mbrown/dps.html |archive-url=https://web.archive.org/web/20111018154917/http://www.gps.caltech.edu/~mbrown/dps.html |archive-date=October 18, 2011 |url-status=live |access-date=November 15, 2013 |website=CalTech}}{{cite web |last=Stern |first=Alan |author-link=Alan Stern |date=August 24, 2012 |title=The PI's Perspective. |url=https://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012 |url-status=dead |archive-url=https://web.archive.org/web/20141113225430/http://pluto.jhuapl.edu/overview/piPerspective.php?page=piPerspective_08_24_2012 |archive-date=November 13, 2014 |access-date=August 24, 2012}}

This was one of the reasons (keeping the roster of 'planets' to a reasonable number) that Pluto was reclassified in the first place.

Research since then has cast doubt on the idea that bodies that small could have achieved or maintained equilibrium under the typical conditions of the Kuiper belt and beyond.

Individual astronomers have recognized a number of objects as dwarf planets or as likely to prove to be dwarf planets. In 2008, Tancredi et al. advised the IAU to officially accept Orcus, Sedna and Quaoar as dwarf planets (Gonggong was not yet known), though the IAU did not address the issue then and has not since. Tancredi also considered the five TNOs {{dp|Varuna}}, {{dp|Ixion}}, {{mpl-|208996|2003 AZ|84}}, {{mpl-|90568|2004 GV|9}}, and {{mpl-|55565|2002 AW|197}} to most likely be dwarf planets as well.{{Cite journal |last1=Tancredi |first1=G. |last2=Favre |first2=S. A. |doi=10.1016/j.icarus.2007.12.020 |title=Which are the dwarfs in the Solar System? |journal=Icarus |volume=195 |issue=2 |pages=851–862 |year=2008 |bibcode=2008Icar..195..851T}}

Since 2011, Brown has maintained a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets, based solely on estimated size.{{cite web |last=Brown |first=Michael |author-link=Michael E. Brown |date=August 23, 2011 |title=Free the Dwarf Planets! |url=http://www.mikebrownsplanets.com/2011/08/free-dwarf-planets.html |website=Mike Brown's Planets |access-date=August 24, 2011 |archive-date=October 5, 2011 |archive-url=https://web.archive.org/web/20111005064654/http://www.mikebrownsplanets.com/2011/08/free-dwarf-planets.html |url-status=live }}

As of September 13, 2019, Brown's list identifies ten trans-Neptunian objects with diameters then thought to be greater than 900 km (the four named by the IAU plus {{dp|Gonggong}}, {{dp|Quaoar}}, {{dp|Sedna}}, {{dp|Orcus}}, {{mpl|(307261) 2002 MS|4}}, and {{dp|Salacia}}) as "near certain" to be dwarf planets, and another 16, with diameter greater than 600 km, as "highly likely". Notably, Gonggong may have a larger diameter ({{val|1230|50|u=km}}) than Pluto's round moon Charon (1212 km).

But in 2019 Grundy et al. proposed, based on their studies of Gǃkúnǁʼhòmdímà, that dark, low-density bodies smaller than about 900–1000 km in diameter, such as Salacia and {{dp|Varda}}, never fully collapsed into solid planetary bodies and retain internal porosity from their formation (in which case they could not be dwarf planets). They accept that brighter (albedo > ≈0.2)Of bodies smaller than 900 km in diameter, the only ones thought to have albedos much greater than this are fragments in the Haumea collisional family and possibly {{mpl|2005 QU|182}} (albedo between 0.2 and 0.5). or denser (> ≈1.4 g/cc) Orcus and Quaoar probably were fully solid:{{cite journal

|first1=W.M. |last1=Grundy

|first2=K.S. |last2=Noll

|first3=M.W. |last3=Buie

|first4=S.D. |last4=Benecchi

|first5=D. |last5=Ragozzine

|first6=H.G. |last6=Roe

|title=The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{mp|(229762) 2007 UK|126}})

|url=http://www2.lowell.edu/users/grundy/abstracts/2019.G-G.html

|journal=Icarus

|doi=10.1016/j.icarus.2018.12.037

|year= 2019

|volume=334

|pages=30–38

|bibcode=2019Icar..334...30G

|s2cid=126574999

|archive-url=https://web.archive.org/web/20190407045339/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf

|url-status=live

|archive-date=April 7, 2019}}

{{blockquote|Orcus and Charon probably melted and differentiated, considering their higher densities and spectra indicating surfaces made of relatively clean H₂O ice. But the lower albedos and densities of Gǃkúnǁʼhòmdímà, 55637, Varda, and Salacia suggest that they never did differentiate, or if they did, it was only in their deep interiors, not a complete melting and overturning that involved the surface. Their surfaces could remain quite cold and uncompressed even as the interior becomes warm and collapses. The liberation of volatiles could further help transport heat out of their interiors, limiting the extent of their internal collapse. An object with a cold, relatively pristine surface and a partially collapsed interior should exhibit very distinctive surface geology, with abundant thrust faults indicative of the reduction in total surface area as the interior compresses and shrinks.}}

Salacia was later found to have a somewhat higher density, comparable within uncertainties to that of Orcus, though still with a very dark surface. Despite this determination, Grundy et al. call it "dwarf-planet sized", while calling Orcus a dwarf planet.{{cite journal

|title=Mutual Orbit Orientations of Transneptunian Binaries

|url=http://www2.lowell.edu/users/grundy/abstracts/preprints/2019.TNB_orbits.pdf

|first1=W. M.

|last1=Grundy

|first2=K. S.

|last2=Noll

|first3=H. G.

|last3=Roe

|first4=M. W.

|last4=Buie

|first5=S. B.

|last5=Porter

|first6=A. H.

|last6=Parker

|first7=D.

|last7=Nesvorný

|first8=S. D.

|last8=Benecchi

|first9=D. C.

|last9=Stephens

|first10=C. A.

|last10=Trujillo

|journal=Icarus

|volume=334

|pages=62–78

|year=2019

|doi=10.1016/j.icarus.2019.03.035

|bibcode=2019Icar..334...62G

|s2cid=133585837

|issn=0019-1035

|access-date=October 26, 2019

|archive-url=https://web.archive.org/web/20200115091416/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.TNB_orbits.pdf

|archive-date=January 15, 2020

|url-status=dead

}} Later studies on Varda suggest that its density may also be high, though a low density could not be excluded.{{cite journal

|display-authors = etal

|first1 = D. |last1 = Souami

|first2 = F. |last2 = Braga-Ribas

|first3 = B. |last3 = Sicardy

|first4 = B. |last4 = Morgado

|first5 = J. L. |last5 = Ortiz

|first6 = J. |last6 = Desmars

|date = August 2020

|title = A multi-chord stellar occultation by the large trans-Neptunian object (174567) Varda

|journal = Astronomy & Astrophysics |volume = 643 |pages = A125 |doi = 10.1051/0004-6361/202038526 |arxiv = 2008.04818

|bibcode = 2020A&A...643A.125S |s2cid = 221095753 }}

In 2023, Emery et al. wrote that near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 suggests that Sedna, Gonggong, and Quaoar underwent internal melting, differentiation, and chemical evolution, like the larger dwarf planets Pluto, Eris, Haumea, and Makemake, but unlike "all smaller KBOs". This is because light hydrocarbons are present on their surfaces (e.g. ethane, acetylene, and ethylene), which implies that methane is continuously being resupplied, and that methane would likely come from internal geochemistry. On the other hand, the surfaces of Sedna, Gonggong, and Quaoar have low abundances of CO and CO₂, similar to Pluto, Eris, and Makemake, but in contrast to smaller bodies. This suggests that the threshold for dwarf planethood in the trans-Neptunian region is a diameter of ~900 km (thus including only Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Sedna), and that even Salacia may not be a dwarf planet.{{cite journal|last1=Emery|first1=J. P. |first2=I. |last2=Wong |first3=R. |last3=Brunetto |first4=J. C. |last4=Cook |first5=N. |last5=Pinilla-Alonso |first6=J. A. |last6=Stansberry |first7=B. J. |last7=Holler |first8=W. M. |last8=Grundy |first9=S. |last9=Protopapa |first10=A. C. |last10=Souza-Feliciano |first11=E. |last11=Fernández-Valenzuela |first12=J. I. |last12=Lunine |first13=D. C. |last13=Hines |author-link=|date=2024|title=A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy|journal=Icarus |volume=414 |doi=10.1016/j.icarus.2024.116017 |arxiv=2309.15230|bibcode=2024Icar..41416017E }} A 2023 study of {{mp|(307261) 2002 MS|4}} shows that it probably has an extremely large crater, whose depth takes up 5.7% of its diameter: this is proportionally larger than the Rheasilvia crater on Vesta, which is the reason Vesta is not usually considered a dwarf planet today.{{cite journal

|display-authors = etal

|first1 = F. L. |last1 = Rommel

|first2 = F. |last2 = Braga-Ribas

|first3 = J. L. |last3 = Ortiz

|first4 = B. |last4 = Sicardy

|first5 = P. |last5 = Santos-Sanz

|first6 = J. |last6 = Desmars

|title = A large topographic feature on the surface of the trans-Neptunian object (307261) 2002 MS4 measured from stellar occultations

|journal = Astronomy & Astrophysics

|date = October 2023

|volume = 678

|id = A167

|pages = 25

|doi-access = free

|doi = 10.1051/0004-6361/202346892

|arxiv = 2308.08062

|bibcode = 2023A&A...678A.167R

|s2cid = 260926329}}

In 2024, Kiss et al. found that Quaoar has an ellipsoidal shape incompatible with hydrostatic equilibrium for its current spin. They hypothesized that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon Weywot.{{cite journal

|display-authors = etal

|first1 = C. |last1 = Kiss

|first2 = T. G. |last2 = Müller

|first3 = G. |last3 = Marton

|first4 = R. |last4 = Szakáts

|first5 = A. |last5 = Pál

|first6 = L. |last6 = Molnár

|title = The visible and thermal light curve of the large Kuiper belt object (50000) Quaoar

|journal = Astronomy & Astrophysics

|date = March 2024

|volume = 684

|issue =

|pages = A50 |doi = 10.1051/0004-6361/202348054

|arxiv = 2401.12679

|bibcode = 2024A&A...684A..50K}} If so, this would resemble the situation of Saturn's moon Iapetus, which is too oblate for its current spin.Cowen, R. (2007). Idiosyncratic Iapetus, Science News vol. 172, pp. 104–106. [http://www.sciencenews.org/articles/20070818/bob8ref.asp references] {{Webarchive|url=https://web.archive.org/web/20071013165655/http://www.sciencenews.org/articles/20070818/bob8ref.asp |date=October 13, 2007 }}{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = September 25, 2015| archive-date = December 23, 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}} Iapetus is generally still considered a planetary-mass moon nonetheless, though not always.{{cite journal |last1=Chen |first1=Jingjing |last2=Kipping |first2=David |date=2016 |title=Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal=The Astrophysical Journal |volume=834 |issue=1 |page=17 |doi= 10.3847/1538-4357/834/1/17|arxiv=1603.08614 |s2cid=119114880 |doi-access=free |bibcode=2017ApJ...834...17C }}

{{main|List of possible dwarf planets}}

File:Density vs albedo of dwarf planets.png

The trans-Neptunian objects in the following tables, except Salacia, are agreed by Brown, Tancredi et al., Grundy et al., and Emery et al. to be probable dwarf planets, or close to it. Salacia has been included as the largest TNO not generally agreed to be a dwarf planet; it is a borderline body by many criteria, and is therefore italicized. Charon, a moon of Pluto that was proposed as a dwarf planet by the IAU in 2006, is included for comparison. Those objects that have absolute magnitude greater than +1, and so meet the threshold of the joint planet–minor planet naming committee of the IAU, are highlighted, as is Ceres, which the IAU has assumed is a dwarf planet since they first debated the concept.

The masses of given dwarf planets are listed for their systems (if they have satellites) with exceptions for Pluto and Orcus.

{{Main|Astronomical symbol#Symbols for trans-Neptunian objects}}

Ceres File:Ceres symbol (fixed width).svg{{cite book

| editor-last = Bode

| editor-first = J.E.

| date = 1801

| title = Berliner astronomisches Jahrbuch führ das Jahr 1804

| trans-title = The Berlin Astronomical Yearbook for 1804

| pages = 97–98

| url = https://books.google.com/books?id=6GElAQAAIAAJ&pg=RA2-PA98

| access-date = October 19, 2022

| archive-date = December 14, 2023

| archive-url = https://web.archive.org/web/20231214145307/https://books.google.com/books?id=6GElAQAAIAAJ&pg=RA2-PA98#v=onepage&q&f=false

| url-status = live

}} and Pluto File:Pluto monogram (fixed width).svg{{cite magazine

| last = Slipher

| first = V.M.

| date = 1930

| title = The trans-Neptunian planet

| magazine = Popular Astronomy

| volume = 38

| page = 415

| url = http://articles.adsabs.harvard.edu/cgi-bin/nph-journal_query?volume=38&plate_select=NO&page=415&plate=&cover=&journal=PA...

| access-date = October 19, 2022

| archive-date = December 11, 2021

| archive-url = https://web.archive.org/web/20211211143952/http://articles.adsabs.harvard.edu/cgi-bin/nph-journal_query?volume=38&plate_select=NO&page=415&plate=&cover=&journal=PA...

| url-status = live

}} received planetary symbols, as they were considered to be planets when they were discovered. By the time the others were discovered, planetary symbols had mostly fallen out of use among astronomers. Unicode includes symbols for Quaoar {{nobr|File:Quaoar symbol (fixed width).svg,}} Sedna {{nobr|File:Sedna symbol (fixed width).svg,}} Orcus {{nobr|File:Orcus symbol (fixed width).svg,}} Haumea {{nobr|File:Haumea symbol (fixed width).svg,}} Eris {{nobr|File:Eris symbol (fixed width).svg,}} Makemake {{nobr|File:Makemake symbol (fixed width).svg,}} and Gonggong {{nobr|File:Gonggong symbol (fixed width).svg,}} that are primarily used by astrologers: they were devised by Denis Moskowitz, a software engineer in Massachusetts.{{cite web |url=http://blog.unicode.org/2022/05/out-of-this-world-new-astronomy-symbols.html |title=Out of this World: New Astronomy Symbols Approved for the Unicode Standard |last=Anderson |first=Deborah |date=May 4, 2022 |website=unicode.org |publisher=The Unicode Consortium |access-date=August 6, 2022 |archive-date=August 6, 2022 |archive-url=https://web.archive.org/web/20220806075352/http://blog.unicode.org/2022/05/out-of-this-world-new-astronomy-symbols.html |url-status=live }}{{cite web|url=https://www.unicode.org/L2/L2021/21224-dwarf-planet-syms.pdf|title=Unicode request for dwarf-planet symbols|last=Miller|first=Kirk|date=October 26, 2021|website=unicode.org|access-date=October 19, 2022|archive-date=March 23, 2022|archive-url=https://web.archive.org/web/20220323174107/https://www.unicode.org/L2/L2021/21224-dwarf-planet-syms.pdf|url-status=live}}{{cite web

| title = Alchemical Symbols

| date = 2022

| website = unicode.org

| publisher = The Unicode Consortium

| url = https://www.unicode.org/charts/PDF/U1F700.pdf

| access-date = October 19, 2022

| archive-date = April 2, 2020

| archive-url = https://web.archive.org/web/20200402042357/http://www.unicode.org/charts/PDF/U1F700.pdf

| url-status = live

}} NASA has used his Haumea, Eris, and Makemake symbols, as well as the traditional astrological symbol for Pluto File:Pluto symbol (large orb, fixed width).svg{{cite web |url= https://www.jpl.nasa.gov/infographics/what-is-a-dwarf-planet |date= April 22, 2015 |website= Jet Propulsion Laboratory |publisher=NASA |title= What is a Dwarf Planet? |access-date= September 24, 2021 |archive-date= December 8, 2021 |archive-url= https://web.archive.org/web/20211208181916/https://www.jpl.nasa.gov/infographics/what-is-a-dwarf-planet |url-status= live }} when referring to it as a dwarf planet. Symbols for smaller objects are less established; a Unicode proposal notes the Moskowitz symbol 20px for Salacia.{{cite web |url=https://www.unicode.org/L2/L2024/24235-constellation-symbols.pdf |title=Preliminary presentation of constellation symbols |last=Miller |first=Kirk |date=18 October 2024 |website=unicode.org |publisher=The Unicode Consortium |access-date=22 October 2024 |quote=}} Moskowitz additionally devised a symbol for Charon File:Charon symbol (fixed width).svg.{{cite web |url=https://www.unicode.org/L2/L2025/25079-phobos-and-deimos.pdf |title=Phobos and Deimos symbols |last1=Bala |first1=Gavin Jared |last2=Miller |first2=Kirk |date=7 March 2025 |website=unicode.org |publisher=The Unicode Consortium |access-date=14 March 2025 |quote=}}

File:PIA19562-Ceres-DwarfPlanet-Dawn-RC3-image19-20150506.jpg spacecraft]]

As of 2024, only two missions have targeted and explored dwarf planets up close. On March 6, 2015, the Dawn spacecraft entered orbit around Ceres, becoming the first spacecraft to visit a dwarf planet.{{cite web |last1=Landau |first1=Elizabeth |last2=Brown |first2=Dwayne |title=NASA Spacecraft Becomes First to Orbit a Dwarf Planet |url=http://www.jpl.nasa.gov/news/news.php?feature=4503 |date=March 6, 2015 |work=NASA |access-date=March 6, 2015 |archive-date=March 7, 2015 |archive-url=https://web.archive.org/web/20150307062258/http://www.jpl.nasa.gov/news/news.php?feature=4503 |url-status=live }} On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons.

Ceres displays such evidence of an active geology as salt deposits and cryovolcanos, while Pluto has water-ice mountains drifting in nitrogen-ice glaciers, as well as a significant atmosphere.

Ceres evidently has brine percolating through its subsurface, while there is evidence that Pluto has an actual subsurface ocean.

Dawn had previously orbited the asteroid Vesta. Saturn's moon Phoebe has been imaged by Cassini and before that by Voyager 2, which also encountered Neptune's moon Triton. All three bodies show evidence of once being dwarf planets, and their exploration helps clarify the evolution of dwarf planets.

New Horizons has captured distant images of Triton, Quaoar, Haumea, Eris, and Makemake, as well as the smaller candidates Ixion, {{mp|2002 MS|4}}, and {{mpl-|556416|2014 OE|394}}.{{cite journal

|display-authors = etal

|first1 = Anne J. |last1 = Verbiscer

|first2 = Paul |last2 = Helfenstein

|first3 = Simon B. |last3 = Porter

|first4 = Susan D. |last4 = Benecchi

|first5 = J. J. |last5 = Kavelaars

|first6 = Tod R. |last6 = Lauer

|title = The Diverse Shapes of Dwarf Planet and Large KBO Phase Curves Observed from New Horizons

|journal = The Planetary Science Journal

|date = April 2022

|volume = 3

|issue = 4

|id = 95

|pages = 31

|doi-access = free

|doi = 10.3847/PSJ/ac63a6

|bibcode = 2022PSJ.....3...95V}}

Quaoar has been proposed as a potential flyby target of the China National Space Administration's two Shensuo probes.{{cite news |last1=Jones |first1=Andrew |title=China to launch a pair of spacecraft towards the edge of the solar system |url=https://spacenews.com/china-to-launch-a-pair-of-spacecraft-towards-the-edge-of-the-solar-system/ |access-date=April 29, 2021 |work=SpaceNews |publisher=SpaceNews |date=April 16, 2021 |archive-date=September 29, 2021 |archive-url=https://web.archive.org/web/20210929080320/https://spacenews.com/china-to-launch-a-pair-of-spacecraft-towards-the-edge-of-the-solar-system/ |url-status=live }}

A number of bodies physically resemble dwarf planets. These include former dwarf planets, which may still have equilibrium shape or evidence of active geology; planetary-mass moons, which meet the physical but not the orbital definition for dwarf planet; and Charon in the Pluto–Charon system, which is arguably a binary dwarf planet. The categories may overlap: Triton, for example, is both a former dwarf planet and a planetary-mass moon.

file:Triton.usgs23.png

Vesta, the next-most-massive body in the asteroid belt after Ceres, was once in hydrostatic equilibrium and is roughly spheroidal, deviating mainly due to massive impacts that formed the Rheasilvia and Veneneia craters after it solidified.{{cite journal |first=Peter C. |last=Thomas |author2=Binzelb, Richard P. |author3=Gaffeyc, Michael J. |author4=Zellnerd, Benjamin H. |author5=Storrse, Alex D. |author6=Wells, Eddie |title=Vesta: Spin Pole, Size, and Shape from HST Images |journal=Icarus |volume=128 |pages=88–94 |date=1997 |issue=1 |doi=10.1006/icar.1997.5736 |bibcode=1997Icar..128...88T|doi-access=free }}

Its dimensions are not consistent with it currently being in hydrostatic equilibrium.{{cite journal |last=Asmar |first=S. W. |author2=Konopliv, A. S. |author3=Park, R. S. |author4=Bills, B. G. |author5=Gaskell, R. |author6=Raymond, C. A. |author7=Russell, C. T. |author8=Smith, D. E. |author9=Toplis, M. J. |author10=Zuber, M. T. |title=The Gravity Field of Vesta and Implications for Interior Structure |journal=43rd Lunar and Planetary Science Conference |issue=1659 |date=2012 |page=2600 |url=http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2600.pdf |bibcode=2012LPI....43.2600A |access-date=July 15, 2015 |archive-date=October 20, 2013 |archive-url=https://web.archive.org/web/20131020181320/http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2600.pdf |url-status=live }}{{cite journal |last=Russel |first=C. T. |title=Dawn at Vesta: Testing the Protoplanetary Paradigm |journal=Science |date=2012 |volume=336 |issue=6082 |pages=684–686 |doi=10.1126/science.1219381 |url=http://www.phyast.pitt.edu/~wmwv/Classes/A1122/Literature/Science-2012-Russell-684-6.pdf |bibcode=2012Sci...336..684R |display-authors=etal |pmid=22582253 |s2cid=206540168 |access-date=July 15, 2015 |archive-date=July 15, 2015 |archive-url=https://web.archive.org/web/20150715181221/http://www.phyast.pitt.edu/~wmwv/Classes/A1122/Literature/Science-2012-Russell-684-6.pdf |url-status=live }}

Triton is more massive than Eris or Pluto, has an equilibrium shape, and is thought to be a captured dwarf planet (likely a member of a binary system), but no longer directly orbits the sun.{{Cite journal |doi=10.1038/nature04792 |url=http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf |title=Neptune's capture of its moon Triton in a binary–planet gravitational encounter |journal=Nature |volume=441 |issue=7090 |pages=192–194 |year=2006 |last1=Agnor |first1=C. B. |last2=Hamilton |first2=D. P. |pmid=16688170 |bibcode=2006Natur.441..192A |s2cid=4420518 |access-date=August 29, 2015 |archive-date=October 14, 2016 |archive-url=https://web.archive.org/web/20161014134243/http://extranet.on.br/rodney/curso2010/aula9/tritoncapt_hamilton.pdf |url-status=dead }}

Phoebe is a captured centaur that, like Vesta, is no longer in hydrostatic equilibrium, but is thought to have been so early in its history due to radiogenic heating.{{cite news |last1=Cook |first1=Jia-Rui C. |last2=Brown |first2=Dwayne |date=April 26, 2012 |url=http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ |url-status=dead |title=Cassini Finds Saturn Moon Has Planet-Like Qualities |archive-url=https://web.archive.org/web/20150713125236/http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ |archive-date=July 13, 2015 |location=Pasadena, California|work=Jey Propoulsion Laboratory |publisher=NASA}}

{{Main|Planetary-mass moon}}

At least nineteen moons have equilibrium shapes from having collapsed into a solid body (or in a few cases into a nearly solid body) or even relaxed under self-gravity at some point, though some of the latter have since frozen solid and are no longer in equilibrium. Seven are more massive than either Eris or Pluto. These larger moons are not physically distinct from the dwarf planets, but do not fit the IAU definition because they do not directly orbit the Sun. (Indeed, Neptune's moon Triton is a captured dwarf planet, and Ceres formed in the same region of the Solar System as the moons of Jupiter and Saturn.) Alan Stern calls planetary-mass moons "satellite planets", one of three categories of planet, together with dwarf planets and classical planets.{{cite web |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |title=Should Large Moons Be Called 'Satellite Planets'? |publisher=News.discovery.com |date=May 14, 2010 |access-date=November 4, 2011 |archive-date=May 5, 2012 |archive-url=https://web.archive.org/web/20120505221146/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |url-status=dead }} The term planemo ("planetary-mass object") also covers all three populations.{{cite journal |last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |date=2006 |title=Planetesimals to Brown Dwarfs: What is a Planet? |url=http://siba.unipv.it/fisica/articoli/A/Annual%20Review%20Earth%20Planetary%20Sciences_vol.34_2006_pp.193-216.pdf |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=193–216 |doi=10.1146/annurev.earth.34.031405.125058 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B |s2cid=119338327 |url-status=dead |archive-url=https://web.archive.org/web/20130731133553/http://siba.unipv.it/fisica/articoli/A/Annual%20Review%20Earth%20Planetary%20Sciences_vol.34_2006_pp.193-216.pdf |archive-date=July 31, 2013 }}

There has been some debate as to whether the Pluto–Charon system should be considered a double dwarf planet.

In a draft resolution for the IAU definition of planet, both Pluto and Charon were considered planets in a binary system.{{efn| The footnote in the original text reads: "For two or more objects comprising a multiple object system. ... A secondary object satisfying these conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto–Charon a double planet."}} The IAU currently says Charon is not considered a dwarf planet but rather a satellite of Pluto, though the idea that Charon might qualify as a dwarf planet may be considered at a later date.{{cite web |title=Pluto and the Solar System |website=iau.org |publisher=International Astronomical Union |url=http://www.iau.org/public/themes/pluto/ |access-date=July 10, 2013 |archive-date=April 17, 2020 |archive-url=https://web.archive.org/web/20200417193151/https://www.iau.org/public/themes/pluto/ |url-status=live }} Nonetheless, it is no longer clear that Charon is in hydrostatic equilibrium. Also, the location of the barycentre depends not only on the relative masses of the bodies, but also on the distance between them; the barycentre of the Sun–Jupiter orbit, for example, lies outside the Sun, but they are not considered a binary object. Thus, a formal definition of what constitutes a binary (dwarf) planet must be established before Pluto and Charon are formally defined as binary dwarf planets.

{{portal|Solar System|Outer Space|Astronomy

}}

{{div col|colwidth=20em}}

• Centaur
• Lists of astronomical objects
• List of former planets
• List of gravitationally rounded objects of the Solar System
• List of planetary bodies
• List of possible dwarf planets
• Lists of small Solar System bodies
• Solar System belts

{{div col end}}

{{clear}}

{{notelist}}

{{reflist|refs=

{{cite web

|title = From discovery to demotion: How a dwarf planet changed astronomy

|url = https://www.aaas.org/taxonomy/term/8/discovery-demotion-how-dwarf-planet-changed-astronomy

|first = Brad |last = Hooker

|publisher = American Association for the Advancement of Science

|date = September 24, 2013

|accessdate = April 15, 2025

|url-status = live

|archive-url = https://web.archive.org/web/20240604163946/https://www.aaas.org/taxonomy/term/8/discovery-demotion-how-dwarf-planet-changed-astronomy

|archive-date = June 4, 2024}}

{{cite interview

|title = NASA at 50 Oral History Project Edited Oral History Transcript

|url = https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/NASA_HQ/NAF/SternA/SternA_4-15-08.htm

|first = Alan |last = Stern

|interviewer-first = Steven J. |interviewer-last = Dick

|work = Johnson Space Center

|publisher = NASA

|date = April 15, 2008

|accessdate = April 15, 2025

|url-status = live

|archive-url = https://web.archive.org/web/20250320054206/https://historycollection.jsc.nasa.gov/JSCHistoryPortal/history/oral_histories/NASA_HQ/NAF/SternA/SternA_4-15-08.htm

|archive-date = March 20, 2025

|quote = I coined the term dwarf planet in 1990 in the academic journal Icarus. It is a dwarf planet. But that's a kind of planet, just like dwarf human beings are still human beings, they're just smaller.}}

}}

{{Wiktionary}}

• [https://www.visualcapitalist.com/a-visual-introduction-to-the-dwarf-planets-in-our-solar-system/ A Visual Introduction to the Dwarf Planets in our Solar System] (Anshool Deshmukh, Visual Capitalist, October 8, 2021, graphics by Mark Belan)
• NPR: [https://www.npr.org/templates/story/story.php?storyId=5631291 Dwarf Planets May Finally Get Respect] (David Kestenbaum, Morning Edition)
• BBC News: [http://news.bbc.co.uk/1/hi/sci/tech/4798205.stm Q&A New planets proposal], August 16, 2006
• Ottawa Citizen: [https://web.archive.org/web/20070224221409/http://www.canada.com/ottawacitizen/news/story.html?id=a9591aed-f19f-4ac3-a324-1f8bb46d9379&p=2 The case against Pluto] (P. Surdas Mohit) August 24, 2006
• James L. Hilton: [https://web.archive.org/web/20070921162818/http://aa.usno.navy.mil/faq/docs/minorplanets.php When Did the Asteroids Become Minor Planets?]
• NASA: [https://web.archive.org/web/20100211161824/http://astronomy2009.nasa.gov/topics_sep.htm IYA 2009 Dwarf Planets]

{{Dwarf planets}}

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Category:1990s neologisms

Category:Minor planets

Category:Planetary-mass objects

Category:Types of planet

Category:Concepts in astronomy

Category:Solar System