Editing Definition of planet (section)

== IAU definition ==
{{main|IAU definition of planet}}The discovery of [[Eris (dwarf planet)|Eris]] forced the [[International Astronomical Union|IAU]] to act on a definition. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of [[90377 Sedna|Sedna]] in 2003, narrowed their choices to a shortlist of three, using [[approval voting]]. The definitions were:

* A planet is any object in [[orbit]] around the Sun with a diameter greater than 2,000&nbsp;km. (Eleven votes in favor)
* A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. (Eight votes in favor)
* A planet is any object in orbit around the Sun that is dominant in its immediate neighbourhood. (Six votes in favor)<ref>{{cite web | author=McKee, Maggie| year=2006 | title=Xena reignites a planet-sized debate | work=NewScientistSpace | url=http://www.newscientistspace.com/article.ns?id=dn8681| access-date=May 25, 2006}}</ref><ref>{{cite web | author=Croswell, Ken| year=2006 | title=The Tenth Planet's First Anniversary| url=http://kencroswell.com/TenthPlanetFirstAnniversary.html| access-date=May 25, 2006}}</ref>

Since no consensus could be reached, the committee decided to put these three definitions to a wider vote at the IAU General Assembly meeting in [[Prague]] in August 2006,<ref>{{cite web|year=2006 |title=Planet Definition |work=IAU |url=http://www.astronomy2006.com/planet-definition.php |archive-url=https://web.archive.org/web/20060826193101/http://www.astronomy2006.com/planet-definition.php |url-status=dead |archive-date=August 26, 2006 |access-date=August 14, 2006 }}</ref> and on August 24, the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between ''planet'' and ''rock'' (or, in the new parlance, ''[[small Solar System body]]''), called ''[[dwarf planet]]'' and placed [[Pluto]] in it, along with Ceres and Eris.<ref>{{cite web |date=August 24, 2006 |title=IAU General Assembly Newspaper |url=http://astro.cas.cz/nuncius/nsiii_09.pdf |archive-url=https://web.archive.org/web/20060926234634/http://astro.cas.cz/nuncius/nsiii_09.pdf |url-status=dead |archive-date=2006-09-26 |access-date=March 31, 2025}}</ref><ref name="IAU0602">{{cite web |date=August 24, 2006 |title=The Final IAU Resolution on the Definition of "Planet" Ready for Voting |url=https://www.iau.org/news/pressreleases/detail/iau0602/ |archive-url=https://web.archive.org/web/20231103173133/https://www.iau.org/news/pressreleases/detail/iau0602/ |url-status=dead |archive-date=2023-11-03 |access-date=March 31, 2025 |publisher=IAU (News Release&nbsp;— IAU0602)}}</ref>

{{blockquote|The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

# A "planet"{{efn|name=fn1}} is a celestial body that: {{Ordered list
|is in orbit around the Sun,
|has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a [[hydrostatic equilibrium]] (nearly round) shape, and
|has [[cleared the neighbourhood]] around its orbit.
|list_style_type=lower-alpha}}
# A "dwarf planet" is a celestial body that: {{Ordered list
|is in orbit around the Sun,
|has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,{{efn|name=fn2}}
|has {{em|not}} cleared the neighbourhood around its orbit, and
|is not a satellite.
|list_style_type=lower-alpha}}
# All other objects,{{efn|name=fn3}} except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

{{notelist|refs=
{{efn|name=fn1|The planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.}}
{{efn|name=fn2|An IAU process will be established to assign borderline objects into either "dwarf planet" and other categories.}}
{{efn|name=fn3|These currently include most of the Solar System asteroids, most [[trans-Neptunian object]]s (TNOs), comets, and other small bodies.}}
}}

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The IAU further resolves:
* [[Pluto]] is a "dwarf planet" by the above definition and is recognised as the prototype of a new category of trans-Neptunian objects.|multiline=true}}

The vote was passed, with 424 astronomers taking part in the ballot.<ref name="britt" /><ref>{{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=August 24, 2006|publisher=IAU|access-date=June 23, 2009}}</ref><ref name="IAU0603">{{cite press release|date=August 24, 2006 |publisher=International Astronomical Union (News Release&nbsp;— IAU0603) |title=IAU 2006 General Assembly: Result of the IAU Resolution votes |url=http://www.iau.org/public_press/news/release/iau0603/ |access-date=December 31, 2007 }} ([http://www.iau.org/iau0603.414.0.html orig link] {{webarchive|url=https://web.archive.org/web/20070103145836/http://www.iau.org/iau0603.414.0.html |date=January 3, 2007 }})</ref> The IAU also resolved that "''planets'' and ''dwarf planets'' are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets.<ref name=IAU0603/>

On September 13, 2006, the IAU placed Eris, its moon Dysnomia, and Pluto into their [[Minor Planet Catalogue]], giving them the official minor planet designations [[Pluto|(134340) Pluto]], [[Eris (dwarf planet)|(136199) Eris]], and [[Dysnomia (moon)|(136199) Eris I Dysnomia]].<ref>{{cite web|title=Circular No. 8747 |author=Central Bureau for Astronomical Telegrams, International Astronomical Union |url=http://www.cbat.eps.harvard.edu/iauc/08700/08747.html |year=2006 |access-date=July 3, 2011 }} [https://web.archive.org/web/20070205035336/http://www.cfa.harvard.edu/iau/special/08747.pdf web.archive]</ref> Other [[possible dwarf planets]], such as [[Haumea (dwarf planet)|2003 EL<sub>61</sub>]], [[Makemake (dwarf planet)|2005 FY<sub>9</sub>]], [[90377 Sedna|Sedna]] and [[50000 Quaoar|Quaoar]], were left in temporary limbo until a formal decision could be reached regarding their status.

On June 11, 2008, the IAU executive committee announced the establishment of a subclass of dwarf planets comprising the aforementioned "new category of trans-Neptunian objects" to which Pluto is a prototype. This new class of objects, termed [[plutoid]]s, would include Pluto, Eris and any other trans-Neptunian dwarf planets, but excluded Ceres. The IAU decided that those TNOs with an [[absolute magnitude]] brighter than +1 would be named by a joint commissions of the planetary and minor-planet naming committees, under the assumption that they were likely to be dwarf planets. To date, only two other TNOs, 2003&nbsp;EL<sub>61</sub> and 2005&nbsp;FY<sub>9</sub>, have met the absolute magnitude requirement, while other possible dwarf planets, such as Sedna, Orcus and Quaoar, were named by the minor-planet committee alone.<ref>{{cite web
 |date=June 11, 2008 |location=Paris 
 |title=Plutoid chosen as name for Solar System objects like Pluto 
 |publisher=[[International Astronomical Union]] (News Release&nbsp;— IAU0804) 
 |url=http://www.iau.org/public_press/news/release/iau0804 
 |access-date=June 11, 2008 |url-status=dead 
 |archive-url=https://web.archive.org/web/20080613121232/http://www.iau.org/public_press/news/release/iau0804/ 
 |archive-date=June 13, 2008 }}</ref> On July 11, 2008, the Working Group on Planetary Nomenclature named 2005&nbsp;FY<sub>9</sub> ''[[Makemake (dwarf planet)|Makemake]]'',<ref name="name">{{cite web  |date=July 11, 2008 <!-- 11:42:58 -->|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=July 13, 2008}}</ref> and on September 17, 2008, they named 2003&nbsp;EL<sub>61</sub> ''[[Haumea (dwarf planet)|Haumea]]''.<ref name=usgs>{{cite news| title =USGS Gazetteer of Planetary Nomenclature| url=http://planetarynames.wr.usgs.gov/append7.html| access-date=September 17, 2008}}</ref>