Editing Pluto (section)

=== Classification ===
{{Further|Definition of planet}}

From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto is part of a population of objects called the [[Kuiper belt]]. This made its official status as a planet controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population. Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the [[Solar System]]. In February 2000 the [[Hayden Planetarium]] in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later.<ref name="Tyson2001" />

[[Ceres (dwarf planet)|Ceres]], [[2 Pallas|Pallas]], [[3 Juno|Juno]] and [[4 Vesta|Vesta]] lost their planet status among most astronomers after the discovery of many other [[asteroid]]s in the 1840s. On the other hand, planetary geologists often regarded Ceres, and less often Pallas and Vesta, as being different from smaller asteroids because they were large enough to have undergone geological evolution.<ref name=metzger19>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Sykes |first2=Mark V. |last3=Stern |first3=Alan |last4=Runyon |first4=Kirby |date=2019 |title=The Reclassification of Asteroids from Planets to Non-Planets |journal=Icarus |volume=319 |pages=21–32 |doi=10.1016/j.icarus.2018.08.026|arxiv=1805.04115 |bibcode=2019Icar..319...21M |s2cid=119206487 }}</ref> Although the first Kuiper belt objects discovered were quite small, objects increasingly closer in size to Pluto were soon discovered, some large enough (like Pluto itself) to satisfy geological but not dynamical ideas of planethood.<ref name=metzger22>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Grundy |first2=W. M. |first3=Mark V. |last3=Sykes |first4=Alan |last4=Stern |first5=James F. |last5=Bell III |first6=Charlene E. |last6=Detelich |first7=Kirby |last7=Runyon |first8=Michael |last8=Summers |date=2022 |title=Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science |url=https://www.sciencedirect.com/science/article/pii/S0019103521004206 |journal=Icarus |volume=374 |issue= |page=114768 |doi=10.1016/j.icarus.2021.114768 |arxiv=2110.15285 |bibcode=2022Icar..37414768M |s2cid=240071005 |access-date=August 8, 2022 |archive-date=September 11, 2022 |archive-url=https://web.archive.org/web/20220911060134/https://www.sciencedirect.com/science/article/pii/S0019103521004206 |url-status=live }}</ref> On July 29, 2005, the debate became unavoidable when astronomers at [[Caltech]] announced the discovery of a new [[trans-Neptunian object]], [[Eris (dwarf planet)|Eris]], which was substantially more massive than Pluto and the most massive object discovered in the Solar System since [[Triton (moon)|Triton]] in 1846. Its discoverers and the press initially called it the [[tenth planet]], although there was no official consensus at the time on whether to call it a planet.<ref name="NASA-JPL press release 07-29-2005" /> Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.<ref>{{cite journal |title=What Is a Planet? |journal=The Astronomical Journal |volume=132 |issue=6 |pages=2513–2519 |date=November 2, 2006 |doi=10.1086/508861 |last1 = Soter|first1 = Steven|bibcode=2006AJ....132.2513S |arxiv=astro-ph/0608359 |s2cid=14676169 }}</ref>

==== IAU classification ====

{{Main|IAU definition of planet}}

The debate came to a head in August 2006 during the triennial meeting of the [[International Astronomical Union|IAU]], when Uruguayan astronomers [[Julio Ángel Fernández]] and [[Gonzalo Tancredi]] first proposed the new definition for the term "planet".<ref>{{Cite web |last=Federation |first=International Astronautical |title=IAF : Gonzalo Tancredi |url=https://www.iafastro.org/biographie/gonzalo-tancredi.html |access-date=2025-03-01 |website=www.iafastro.org |language=en}}</ref><ref>{{Cite news |title=Cómo fue el día en que dos uruguayos lograron que Plutón dejara de ser considerado un planeta |url=https://www.bbc.com/mundo/noticias-37164992 |access-date=2025-03-01 |work=BBC News Mundo |language=es}}</ref> According to their proposal, there are three conditions for an object in the [[Solar System]] to be considered a planet:
* The object must be in orbit around the [[Sun]].
* The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by [[hydrostatic equilibrium]].
* It must have [[cleared the neighborhood]] around its orbit.<ref name="IAU2006 GA26-5-6" /><ref name="IAU0603" />
Pluto fails to meet the third condition.<ref name="Margot2015">{{cite journal|last1=Margot|first1=Jean-Luc|title=A Quantitative Criterion for Defining Planets|journal=The Astronomical Journal|volume=150|issue=6|year=2015|pages=185 |doi=10.1088/0004-6256/150/6/185|bibcode=2015AJ....150..185M|arxiv=1507.06300|s2cid=51684830}}</ref> Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7&nbsp;million times the remaining mass in its orbit (excluding the moon).<ref name="what" /><ref name="IAU0603" /> The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called [[dwarf planet]]s. In September 2006, the IAU included Pluto, and Eris and its moon [[Dysnomia (moon)|Dysnomia]], in their [[Minor Planet Catalogue]], giving them the official [[minor-planet designation]]s "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia".<ref name="IAUC 8747" /> Had Pluto been included upon its discovery in 1930, it would have likely been designated 1164, following [[1163 Saga]], which was discovered a month earlier.<ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi#top |title=JPL Small-Body Database Browser |publisher=California Institute of Technology |access-date=July 15, 2015 |archive-date=July 21, 2011 |archive-url=https://web.archive.org/web/20110721054158/http://ssd.jpl.nasa.gov/sbdb.cgi#top |url-status=live }}</ref>

There has been some resistance within the astronomical community toward the reclassification, and in particular planetary scientists often continue to reject it, considering Pluto, Charon, and Eris to be planets for the same reason they do so for Ceres. In effect, this amounts to accepting only the second clause of the IAU definition.<ref name="geoff2006c" /><ref name="Ruibal-1999" /><ref name="Britt-2006" /> [[Alan Stern]], principal investigator with [[NASA]]'s ''New Horizons'' mission to Pluto, derided the IAU resolution.<ref name="geoff2006a" /><ref name="newscientistspace" /><ref>{{cite web |url=https://time.com/7221576/what-makes-pluto-intriguing/ |publisher=Time |quote=Stern thinks the dwarf planet distinction is nonsensical—an arbitrary parsing of cosmic definitions. “Small planets are planets too,” he says. “Just because the sun is a small star we don’t call it a dwarf star. We’re not afraid of large numbers of planets; we’re not afraid of schoolchildren having to learn all their names. After all, kids don’t have to memorize every element in the periodic table.” |title=What Makes Pluto So Intriguing |first=Jeffrey |last=Kluger|date=February 18, 2025 }}</ref> He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.<ref name="newscientistspace" /> [[Marc W. Buie]], then at the Lowell Observatory, petitioned against the definition.<ref name="Buie2006 IAU response" /> Others have supported the IAU, for example [[Michael E. Brown|Mike Brown]], the astronomer who discovered Eris.<ref name="Overbye2006" />

Public reception to the IAU decision was mixed. A resolution introduced in the [[California State Assembly]] facetiously called the IAU decision a "scientific heresy".<ref name="DeVore2006" /> The [[New Mexico House of Representatives]] passed a resolution in honor of Clyde Tombaugh, the discoverer of Pluto and a longtime resident of that state, that declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007, was Pluto Planet Day.<ref name="Holden2007" /><ref name="Gutierrez2007" /> The [[Illinois Senate]] passed a similar resolution in 2009 on the basis that Tombaugh was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU."<ref name="ILGA SR0046" /> Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.<ref name="Sapa-AP" /> In 2006, in its 17th annual words-of-the-year vote, the [[American Dialect Society]] voted ''[[plutoed]]'' as the word of the year. To "pluto" is to "demote or devalue someone or something".<ref name="msnbc" /> In April 2024, [[Arizona]] (where Pluto was first discovered in 1930) passed a law naming Pluto as the official state planet.<ref>{{cite web |last1=Sanchez |first1=Cameron |title=Pluto is a planet again — at least in Arizona |url=https://www.npr.org/2024/04/06/1243230463/pluto-was-discovered-at-an-arizona-observatory-it-might-be-named-the-state-plane |website=npr.org |publisher=NPR |access-date=April 12, 2024}}</ref>

Researchers on both sides of the debate gathered in August 2008, at the Johns Hopkins University [[Applied Physics Laboratory]] for a conference that included back-to-back talks on the IAU definition of a planet.<ref name="Minkel2008" /> Entitled "The Great Planet Debate",<ref name="The Great Planet Debate" /> the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet.<ref name="PSIedu press release 2008-09-19" /> In June 2008, the IAU had announced in a press release that the term "[[plutoid]]" would henceforth be used to refer to Pluto and other planetary-mass objects that have an orbital [[semi-major axis]] greater than that of Neptune, though the term has not seen significant use.<ref name="IAU0804" /><ref name="Discover 2009-JANp76" /><ref name="Science News, July 5, 2008 p. 7" />