Editing Pluto (section)

== History ==

=== Discovery ===
{{further|Planets beyond Neptune}}
[[File:Pluto discovery plates.png|left|thumb|alt=The same area of night sky with stars, shown twice, side by side. One of the bright points, located with an arrow, changes position between the two images.|Discovery photographs of Pluto]]
In the 1840s, [[Urbain Le Verrier]] used [[Classical mechanics|Newtonian mechanics]] to predict the position of the then-undiscovered planet [[Neptune]] after analyzing perturbations in the orbit of [[Uranus]]. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune.<ref>{{cite book |last=Croswell |first=Ken |title=Planet Quest: The Epic Discovery of Alien Solar Systems |url=https://books.google.com/books?id=60sPD6yjbVAC |location=New York |publisher=The Free Press |year=1997 |isbn=978-0-684-83252-4 |page=43 |access-date=April 15, 2022 |archive-date=February 26, 2024 |archive-url=https://web.archive.org/web/20240226151141/https://books.google.com/books?id=60sPD6yjbVAC |url-status=live }}</ref>

In 1906, [[Percival Lowell]]—a wealthy [[Boston|Bostonian]] who had founded [[Lowell Observatory]] in [[Flagstaff, Arizona]], in 1894—started an extensive project in search of a possible ninth planet, which he termed "[[Planet X]]".<ref name="Tombaugh1946" /> By 1909, Lowell and [[William Henry Pickering|William H. Pickering]] had suggested several possible celestial coordinates for such a planet.<ref name="Hoyt" /> Lowell and his observatory conducted his search, using mathematical calculations made by [[Elizabeth Langdon Williams|Elizabeth Williams]], until his death in 1916, but to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were.<ref name="Hoyt" /><ref name="Littman1990" /> There are fourteen other known [[precovery]] observations, with the earliest made by the [[Yerkes Observatory]] on August 20, 1909.<ref name="BuchwaldDimarioWild2000" />

[[File:Clyde W. Tombaugh.jpeg|left|thumb|upright|Clyde Tombaugh, in Kansas]]

Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory over her husband's legacy, and the search for Planet X did not resume until 1929.{{sfn|Croswell|1997|p=50}} [[Vesto Melvin Slipher]], the observatory director, gave the job of locating Planet X to 23-year-old [[Clyde Tombaugh]], who had just arrived at the observatory after Slipher had been impressed by a sample of his astronomical drawings.{{sfn|Croswell|1997|p=50}}

Tombaugh's task was to systematically image the night sky in pairs of photographs, then examine each pair and determine whether any objects had shifted position. Using a [[blink comparator]], he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29. A lesser-quality photograph taken on January 21 helped confirm the movement.{{sfn|Croswell|1997|p=52}} After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the [[Harvard College Observatory]] on March 13, 1930.<ref name="Hoyt" />

One Plutonian year corresponds to 247.94 Earth years;<ref name="Pluto Fact Sheet" /> thus, in 2178, Pluto will complete its first orbit since its discovery.

=== Name and symbol ===
The name ''Pluto'' came from the Roman [[Pluto (mythology)|god of the underworld]]; and it is also an [[epithet]] for [[Hades]] (the Greek equivalent of Pluto).

Upon the announcement of the discovery, Lowell Observatory received over a thousand suggestions for names.<ref name="pluto guide" /> Three names topped the list: [[Minerva]], Pluto and [[Cronus]]. 'Minerva' was the Lowell staff's first choice<ref name=S&G/> but was rejected because it had already been used for [[93 Minerva|an asteroid]]; Cronus was disfavored because it was promoted by an unpopular and egocentric astronomer, [[Thomas Jefferson Jackson See]]. A vote was then taken and 'Pluto' was the unanimous choice. To make sure the name stuck, and that the planet would not suffer changes in its name as Uranus had, Lowell Observatory proposed the name to the [[American Astronomical Society]] and the [[Royal Astronomical Society]]; both approved it unanimously.<ref name=T&M>Clyde Tombaugh & Patrick Moore (2008) ''Out of the Darkness: The Planet Pluto''</ref>{{rp|136}}{{sfn|Croswell|1997|pp=54–55}} The name was published on May 1, 1930.<ref name="Venetia" /><ref>{{cite web |title=Pluto Research at Lowell |url=https://lowell.edu/in-depth/pluto/pluto-research-at-lowell/ |website=Lowell Observatory |access-date=March 22, 2017 |archive-url=https://web.archive.org/web/20160418140312/http://lowell.edu/in-depth/pluto/pluto-research-at-lowell/ |archive-date=April 18, 2016 |url-status=dead }}</ref>

The name ''Pluto'' had received some 150 nominations among the letters and telegrams sent to Lowell. The first{{efn|A French astronomer had suggested the name ''Pluto'' for Planet X in 1919, but there is no indication that the Lowell staff knew of this.<ref>Ferris (2012: 336) ''Seeing in the Dark''</ref>}} had been from [[Venetia Burney]] (1918–2009), an eleven-year-old schoolgirl in [[Oxford]], England, who was interested in [[classical mythology]].<ref name=T&M/><ref name="Venetia" /> She had suggested it to her grandfather [[Falconer Madan]] when he read the news of Pluto's discovery to his family over breakfast; Madan passed the suggestion to astronomy professor [[Herbert Hall Turner]], who cabled it to colleagues at Lowell on March 16, three days after the announcement.<ref name=S&G>Kevin Schindler & William Grundy (2018) ''Pluto and Lowell Observatory'', pp. 73–79.</ref><ref name="Venetia" />

The name 'Pluto' was mythologically appropriate: the god Pluto was one of six surviving children of [[Saturn (mythology)|Saturn]], and the others had already all been chosen as names of major or minor planets (his brothers [[Jupiter (mythology)|Jupiter]] and [[Neptune (mythology)|Neptune]], and his sisters [[Ceres (mythology)|Ceres]], [[Juno (mythology)|Juno]] and [[Vesta (mythology)|Vesta]]). Both the god and the planet inhabited "gloomy" regions, and the god was able to make himself invisible, as the planet had been for so long.<ref>Scott & Powell (2018) ''The Universe as It Really Is''</ref> 
The choice was further helped by the fact that the first two letters of ''Pluto'' were the initials of Percival Lowell; indeed, 'Percival' had been one of the more popular suggestions for a name for the new planet.<ref name=S&G/><ref>Coincidentally, as popular science author [[Martin Gardner]] and others have noted of the name "Pluto", "the last two letters are the first two letters of Tombaugh's name" Martin Gardner, ''Puzzling Questions about the Solar System'' (Dover Publications, 1997) p. 55</ref>

=== Symbol ===
Once named, Pluto's [[planetary symbol]] {{angbr|[[File:Pluto monogram (fixed width).svg|20px|♇|link=wikt:♇]]}} was then created as a [[monogram]] of the letters "PL".<ref name="JPL/NASA Pluto's Symbol" /> This symbol is rarely used in astronomy anymore,{{efn|name = PL |For example, {{angbr|♇}} (in [[Unicode]]: {{unichar|2647|PLUTO}}) occurs in a table of the planets identified by their symbols in a 2004 article written before the 2006 IAU definition,<ref>{{cite book |editor=John Lewis |date=2004 |title=Physics and chemistry of the solar system |edition= 2 |page=64 |publisher=Elsevier}}</ref> but not in a graph of planets, dwarf planets and moons from 2016, where only the eight IAU planets are identified by their symbols.<ref>{{cite journal |author1= Jingjing Chen |author2= David Kipping |year=2017 |title= Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal= The Astrophysical Journal |volume= 834 |issue= 17 |page= 8 |publisher= The American Astronomical Society|doi= 10.3847/1538-4357/834/1/17 |arxiv= 1603.08614 |bibcode= 2017ApJ...834...17C |s2cid= 119114880 |doi-access= free }}</ref>
(Planetary symbols in general are uncommon in astronomy, and are discouraged by the IAU.)<ref name="iau_1989">{{cite book|date=1989|language=en|page=27|title=The IAU Style Manual|url=http://www.iau.org/static/publications/stylemanual1989.pdf|access-date=January 29, 2022|archive-date=July 26, 2011|archive-url=https://web.archive.org/web/20110726170213/http://www.iau.org/static/publications/stylemanual1989.pdf|url-status=live}}</ref>}} though it is still common in astrology. However, the most common [[astrological symbol]] for Pluto, occasionally used in astronomy as well, is an orb (possibly representing Pluto's invisibility cap) over Pluto's [[bident]] {{angbr|[[File:Pluto symbol (large orb, fixed width).svg|20px|⯓|link=wikt:⯓]]}}, which dates to the early 1930s.<ref>Dane Rudhyar (1936) ''The Astrology of Personality'', credits it to Paul Clancy Publications, founded in 1933.</ref>{{efn|name = bident|The bident symbol ({{Unichar|2BD3|PLUTO FORM TWO}}) has seen some astronomical use as well since the IAU decision on dwarf planets, for example in a public-education poster on dwarf planets published by the NASA/JPL ''Dawn'' mission in 2015, in which each of the five dwarf planets announced by the IAU receives a symbol.<ref>NASA/JPL, [https://www.jpl.nasa.gov/infographics/what-is-a-dwarf-planet What is a Dwarf Planet?] {{Webarchive|url=https://web.archive.org/web/20211208181916/https://www.jpl.nasa.gov/infographics/what-is-a-dwarf-planet |date=December 8, 2021 }} 2015 Apr 22</ref> There are in addition several other symbols for Pluto found in astrological sources,<ref>Fred Gettings (1981) ''Dictionary of Occult, Hermetic and Alchemical Sigils.'' Routledge & Kegan Paul, London.</ref> including three accepted by Unicode: [[File:Pluto symbol (southern Europe).svg|20px|⯔]], {{unichar|2BD4|PLUTO FORM THREE}}, used principally in southern Europe; [[File:Pluto symbol (northern Europe).svg|20px|⯖]]/[[File:Pluto symbol (northern Europe, variant).svg|20px|⯖]], {{unichar|2BD6|PLUTO FORM FIVE}} (found in various orientations, showing Pluto's orbit cutting across that of Neptune), used principally in northern Europe; and [[File:Charon symbol (fixed width).svg|20px|⯕]], {{unichar|2BD5|PLUTO FORM FOUR}}, used in [[Uranian astrology]].<ref>{{cite web |last1=Faulks |first1=David |title=Astrological Plutos |url=https://www.unicode.org/L2/L2016/16067r-astrological-plutos.pdf |website=www.unicode.org |publisher=Unicode |access-date=October 1, 2021 |archive-date=November 12, 2020 |archive-url=https://web.archive.org/web/20201112010819/https://www.unicode.org/L2/L2016/16067r-astrological-plutos.pdf |url-status=live }}</ref>}}

The name 'Pluto' was soon embraced by wider culture. In 1930, [[Walt Disney]] was apparently inspired by it when he introduced for [[Mickey Mouse]] a canine companion named [[Pluto (Disney)|Pluto]], although [[Disney]] animator [[Ben Sharpsteen]] could not confirm why the name was given.<ref name="Heinrichs2006" /> In 1941, [[Glenn T. Seaborg]] named the newly created [[Chemical element|element]] [[plutonium]] after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following [[uranium]], which was named after Uranus, and [[neptunium]], which was named after Neptune.<ref name="ClarkHobart2000" />

Most languages use the name "Pluto" in various transliterations.{{efn|The equivalence is less close in languages whose [[phonology]] differs widely from [[Ancient Greek phonology|Greek's]], such as [[Somali language|Somali]] ''Buluuto'' and [[Navajo language|Navajo]] ''Tłóotoo''.}} In Japanese, [[Houei Nojiri]] suggested the [[calque]] {{nihongo3|"Star of the King (God) of the Underworld"|冥王星|Meiōsei}}, and this was borrowed into Chinese and Korean. Some [[languages of India]] use the name Pluto, but others, such as [[Hindi]], use the name of ''[[Yama]]'', the God of Death in Hinduism.<ref name="nineplan" /> [[Polynesian languages]] also tend to use the indigenous god of the underworld, as in [[Māori language|Māori]] ''[[Whiro]]''.<ref name="nineplan" />
Vietnamese might be expected to follow Chinese, but does not because the [[Sino-Vietnamese vocabulary|Sino-Vietnamese]] word 冥 ''minh'' "dark" is homophonous with 明 ''minh'' "bright". Vietnamese instead uses Yama, which is also a Buddhist deity, in the form of ''Sao Diêm Vương'' 星閻王 "Yama's Star", derived from Chinese 閻王 ''[[Yama (Buddhism)|Yán Wáng / Yìhm Wòhng]]'' "King Yama".<ref name="nineplan" /><ref name="RenshawIhara2000" /><ref name="Bathrobe" />

=== Planet X disproved ===
Once Pluto was found, its faintness and lack of a [[Angular diameter|viewable disc]] cast doubt on the idea that it was Lowell's [[planets beyond Neptune|Planet X]].<ref name="Tombaugh1946" /> Estimates of Pluto's mass were revised downward throughout the 20th century.<ref>{{cite book|last1 = Stern|first1 = Alan|last2 = Tholen|first2 = David James|title = Pluto and Charon|date = 1997|publisher=University of Arizona Press|isbn = 978-0-8165-1840-1|pages=206–208}}</ref>

{| class="wikitable plainrowheaders floatright" style="clear:right;"
|+ Mass estimates for Pluto
!scope="col"| Year
!scope="col"| Mass
!scope="col"| Estimate by
|-
!scope="row"| 1915
| 7 Earths
| [[Percival Lowell|Lowell]] (prediction for [[Planet X]])<ref name="Tombaugh1946" />
|-
!scope="row"| 1931
| 1 Earth
| [[Seth Barnes Nicholson|Nicholson]] & [[Nicholas U. Mayall|Mayall]]<ref name="RAS1931.91" /><ref name="Nicholson et al 1930">{{cite journal
 | bibcode = 1930PASP...42..350N
 | title = The Probable Value of the Mass of Pluto
 | first1 = Seth B.
 | last1 = Nicholson
 | author-link1 = Seth Barnes Nicholson
 | first2 = Nicholas U.
 | last2 = Mayall
 | author-link2 = Nicholas U. Mayall
 | journal = Publications of the Astronomical Society of the Pacific
 | volume = 42
 | issue = 250
 | page = 350
 | date = December 1930
 | doi = 10.1086/124071
 | doi-access = free
}}</ref><ref name="Nicholson et al. 1931" />
|-
!scope="row"| 1948
| 0.1 (1/10) Earth
| [[Gerard Kuiper|Kuiper]]<ref name="Kuiper 10.1086/126255" />
|-
!scope="row"| 1976
| 0.01 (1/100) Earth
| [[Dale Cruikshank|Cruikshank]], <!-- Carl -->Pilcher, & [[David Morrison (astrophysicist)|Morrison]]{{sfn|Croswell|1997|p=57}}
|-
!scope="row"| 1978
| 0.0015 (1/650) Earth
| [[James W. Christy|Christy]] & [[Robert Sutton Harrington|Harrington]]<ref name="ChristyHarrington1978" />
|-
!scope="row"| 2006
| 0.00218 (1/459) Earth
| [[Marc W. Buie|Buie]] et al.<!-- William M. Grundy, Eliot F. Young, Leslie A. Young, S. Alan Stern --><ref name="BuieGrundyYoung_2006" />
|}
Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931, Pluto was calculated to be roughly the mass of [[Earth]], with further calculations in 1948 bringing the mass down to roughly that of [[Mars]].<ref name="Nicholson et al 1930" /><ref name="Kuiper 10.1086/126255" /> In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the [[University of Hawaiʻi]] calculated Pluto's [[Albedo#Astronomical albedo|albedo]] for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of Earth.{{sfn|Croswell|1997|p=57}} (Pluto's albedo is {{nowrap|1.4–1.9}} times that of Earth.<ref name="Pluto Fact Sheet" />)

In 1978, the discovery of Pluto's moon [[Charon (moon)|Charon]] allowed the measurement of Pluto's mass for the first time: roughly 0.2% that of Earth, and far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by [[Robert Sutton Harrington]],<ref name="SeidelmannHarrington1988" /> failed. In 1992, [[E. Myles Standish|Myles Standish]] used data from ''[[Voyager 2]]'''s flyby of Neptune in 1989, which had revised the estimates of Neptune's mass downward by 0.5%—an amount comparable to the mass of Mars—to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.<ref name="Standish1993" /> {{as of|2000}} the majority of scientists agree that Planet X, as Lowell defined it, does not exist.<ref name="Standage2000" /> Lowell had made a prediction of Planet X's orbit and position in 1915 that was fairly close to Pluto's actual orbit and its position at that time; [[Ernest W. Brown]] concluded soon after Pluto's discovery that this was a coincidence.<ref>Ernest W. Brown, [https://www.pnas.org/content/16/5/364 On the predictions of trans-Neptunian planets from the perturbations of Uranus] {{Webarchive|url=https://web.archive.org/web/20220118073827/https://www.pnas.org/content/16/5/364 |date=January 18, 2022 }}, PNAS May 15, 1930, 16 (5) 364–371.</ref>

=== 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" />