View source for Pluto

{{Short description|Dwarf planet}}
{{About|the dwarf planet|the deity|Pluto (mythology)|other uses|Pluto (disambiguation)}}
{{Featured article}}
{{Pp-semi-indef}}
{{Pp-move}}
{{Use American English|date=August 2024}}
{{Use mdy dates|date=August 2024}}
{{Infobox planet
| name = 134340 Pluto
| minorplanet = yes
| symbol = [[File:Pluto monogram (bold).svg|24px|♇|class=skin-invert]] or [[File:Pluto symbol (large orb, bold).svg|24px|⯓|class=skin-invert]]<!--Do not delete the bident symbol; it is not merely astrological. NASA has used it; moreover, the IAU specifically discourages use of symbols-->
| image = Pluto in True Color - High-Res.png
| caption = Pluto, imaged by the ''[[New Horizons]]'' spacecraft, July 2015.{{efn|name = caption|This photograph was taken by the [[Ralph (New Horizons)|Ralph]] telescope aboard ''[[New Horizons]]'' on July 14, 2015, from a distance of {{convert|35,445|km|mi|abbr=on}}}} The most prominent feature in the image, the bright, youthful plains of [[Tombaugh Regio]] and [[Sputnik Planitia]], can be seen at right. It contrasts the darker, cratered terrain of [[Belton Regio]] at lower left
| background = PapayaWhip
| discoverer = [[Clyde W. Tombaugh]]
| discovered = February 18, 1930
| discovery_site = [[Lowell Observatory]]
| mpc_name = (134340) Pluto
| named_after = [[Pluto (mythology)|Pluto]]
| mp_category = {{Plain list|
* [[Dwarf planet]]
* [[Trans-Neptunian object]]
* [[Kuiper belt]] object
* [[Plutino]]
}}
| orbit_ref = <ref name="TOP2013" />{{efn|name = MeanElements|The mean elements here are from the Theory of the Outer Planets (TOP2013) solution by the Institut de mécanique céleste et de calcul des éphémérides (IMCCE). They refer to the standard equinox J2000, the barycenter of the Solar System, and the epoch J2000.}}
| epoch = [[J2000]]
| earliest_precovery_date = August 20, 1909
| aphelion = {{Plain list|
* {{val|49.305|ul=AU}}
* ({{nowrap|{{val|fmt=commas|7.37593|u=billion km}}}})
* February 2114
}}
| perihelion = {{Plain list|
* {{val|29.658|u=AU}}
* ({{nowrap|{{val|fmt=commas|4.43682|u=billion km}}}})<ref name="Pluto Fact Sheet" />
* (September 5, 1989)<ref name="jpl-ssd-horizons" />
}}
| semimajor = {{Plain list|
* {{val|39.482|u=AU}}
* ({{nowrap|{{val|fmt=commas|5.90638|u=billion km}}}})
}}
| eccentricity = {{val|0.2488}}
| inclination = {{Plain list|
* {{val|17.16|u=°}}
* (11.88° to Sun's equator)
}}
| asc_node = {{val|110.299|u=°}}
| arg_peri = {{val|113.834|u=°}}
| period = {{Plain list|
* {{val|247.94}} [[julian year (astronomy)|years]]<ref name="Pluto Fact Sheet" />
* {{nowrap|{{val|fmt=commas|90560|u=days}}}}<ref name="Pluto Fact Sheet" />
<!-- * {{nowrap|{{val|fmt=commas|14164.4}}}} Plutonian [[solar day]]s<ref name="planet_years" /> -->
}}
| synodic_period = 366.73 days<ref name="Pluto Fact Sheet" />
| avg_speed = 4.743&nbsp;km/s<ref name="Pluto Fact Sheet" />
| mean_anomaly = {{val|14.53|u=[[Degree (angle)|deg]]}}
| satellites = [[Moons of Pluto|5]]
| mean_radius = {{Plain list|
* {{nowrap|{{val|fmt=commas|1188.3|0.8|u=km}}}}<ref name = "Pluto System after New Horizons">{{cite journal |last1=Stern |first1=S.A. |last2=Grundy |first2=W. |last3=McKinnon |first3=W.B. |last4=Weaver |first4=H.A. |last5=Young |first5=L.A. |title=The Pluto System After New Horizons |journal=Annual Review of Astronomy and Astrophysics |volume=2018 |pages=357–392 |arxiv=1712.05669 |year=2017 |bibcode=2018ARA&A..56..357S |s2cid=119072504 |doi=10.1146/annurev-astro-081817-051935|issn = 0066-4146}}</ref><ref name="Nimmo2017">{{cite journal |last=Nimmo |first=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 |bibcode=2017Icar..287...12N |arxiv=1603.00821 |s2cid=44935431 |doi=10.1016/j.icarus.2016.06.027}}</ref>
* 0.1868 [[Earth radius|Earths]]}}
| dimensions = {{val|fmt=commas|2376.6|1.6|u=km}} (observations consistent with a sphere, predicted deviations too small to be observed)<ref name="Nimmo2017"/>
| surface_area = {{Plain list|
* {{val|1.774443|e=7|u=km2}}{{efn|name=Surface area}}
* 0.035 Earths
}}
| volume = {{Plain list|
* {{val|7.057|0.004|e=9|u=km3}}{{efn|name=Volume}}
* {{val|0.00651|u=Earths}}
}}
| mass = {{Plain list|
* {{val|1.3025|0.0006|e = 22|u=kg}}<ref name="Brozovic2024"/><!-- Calculated from GM=869.3 ± 0.4 km3 s–2 -->
* {{val|0.00218|u=[[Earth mass|Earths]]|fmt=none}}
* 0.177 [[Moon mass|Moons]]
}}
| density = {{val|1.853|0.004|u=g/cm3}}<ref name="Brozovic2024"/>
| surface_grav = {{cvt|0.620|m/s2|g0|lk=out}}{{efn|name = Surface gravity}}
| escape_velocity = {{val|1.212}}&nbsp;km/s{{efn|name = Escape velocity}}
| rotation = {{Plain list|
* {{val|−6.38680|u=day}}
* −6 d, 9 h, 17&nbsp;m, 00 s
}}<ref name="planet_years">{{cite web |url=http://cseligman.com/text/sky/rotationvsday.htm |title=Rotation Period and Day Length |last=Seligman |first=Courtney |access-date=June 12, 2021 |archive-date=September 29, 2018 |archive-url=https://web.archive.org/web/20180929010908/http://cseligman.com/text/sky/rotationvsday.htm |url-status=live }}</ref>
| sidereal_day = {{Plain list|
* {{val|−6.387230|u=day}}
* −6 d, 9 h, 17&nbsp;m, 36 s
}}
| rot_velocity = {{cvt|47.18|km/h|m/s|disp=out}}{{fact|date=July 2024}}
| axial_tilt = {{val|122.53|u=°}} (to orbit)<ref name="Pluto Fact Sheet" />
| right_asc_north_pole = 132.993°<ref name="Archinal" />
| declination = −6.163°<ref name="Archinal" />
| albedo = 0.52 [[Geometric albedo|geometric]]<ref name="Pluto Fact Sheet" /><br />0.72 [[Bond albedo|Bond]]<ref name="Pluto Fact Sheet" />
| magnitude = 13.65<ref name="Pluto Fact Sheet" /> to 16.3<ref name="AstDys-Pluto" /> <br /> (mean is 15.1)<ref name="Pluto Fact Sheet" />
| abs_magnitude = −0.44<ref name="jpldata" />
| angular_size = 0.06{{pprime}} to 0.11{{pprime}}<ref name="Pluto Fact Sheet" />{{efn|name = Angular size}}
| pronounced = {{IPAc-en|audio=en-us-Pluto.ogg|ˈ|p|l|uː|t|oʊ}}
| adjectives = [[Wiktionary:Plutonian#Adjective 2|Plutonian]] {{IPAc-en|p|l|uː|ˈ|t|oʊ|n|i|ə|n}}<ref>{{OED|Plutonian}}</ref>
| atmosphere = yes
| temp_name1 = [[Kelvin]]
| min_temp_1 = 33 K
| mean_temp_1 = 44 K (−229 °C)
| max_temp_1 = 55 K
| surface_pressure = 1.0 [[pascal (unit)|Pa]] (2015)<ref name=Stern2015 /><ref>{{cite news |last=Amos |first=Jonathan |url=https://www.bbc.com/news/science-environment-33657447 |title=New Horizons: Pluto may have 'nitrogen glaciers' |publisher=BBC News |date=July 23, 2015 |access-date=July 26, 2015 |quote=It could tell from the passage of sunlight and radiowaves through the Plutonian "air" that the pressure was only about 10 microbars at the surface |archive-date=October 27, 2017 |archive-url=https://web.archive.org/web/20171027182142/http://www.bbc.com/news/science-environment-33657447 |url-status=live }}</ref>
| atmosphere_composition = [[Nitrogen]], [[methane]], [[carbon monoxide]]<ref name="Physorg April 19, 2011" />
| note = no
| scale_height = 
| flattening = <1%<ref name="Stern2015" />
}}
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'''Pluto''' ([[minor-planet designation]]: '''134340 Pluto''') is a [[dwarf planet]] in the [[Kuiper belt]], a ring of [[Trans-Neptunian object|bodies beyond the orbit of Neptune]]. It is the ninth-largest and tenth-most-[[mass]]ive known object to directly orbit the [[Sun]]. It is the largest known [[trans-Neptunian object]] by volume, by a small margin, but is less massive than [[Eris (dwarf planet)|Eris]]. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the [[inner planet]]s. Pluto has roughly one-sixth the mass of the [[Moon]], and one-third its volume.

Pluto has a moderately [[Orbital eccentricity|eccentric]] and [[Inclination|inclined]] orbit, ranging from {{convert|30|to|49|AU|e9km e9mi|lk=on|abbr=off}} from the Sun. Light from the Sun takes 5.5 hours to reach Pluto at its orbital distance of {{convert|39.5|AU|e9km e9mi|abbr=unit|}}. Pluto's eccentric orbit periodically brings it closer to the Sun than [[Neptune]], but a stable [[orbital resonance]] prevents them from colliding.

Pluto has [[moons of Pluto|five known moons]]: [[Charon (moon)|Charon]], the largest, whose diameter is just over half that of Pluto; [[Styx (moon)|Styx]]; [[Nix (moon)|Nix]]; [[Kerberos (moon)|Kerberos]]; and [[Hydra (moon)|Hydra]]. Pluto and Charon are sometimes considered a [[binary system]] because the [[barycenter]] of their orbits does not lie within either body, and they are [[tidally locked]]. ''[[New Horizons]]'' was the first spacecraft to visit Pluto and its moons, making a [[Planetary flyby|flyby]] on July&nbsp;14,&nbsp;2015, and taking detailed measurements and observations.

Pluto was discovered in 1930 by [[Clyde W. Tombaugh]], making it by far the first known object in the Kuiper belt. It was immediately hailed as the [[ninth planet]], but it never fit well with the other eight,<ref name=T&M/>{{rp|27}} and its planetary status was questioned when it was found to be much smaller than expected. These doubts increased following the discovery of additional objects in the Kuiper belt starting in the 1990s, and particularly the more massive [[scattered disk]] object [[Eris (dwarf planet)|Eris]] in 2005. In 2006, the [[International Astronomical Union]] (IAU) formally [[IAU definition of planet|redefined the term ''planet'']] to exclude dwarf planets such as Pluto. Many planetary astronomers, however, continue to consider Pluto and other dwarf planets to be planets.

== 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 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>
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" style="float: right; margin-right: 0; margin-left: 1em; text-align: center; clear:right;"
|+ Mass estimates for Pluto
! scope="col" | Year
! scope="col" | Mass
! scope="col" | Estimate by
|-
! scope="row" | 1915
| {{left|7 Earths}}
| [[Percival Lowell|Lowell]] (prediction for [[Planet X]])<ref name="Tombaugh1946" />
|-
! scope="row" | 1931
|  {{left|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 B. 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
| {{left|0.1 (1/10) Earth}}
| [[Gerard Kuiper|Kuiper]]<ref name="Kuiper 10.1086/126255" />
|-
! scope="row" | 1976
|  {{left|0.01 (1/100) Earth}}
| [[Dale Cruikshank|Cruikshank]], <!-- Carl -->Pilcher, & [[David Morrison (astrophysicist)|Morrison]]{{sfn|Croswell|1997|p=57}}
|-
! scope="row" | 1978
|  {{left|0.0015 (1/650) Earth}}
| [[James W. Christy|Christy]] & [[Robert Sutton Harrington|Harrington]]<ref name="ChristyHarrington1978" />
|-
! scope="row" | 2006
|  {{left|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, with an [[IAU definition of planet|IAU resolution]] that created an official definition for the term "planet". According to this resolution, 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" /> 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" />

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" />

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>

== Orbit ==
[[File:Animation of Pluto orbit.gif|thumb|upright=1.5|Animation of Pluto{{'s}} orbit from 1850 to 2097<br />{{legend2|Yellow| Sun}}{{·}}{{legend2|Gold | Saturn}}{{·}}{{legend2|Cyan | Uranus}}{{·}}{{legend2| RoyalBlue | Neptune}}{{·}}{{legend2|Magenta| Pluto}}]]
Pluto's orbital period is about 248 years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference [[plane (mathematics)|plane]] called the [[ecliptic]]. In contrast, Pluto's orbit is moderately [[orbital inclination|inclined]] relative to the ecliptic (over 17°) and moderately [[orbital eccentricity|eccentric]] (elliptical). This eccentricity means a small region of Pluto's orbit lies closer to the Sun than Neptune's. The Pluto–Charon barycenter came to [[apsis|perihelion]] on September 5, 1989,<ref name="jpl-ssd-horizons" />{{efn|name = Perihelion}} and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999.<ref name="pluto990209" />

Although the 3:2 resonance with Neptune (see below) is maintained, Pluto's inclination and eccentricity behave in a [[chaos theory|chaotic]] manner. Computer simulations can be used to predict its position for several million years (both [[time reversibility|forward and backward]] in time), but after intervals much longer than the [[Lyapunov time]] of 10–20&nbsp;million years, calculations become unreliable: Pluto is sensitive to immeasurably small details of the Solar System, hard-to-predict factors that will gradually change Pluto's position in its orbit.<ref name="sussman88" /><ref name="wisdom91" />

The [[semi-major axis]] of Pluto's orbit varies between about 39.3 and 39.6&nbsp;[[astronomical unit|AU]] with a period of about 19,951 years, corresponding to an orbital period varying between 246 and 249 years. The semi-major axis and period are presently getting longer.<ref name=williams71 />

=== Relationship with Neptune <span class="anchor" id="Orbits of Pluto and Neptune"></span> ===
[[File:Plutoorbit1.5sideview.gif|left|thumb|Orbit of Pluto&nbsp;– ecliptic view. This "side view" of Pluto's orbit (in red) shows its large inclination to the [[ecliptic]]. Neptune is seen orbiting close to the ecliptic.]]

Despite Pluto's orbit appearing to cross that of Neptune when viewed from north or south of the Solar System, the two objects' orbits do not intersect. When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from such a position, it is also the farthest north of Neptune's path. Pluto's orbit passes about 8 [[astronomical unit|AU]] north of that of Neptune, preventing a collision.<ref name="huainn01" /><ref name="Hunter2004" /><ref name="malhotra-9planets" />{{efn|Because of the eccentricity of Pluto's orbit, some have theorized that it was once a [[satellite of Neptune]].<ref>{{cite book|last1=Sagan|first1=Carl|url=https://books.google.com/books?id=LhkoowKFaTsC|title=Comet|last2=Druyan|first2=Ann|publisher=Random House|year=1997|isbn=978-0-3078-0105-0|location=New York|page=223|author-link1=Carl Sagan|author-link2=Ann Druyan|name-list-style=amp|access-date=October 18, 2021|archive-date=February 26, 2024|archive-url=https://web.archive.org/web/20240226151129/https://books.google.com/books?id=LhkoowKFaTsC|url-status=live}}</ref>}}

This alone is not enough to protect Pluto; [[perturbation (astronomy)|perturbations]] from the planets (especially Neptune) could alter Pluto's orbit (such as its [[apsidal precession|orbital precession]]) over millions of years so that a collision could happen. However, Pluto is also protected by its 2:3 [[orbital resonance]] with [[Neptune]]: for every two orbits that Pluto makes around the Sun, Neptune makes three, in a frame of reference that rotates at the rate that Pluto's perihelion precesses (about {{value|0.97e-4}} degrees per year<ref name=williams71/>). Each cycle lasts about 495 years. (There are many other objects in this same resonance, called [[plutino]]s.) At present, in each 495-year cycle, the first time Pluto is at [[perihelion]] (such as in 1989), Neptune is 57° ahead of Pluto. By Pluto's second passage through perihelion, Neptune will have completed a further one and a half of its own orbits, and will be 123° behind Pluto.<ref name=Horizons>The [https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 ecliptic longitude of Pluto] {{Webarchive|url=https://web.archive.org/web/20240213103809/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27134340%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 |date=February 13, 2024 }} and [https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27899%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 of Neptune] {{Webarchive|url=https://web.archive.org/web/20240213103809/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27899%27&START_TIME=%271700-1-7%27&STOP_TIME=%272097-12-31%27&STEP_SIZE=%271461%20days%27&QUANTITIES=%2718%2019%27 |date=February 13, 2024 }} are available from the [[JPL Horizons On-Line Ephemeris System]].</ref> Pluto and Neptune's minimum separation is over 17&nbsp;AU, which is greater than Pluto's minimum separation from Uranus (11&nbsp;AU).<ref name="malhotra-9planets" /> The minimum separation between Pluto and Neptune actually occurs near the time of Pluto's aphelion.<ref name=williams71 />

[[File:Neptune-Pluto longitude difference.png|thumb|400px|Ecliptic longitude of Neptune minus that of Pluto (blue), and rate of change of Pluto's distance from the sun (red). The red curve crosses zero at perihelion and aphelion.]]
The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years.<ref name="sp-345" /> This prevents their orbits from changing relative to one another, so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide.<ref name="malhotra-9planets" /> When Pluto's period is slightly different from 3/2 of Neptune's, the pattern of its distance from Neptune will drift. Near perihelion Pluto moves interior to Neptune's orbit and is therefore moving faster, so during the first of two orbits in the 495-year cycle, it is approaching Neptune from behind. At present it remains between 50° and 65° behind Neptune for 100 years (e.g. 1937–2036).<ref name=Horizons/> The gravitational pull between the two causes [[angular momentum]] to be transferred to Pluto. This situation moves Pluto into a slightly larger orbit, where it has a slightly longer period, according to [[Kepler's third law]]. After several such repetitions, Pluto is sufficiently delayed that at the second perihelion of each cycle it will not be far ahead of Neptune coming behind it, and Neptune will start to decrease Pluto's period again. The whole cycle takes about 20,000 years to complete.<ref name="malhotra-9planets" /><ref name="sp-345" /><ref name="Cohen_Hubbard_1965">{{cite journal|last1=Cohen|first1=C. J.|last2=Hubbard|first2=E. C.|title=Libration of the close approaches of Pluto to Neptune|journal=Astronomical Journal|date=1965|volume=70|page=10|doi=10.1086/109674|bibcode=1965AJ.....70...10C|doi-access=free}}</ref>

==== Other factors ====
Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto and Neptune does not change.<ref name="huainn01" /><ref name="williams71" /> There are several other resonances and interactions that enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 2:3 mean-motion resonance).

First, Pluto's [[argument of perihelion]], the angle between the point where it crosses the ecliptic (or the [[invariant plane]]) and the point where it is closest to the Sun, [[libration|librates]] around 90°.<ref name="williams71" /> This means that when Pluto is closest to the Sun, it is at its farthest north of the plane of the Solar System, preventing encounters with Neptune. This is a consequence of the [[Kozai mechanism]],<ref name="huainn01" /> which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case, Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° {{nowrap|(90°–38°)}}. The closest such angular separation occurs every 10,000 years.<ref name="sp-345" />

Second, the longitudes of ascending nodes of the two bodies—the points where they cross the [[invariant plane]]—are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and hence it comes closest to the Sun when it is furthest north of Neptune's orbit. This is known as the ''1:1 superresonance''. All the [[Jovian planets]] (Jupiter, Saturn, Uranus, and Neptune) play a role in the creation of the superresonance.<ref name="huainn01" />

===Orcus===
The 2nd-largest known [[plutino]], [[90482 Orcus|Orcus]], has a diameter around 900&nbsp;km and is in a very similar orbit to that of Pluto. However, the orbits of Pluto and Orcus are out of phase, so that the two never approach each other. It has been termed the "anti-Pluto", and is named for the [[Orcus|Etruscan counterpart to the god Pluto]].

== Rotation ==
[[file:Pluto rotation movie2.gif|thumb]]
Pluto's [[rotation period]], its day, is equal to 6.387 [[Earth]] days.<ref name="Pluto Fact Sheet" /><ref name="axis" /> Like [[Uranus]] and [[2 Pallas]], Pluto rotates on its "side" in its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its [[solstice]]s, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness.<ref name="Oregon" /> The reason for this unusual orientation has been debated. Research from the [[University of Arizona]] has suggested that it may be due to the way that a body's spin will always adjust to minimize energy. This could mean a body reorienting itself to put extraneous mass near the equator and regions lacking mass tend towards the poles. This is called ''[[polar wander]]''.<ref>{{Cite journal|last1=Kirschvink|first1=Joseph L.|last2=Ripperdan|first2=Robert L.|last3=Evans|first3=David A.|date=July 25, 1997|title=Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander|journal=Science|language=en|volume=277|issue=5325|pages=541–545|doi=10.1126/science.277.5325.541|s2cid=177135895|issn=0036-8075}}</ref> According to a paper released from the University of Arizona, this could be caused by masses of frozen nitrogen building up in shadowed areas of the dwarf planet. These masses would cause the body to reorient itself, leading to its unusual axial tilt of 120°. The buildup of nitrogen is due to Pluto's vast distance from the Sun. At the equator, temperatures can drop to {{convert|-240|C|F K}}, causing nitrogen to freeze as water would freeze on Earth. The same polar wandering effect seen on Pluto would be observed on Earth were the [[Antarctic ice sheet]] several times larger.<ref>{{Cite journal|last1=Keane|first1=James T.|last2=Matsuyama|first2=Isamu|last3=Kamata|first3=Shunichi|last4=Steckloff|first4=Jordan K.|title=Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia|journal=Nature|volume=540|issue=7631|pages=90–93|doi=10.1038/nature20120|pmid=27851731|bibcode = 2016Natur.540...90K |year=2016|s2cid=4468636}}</ref>

== Geology ==
{{Main|Geology of Pluto|Geography of Pluto}}

=== Surface ===
[[File:Pluto’s Heart - Like a Cosmic Lava Lamp.jpg|thumb|Sputnik Planitia is covered with churning nitrogen ice "cells" that are geologically young and turning over due to [[Convection cell|convection]].]]
The plains on Pluto's surface are composed of more than 98 percent [[nitrogen ice]], with traces of methane and [[carbon monoxide]].<ref name="tobias" /> [[Nitrogen]] and carbon monoxide are most abundant on the anti-Charon face of Pluto (around 180° longitude, where [[Tombaugh Regio]]'s western lobe, [[Sputnik Planitia]], is located), whereas methane is most abundant near 300° east.<ref name=Grundy_2013 /> The mountains are made of water ice.<ref name="drake-natgeo">{{cite magazine | url = http://news.nationalgeographic.com/2015/11/151109-astronomy-pluto-nasa-new-horizons-volcano-moons-science/| archive-url = https://web.archive.org/web/20151113013310/http://news.nationalgeographic.com/2015/11/151109-astronomy-pluto-nasa-new-horizons-volcano-moons-science/| url-status = dead| archive-date = November 13, 2015| title = Floating Mountains on Pluto – You Can't Make This Stuff Up| last = Drake| first = Nadia| author-link = Nadia Drake | date = November 9, 2015| magazine = [[National Geographic]]| access-date = December 23, 2016}}</ref> Pluto's surface is quite varied, with large differences in both brightness and color.<ref name="Buie_2010 light curve" /> Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as [[Saturn]]'s moon [[Iapetus (moon)|Iapetus]].<ref name="Buie_web_map" /> The color varies from charcoal black, to dark orange and white.<ref name="Hubble2010" /> Pluto's color is more similar to that of [[Io (moon)|Io]] with slightly more orange and significantly less red than [[Mars]].<ref name="Buie_2010 surface-maps" /> [[Geography of Pluto|Notable geographical features]] include Tombaugh Regio, or the "Heart" (a large bright area on the side opposite Charon), [[Belton Regio]],<ref name = "Pluto System after New Horizons"/> or the "Whale" (a large dark area on the trailing hemisphere), and the "[[Brass Knuckles (Pluto)|Brass Knuckles]]" (a series of equatorial dark areas on the leading hemisphere).

Sputnik Planitia, the western lobe of the "Heart", is a 1,000&nbsp;km-wide basin of frozen nitrogen and carbon monoxide ices, divided into polygonal cells, which are interpreted as [[convection cell]]s that carry floating blocks of water ice crust and [[Sublimation (phase transition)|sublimation]] pits towards their margins;<ref name="lakdawalla-DPS-2016-10-26">{{cite web| url = http://www.planetary.org/blogs/emily-lakdawalla/2016/10251718-dpsepsc-new-horizons-pluto.html| title = DPS/EPSC update on New Horizons at the Pluto system and beyond| last = Lakdawalla| first = Emily| author-link = Emily Lakdawalla| date = October 26, 2016| publisher = [[The Planetary Society]]| access-date = October 26, 2016| archive-date = October 8, 2018| archive-url = https://web.archive.org/web/20181008021643/http://www.planetary.org/blogs/emily-lakdawalla/2016/10251718-dpsepsc-new-horizons-pluto.html| url-status = live}}</ref><ref name="McKinnon2016">{{cite journal|last1=McKinnon|first1=W. B.|last2=Nimmo|first2= F.|last3=Wong|first3= T.|last4= Schenk|first4=P. M.|last5=White|first5=O. L.|last6=Roberts|first6=J. H.|last7=Moore|first7=J. M.|last8=Spencer|first8=J. R.|last9=Howard|first9=A. D.|last10=Umurhan|first10=O. M.|last11= Stern|first11=S. A.|last12=Weaver|first12=H. A.|last13= Olkin|first13=C. B.|last14=Young|first14=L. A.|last15= Smith|first15=K. E.|last16=Beyer|first16= R.|last17= Buie|first17= M.|last18=Buratti|first18= B.|last19= Cheng|first19= A.|last20=Cruikshank|first20=D.|last21=Dalle Ore|first21= C.|last22= Gladstone|first22= R.|last23= Grundy|first23= W.|last24=Lauer|first24=T.|last25=Linscott|first25= I.|last26= Parker|first26= J.|last27=Porter|first27= S.|last28= Reitsema|first28= H.|last29=Reuter|first29= D.|last30= Robbins|first30= S.|last31= Showalter|first31= M.|last32= Singer|first32= K.|last33=Strobel|first33= D.|last34= Summers|first34= M.|last35= Tyler|first35= L.|last36= Banks|first36= M.|last37=Barnouin|first37= O.|last38= Bray|first38= V.|last39= Carcich|first39= B.|last40=Chaikin|first40= A.|last41= Chavez|first41=C.|last42= Conrad|first42= C.|last43= Hamilton|first43= D.|last44= Howett|first44= C.|last45=Hofgartner|first45= J.|last46= Kammer|first46= J.|last47= Lisse|first47= C.|last48= Marcotte|first48= A.|last49=Parker|first49= A.|last50= Retherford|first50= K.|last51=Saina|first51= M.|last52= Runyon|first52= K.|last53=Schindhelm|first53= E.|last54= Stansberry|first54= J.|last55= Steffl|first55= A.|last56= Stryk|first56=T.|last57=Throop|first57=H.|last58=Tsang|first58=C.|last59=Verbiscer|first59=A.|last60=Winters|first60=H.|last61=Zangari|first61=A.|display-authors=5|title=Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigour|journal= Nature|volume=534|issue= 7605|date=June 1, 2016|pages= 82–85|doi= 10.1038/nature18289|pmid=27251279|bibcode = 2016Natur.534...82M |arxiv=1903.05571|s2cid=30903520}}</ref><ref name="Trowbridge2016">{{cite journal|last1=Trowbridge|first1=A. J.|last2= Melosh|first2=H. J.|last3= Steckloff|first3= J. K.|last4=Freed|first4=A. M.|title=Vigorous convection as the explanation for Pluto's polygonal terrain|journal= Nature|volume= 534|issue=7605|date= June 1, 2016|pages=79–81|doi=10.1038/nature18016|pmid=27251278|bibcode = 2016Natur.534...79T |s2cid=6743360 }}</ref> there are obvious signs of glacial flows both into and out of the basin.<ref name="Pluto updates">{{cite web| url = http://www.planetary.org/blogs/emily-lakdawalla/2015/12211538-pluto-updates-from-agu.html| title = Pluto updates from AGU and DPS: Pretty pictures from a confusing world| last = Lakdawalla| first = Emily| author-link = Emily Lakdawalla| date = December 21, 2015| publisher = [[The Planetary Society]]| access-date = January 24, 2016| archive-date = December 24, 2015| archive-url = https://web.archive.org/web/20151224193036/http://www.planetary.org/blogs/emily-lakdawalla/2015/12211538-pluto-updates-from-agu.html| url-status = live}}</ref><ref name="Umurhan2016-01-08">{{cite web
| url = https://blogs.nasa.gov/pluto/2016/01/08/probing-the-mysterious-glacial-flow-on-plutos-frozen-heart/
| title = Probing the Mysterious Glacial Flow on Pluto's Frozen 'Heart'
| last = Umurhan
| first = O.
| date = January 8, 2016
| website = blogs.nasa.gov
| publisher = NASA
| access-date = January 24, 2016
| archive-date = April 19, 2016
| archive-url = https://web.archive.org/web/20160419182828/https://blogs.nasa.gov/pluto/2016/01/08/probing-the-mysterious-glacial-flow-on-plutos-frozen-heart/
| url-status = live
}}</ref> It has no craters that were visible to ''New Horizons'', indicating that its surface is less than 10&nbsp;million years old.<ref name="Marchis2016">{{cite journal|last1=Marchis|first1=F.|last2=Trilling|first2=D. E.|title=The Surface Age of Sputnik Planum, Pluto, Must Be Less than 10 Million Years|journal=PLOS ONE|volume= 11|issue=1|date= January 20, 2016|pages= e0147386|doi= 10.1371/journal.pone.0147386|arxiv = 1601.02833 |bibcode = 2016PLoSO..1147386T|pmid=26790001|pmc=4720356|doi-access=free}}</ref> Latest studies have shown that the surface has an age of {{val|180000|-90000|+40000}} years.<ref name="LPSC2017Buhler">{{cite conference|last1=Buhler|first1=P. B.|last2=Ingersoll|first2=A. P.|url=https://www.hou.usra.edu/meetings/lpsc2017/pdf/1746.pdf|title=Sublimation pit distribution indicates convection cell surface velocity of ~10 centimeters per year in Sputnik Planitia, Pluto|book-title=48th Lunar and Planetary Science Conference|date=March 23, 2017|access-date=May 11, 2017|archive-date=August 13, 2017|archive-url=https://web.archive.org/web/20170813010426/https://www.hou.usra.edu/meetings/lpsc2017/pdf/1746.pdf|url-status=live}}</ref>
The New Horizons science team summarized initial findings as "Pluto displays a surprisingly wide variety of geological landforms, including those resulting from [[glaciological]] and surface–atmosphere interactions as well as impact, [[plate tectonics|tectonic]], possible [[cryovolcanic]], and [[mass wasting|mass-wasting]] processes."<ref name="Stern2015" />

In Western parts of Sputnik Planitia there are fields of [[transverse dunes]] formed by the winds blowing from the center of Sputnik Planitia in the direction of surrounding mountains. The dune wavelengths are in the range of 0.4–1&nbsp;km and likely consist of methane particles 200–300&nbsp;μm in size.<ref name="Brown2018">{{cite journal|doi=10.1126/science.aao2975|pmid=29853681|title=Dunes on Pluto|journal=Science|volume=360|issue=6392|pages=992–997|year=2018|last1=Telfer|first1=Matt W|last2=Parteli|first2=Eric J. R|last3=Radebaugh|first3=Jani|last4=Beyer|first4=Ross A|last5=Bertrand|first5=Tanguy|last6=Forget|first6=François|last7=Nimmo|first7=Francis|last8=Grundy|first8=Will M|last9=Moore|first9=Jeffrey M|last10=Stern|first10=S. Alan|last11=Spencer|first11=John|last12=Lauer|first12=Tod R|last13=Earle|first13=Alissa M|last14=Binzel|first14=Richard P|last15=Weaver|first15=Hal A|last16=Olkin|first16=Cathy B|last17=Young|first17=Leslie A|last18=Ennico|first18=Kimberly|last19=Runyon|first19=Kirby|bibcode=2018Sci...360..992T|s2cid=44159592|url=https://pearl.plymouth.ac.uk/bitstream/handle/10026.1/11613/UoP_Deposit_Agreement%20v1.1%2020160217.pdf?sequence=2&isAllowed=y|doi-access=free|access-date=April 12, 2020|archive-date=October 23, 2020|archive-url=https://web.archive.org/web/20201023114119/https://pearl.plymouth.ac.uk/bitstream/handle/10026.1/11613/UoP_Deposit_Agreement%20v1.1%2020160217.pdf?sequence=2&isAllowed=y|url-status=live}}</ref>

<gallery mode=packed heights=160>
File:Pluto-01 Stern 03 Pluto Color TXT.jpg|Multispectral Visual Imaging Camera image of Pluto in enhanced color to bring out differences in surface composition.
File:Pluto_Charon_crater_map_Robbins_Dones_2023.jpg|Distribution of numerous impact craters and basins on both Pluto and Charon. The variation in density (with none found in [[Sputnik Planitia]]) indicates a long history of varying geological activity. Precisely for this reason, the confidence of numerous craters on Pluto remain uncertain.<ref name="Robbins2023"/> The lack of craters on the left and right of each map is due to low-resolution coverage of those anti-encounter regions.
File:Pluto's Sputnik Planum geologic map (cropped).jpg|Geologic map of Sputnik Planitia and surroundings ([[:File:Pluto's Sputnik Planum geologic map - context.jpg|context]]), with [[convection cell]] margins outlined in black
NH-Pluto-WaterIceDetected-BlueRegions-Released-20151008.jpg|Regions where water ice has been detected (blue regions)
</gallery>

=== Internal structure ===
{{redirect|Life on Pluto|fiction about aliens from Pluto|Life on Pluto in fiction}}
[[File:Pluto's internal structure2.jpg|thumb|Model of the internal structure of Pluto<ref name="Hussmann2006" />{{Bulleted list|Water ice crust|Liquid water ocean|Silicate core}}]]
Pluto's density is {{val|1.853|0.004|u=g/cm3}}.<ref name="Brozovic2024"/> Because the decay of radioactive elements would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense [[Core (geology)|core]] surrounded by a [[mantle (geology)|mantle]] of water ice. The pre–''New Horizons'' estimate for the diameter of the core is {{val|1,700|u=km}}, 70% of Pluto's diameter.<ref name="Hussmann2006" /> 
It is possible that such heating continues, creating a [[subsurface ocean]] of liquid water {{nowrap|100 to 180 km}} thick at the core–mantle boundary.<ref name="Hussmann2006" /><ref name="pluto.jhuapl Inside Story" /><ref name="Sci Am 2017">[https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/ Overlooked Ocean Worlds Fill the Outer Solar System] {{Webarchive|url=https://web.archive.org/web/20181226133924/https://www.scientificamerican.com/article/overlooked-ocean-worlds-fill-the-outer-solar-system/ |date=December 26, 2018 }}.  John Wenz, ''Scientific American''. October 4, 2017.</ref> In September 2016, scientists at [[Brown University]] simulated the impact thought to have formed [[Sputnik Planitia]], and showed that it might have been the result of liquid water upweling from below after the collision, implying the existence of a subsurface ocean at least 100&nbsp;km deep.<ref>{{cite magazine|title=An Incredibly Deep Ocean Could Be Hiding Beneath Pluto's Icy Heart|author=Samantha Cole|url=http://www.popsci.com/an-incredibly-deep-ocean-could-be-hiding-beneath-plutos-icy-heart|magazine=Popular Science|access-date=September 24, 2016|archive-date=September 27, 2016|archive-url=https://web.archive.org/web/20160927112125/http://www.popsci.com/an-incredibly-deep-ocean-could-be-hiding-beneath-plutos-icy-heart|url-status=live}}</ref> In June 2020, astronomers reported evidence that Pluto may have had a [[Extraterrestrial liquid water|subsurface ocean]], and consequently may have been [[Planetary habitability|habitable]], when it was first formed.<ref name="INV-20200622">{{cite news |last=Rabie |first=Passant |title=New Evidence Suggests Something Strange and Surprising about Pluto - The findings will make scientists rethink the habitability of Kuiper Belt objects. |url=https://www.inverse.com/science/pluto-hot-star |date=June 22, 2020 |work=[[Inverse (website)|Inverse]] |access-date=June 23, 2020 |archive-date=June 23, 2020 |archive-url=https://web.archive.org/web/20200623071829/https://www.inverse.com/science/pluto-hot-star |url-status=live }}</ref><ref name="NGS-20200622">{{cite journal |author=Bierson, Carver |display-authors=et al. |title=Evidence for a hot start and early ocean formation on Pluto |url=https://www.nature.com/articles/s41561-020-0595-0 |date=June 22, 2020 |journal=[[Nature Geoscience]] |volume=769 |issue=7 |pages=468–472 |doi=10.1038/s41561-020-0595-0 |bibcode=2020NatGe..13..468B |s2cid=219976751 |access-date=June 23, 2020 |url-access=subscription |archive-date=June 22, 2020 |archive-url=https://web.archive.org/web/20200622201613/https://www.nature.com/articles/s41561-020-0595-0 |url-status=live }}</ref> In March 2022, a team of researchers proposed that the mountains [[Wright Mons]] and [[Piccard Mons]] are actually a merger of many smaller cryovolcanic domes, suggesting a source of heat on the body at levels previously thought not possible.<ref>{{cite journal |title=Large-scale cryovolcanic resurfacing on Pluto |first=Kelsi N. |last=Singer |journal=[[Nature Communications]] |date=March 29, 2022 |volume=13 |issue=1 |page=1542 |doi=10.1038/s41467-022-29056-3 |pmid=35351895 |pmc=8964750 |arxiv=2207.06557 |bibcode=2022NatCo..13.1542S }}</ref>

== Mass and size ==
[[File:Pluto, Earth & Moon size comparison.jpg|thumb|right|Pluto (bottom left) compared in size to the Earth and the Moon]]Pluto's diameter is {{val|2376.6|3.2|u=km}}<ref name="Nimmo2017" /> and its mass is {{val|1.303|0.003|e=22|u=kg}}, 17.7% that of the [[Moon]] (0.22% that of Earth).<ref name="Davies2001" /> Its [[surface area]] is {{val|1.774443|e=7|u=km2}}, or just slightly bigger than Russia or [[Antarctica]] (particularly including the [[Antarctic sea ice]] during winter). Its [[surface gravity]] is 0.063 ''g'' (compared to 1 ''g'' for Earth and 0.17 ''g'' for the Moon).<ref name="Pluto Fact Sheet"/> This gives Pluto an escape velocity of 4,363.2&nbsp;km per hour / 2,711.167 miles per hour (as compared to Earth's 40,270&nbsp;km per hour / 25,020 miles per hour). Pluto is more than twice the diameter and a dozen times the mass of [[Ceres (dwarf planet)|Ceres]], the largest object in the [[asteroid belt]]. It is less massive than the dwarf planet [[Eris (dwarf planet)|Eris]], a [[trans-Neptunian object]] discovered in 2005, though Pluto has a larger diameter of 2,376.6&nbsp;km<ref name="Nimmo2017" /> compared to Eris's approximate diameter of 2,326&nbsp;km.<ref name="NewHorizons_PlutoSize">{{cite web |url=http://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate |title=How Big Is Pluto? New Horizons Settles Decades-Long Debate |publisher=NASA |date=July 13, 2015 |access-date=July 13, 2015 |archive-date=July 1, 2017 |archive-url=https://web.archive.org/web/20170701005734/https://www.nasa.gov/feature/how-big-is-pluto-new-horizons-settles-decades-long-debate/ |url-status=dead }}</ref>

With less than 0.2 lunar masses, Pluto is much less massive than the [[terrestrial planet]]s, and also less massive than seven [[moons]]: [[Ganymede (moon)|Ganymede]], [[Titan (moon)|Titan]], [[Callisto (moon)|Callisto]], [[Io (moon)|Io]], the [[Moon]], [[Europa (moon)|Europa]], and [[Triton (moon)|Triton]]. The mass is much less than thought before Charon was discovered.<ref>{{cite web |title=Pluto and Charon {{!}} Astronomy |url=https://courses.lumenlearning.com/astronomy/chapter/pluto-and-charon/ |website=courses.lumenlearning.com |access-date=April 6, 2022 |quote=For a long time, it was thought that the mass of Pluto was similar to that of Earth, so that it was classed as a fifth terrestrial planet, somehow misplaced in the far outer reaches of the solar system. There were other anomalies, however, as Pluto's orbit was more eccentric and inclined to the plane of our solar system than that of any other planet. Only after the discovery of its moon Charon in 1978 could the mass of Pluto be measured, and it turned out to be far less than the mass of Earth. |archive-date=March 24, 2022 |archive-url=https://web.archive.org/web/20220324145506/https://courses.lumenlearning.com/astronomy/chapter/pluto-and-charon/ |url-status=live }}</ref>

The discovery of Pluto's satellite [[Charon (moon)|Charon]] in 1978 enabled a determination of the mass of the Pluto–Charon system by application of [[Kepler's laws of planetary motion#Third law|Newton's formulation of Kepler's third law]]. Observations of Pluto in [[occultation]] with Charon allowed scientists to establish Pluto's diameter more accurately, whereas the invention of [[adaptive optics]] allowed them to determine its shape more accurately.<ref name="Close_2000" />

Determinations of Pluto's size have been complicated by its atmosphere<ref name="Young2007" /> and [[hydrocarbon]] haze.<ref name="Plutosize" /> In March 2014, Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with a Plutonian diameter greater than 2,360&nbsp;km, with a "best guess" of 2,368&nbsp;km.<ref name=Lellouch_2015 /> On July 13, 2015, images from NASA's ''New Horizons'' mission [[Long Range Reconnaissance Imager]] (LORRI), along with data from the other instruments, determined Pluto's diameter to be {{convert|2370|km|abbr=on|sigfig=4}},<ref name = NewHorizons_PlutoSize /><ref name="emily">{{cite web |last1=Lakdawalla |first1=Emily |title=Pluto minus one day: Very first New Horizons Pluto encounter science results |url=http://www.planetary.org/blogs/emily-lakdawalla/2015/07131311-pluto-first-science.html |publisher=[[The Planetary Society]] |date=July 13, 2015 |access-date=July 13, 2015 |archive-date=March 2, 2020 |archive-url=https://web.archive.org/web/20200302200913/https://www.planetary.org/blogs/emily-lakdawalla/2015/07131311-pluto-first-science.html |url-status=live }}</ref> which was later revised to be {{convert|2372|km|abbr=on}} on July 24,<ref name="NHPC_20150724">{{cite AV media |url=https://www.youtube.com/watch?v=dWr29KIs2Ns |title=NASA's New Horizons Team Reveals New Scientific Findings on Pluto |date=July 24, 2015 |publisher=NASA |time=52:30 |access-date=July 30, 2015 |archive-url=https://ghostarchive.org/varchive/youtube/20211028/dWr29KIs2Ns |archive-date=October 28, 2021 |quote=We had an uncertainty that ranged over maybe 70 kilometers, we've collapsed that to plus and minus two, and it's centered around 1186}}{{cbignore}}</ref> and later to {{val|2374|8|u=km}}.<ref name="Stern2015" /> Using [[radio occultation]] data from the ''New Horizons'' Radio Science Experiment (REX), the diameter was found to be {{val|2376.6|3.2|u=km}}.<ref name="Nimmo2017" />

{{image frame
|content={{Graph:Chart
   | width=400
   | height=200
   | type=rect
   | x=Triton,Eris,Pluto,Haumea,Titania,Makemake,Oberon,Rhea,Iapetus,Gonggong,Charon,Quaoar,Ceres,Orcus
   | y1=21.39,16.6,13.03,4.01,3.40,3.1,3.08,2.307,1.806,1.75,1.59,1.4,0.94,0.61
   | showValues=format:.1f, offset:1
   | xAxisAngle=45
}}
|width=440
|caption=The masses of Pluto and Charon compared to other dwarf planets ({{dp|Eris}}, {{dp|Haumea}}, {{dp|Makemake}}, {{dp|Gonggong}}, {{dp|Quaoar}}, {{dp|Orcus}}, {{dp|Ceres}}) and to the icy moons Triton (Neptune I), Titania (Uranus III), Oberon (Uranus IV), Rhea (Saturn V) and Iapetus (Saturn VIII). The unit of mass is {{e|21}} kg.
|border=no
}}{{Clear}}

== Atmosphere ==
{{Main|Atmosphere of Pluto}}
[[File:PIA21590 – Blue Rays, New Horizons' High-Res Farewell to Pluto.jpg|thumb|upright=1.28|A near-true-color image taken by ''New Horizons'' after its flyby. Numerous layers of blue haze float in Pluto's atmosphere. Along and near the limb, mountains and their shadows are visible.]]Pluto has a tenuous [[atmosphere]] consisting of [[nitrogen]] (N<sub>2</sub>), [[methane]] (CH<sub>4</sub>), and carbon monoxide (CO), which are in [[equilibrium vapor pressure|equilibrium with their ices]] on Pluto's surface.<ref name="NYT-20150724-ap">{{cite news |title=Conditions on Pluto: Incredibly Hazy With Flowing Ice |url=https://www.nytimes.com/aponline/2015/07/24/science/ap-us-sci-pluto.html |date=July 24, 2015 |work=[[New York Times]] |access-date=July 24, 2015 |archive-date=July 28, 2015 |archive-url=https://web.archive.org/web/20150728081402/http://www.nytimes.com/aponline/2015/07/24/science/ap-us-sci-pluto.html |url-status=live }}</ref><ref name="Croswell1992" /> According to the measurements by ''New Horizons'', the surface pressure is about 1&nbsp;[[Pascal (unit)|Pa]] (10&nbsp;[[μbar]]),<ref name=Stern2015 /> roughly one million to 100,000 times less than Earth's atmospheric pressure. It was initially thought that, as Pluto moves away from the Sun, its atmosphere should gradually freeze onto the surface; studies of ''New Horizons'' data and ground-based occultations show that Pluto's atmospheric density increases, and that it likely remains gaseous throughout Pluto's orbit.<ref name=Olkin_2015 /><ref name="skyandtel">{{cite news|title=Pluto's Atmosphere Confounds Researchers|author=Kelly Beatty|newspaper=Sky & Telescope|year=2016|url=http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/|access-date=April 2, 2016|archive-date=April 7, 2016|archive-url=https://web.archive.org/web/20160407162627/http://www.skyandtelescope.com/astronomy-news/plutos-atmosphere-confounds-researchers-032520166/|url-status=live}}</ref> ''New Horizons'' observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected.<ref name=skyandtel /> Alan Stern has contended that even a small increase in Pluto's surface temperature can lead to exponential increases in Pluto's atmospheric density; from 18&nbsp;hPa to as much as 280&nbsp;hPa (three times that of Mars to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid.<ref name=skyandtel /> Just like sweat cools the body as it evaporates from the skin, the [[sublimation (phase transition)|sublimation]] of Pluto's atmosphere cools its surface.<ref name="KerThan2006-CNN" /> Pluto has no or almost no [[troposphere]]; observations by ''New Horizons'' suggest only a thin tropospheric [[planetary boundary layer|boundary layer]]. Its thickness in the place of measurement was 4&nbsp;km, and the temperature was 37±3 K. The layer is not continuous.<ref name=Gladstone_2016>{{cite journal |title=The atmosphere of Pluto as observed by New Horizons |last1=Gladstone |first1=G. R. |last2=Stern |first2=S. A. |last3=Ennico |first3=K. |display-authors=etal |date=March 2016 |journal=Science |volume=351 |issue=6279 |doi=10.1126/science.aad8866 |bibcode=2016Sci...351.8866G |arxiv=1604.05356 |url=https://www.astro.umd.edu/~dphamil/research/reprints/GlaSteEnn16.pdf |url-status=dead |archive-url=https://web.archive.org/web/20160521090831/https://www.astro.umd.edu/~dphamil/research/reprints/GlaSteEnn16.pdf |archive-date=May 21, 2016 |pages=aad8866 |pmid=26989258 |s2cid=32043359 |access-date=June 12, 2016}} ([https://web.archive.org/web/20160521191420/http://authors.library.caltech.edu/65692/2/Gladstone-SM.pdf Supplementary Material])</ref>

In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016.<ref>{{cite web|title=What is happening to Pluto's Atmosphere|url=https://astronomy.com/news/2020/05/plutos-strange-atmosphere-just-collapsed|date=May 22, 2020|access-date=October 7, 2021|archive-date=October 24, 2021|archive-url=https://web.archive.org/web/20211024063219/https://astronomy.com/news/2020/05/plutos-strange-atmosphere-just-collapsed|url-status=live}}</ref> In 2021, astronomers at the [[Southwest Research Institute]] confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere.<ref>{{cite web|title=SwRI Scientists Confirm Decrease In Pluto's Atmospheric Density|url=https://www.swri.org/press-release/scientists-confirm-decrease-plutos-atmospheric-density|date=October 4, 2021|work=Southwest Research Institute|access-date=October 7, 2021|archive-date=October 15, 2021|archive-url=https://web.archive.org/web/20211015201003/https://www.swri.org/press-release/scientists-confirm-decrease-plutos-atmospheric-density|url-status=live}}</ref>

The presence of methane, a powerful [[greenhouse gas]], in Pluto's atmosphere creates a [[temperature inversion]], with the average temperature of its atmosphere tens of degrees warmer than its surface,<ref name=Lellouch_2009 /> though observations by ''New Horizons'' have revealed Pluto's upper atmosphere to be far colder than expected (70&nbsp;K, as opposed to about 100&nbsp;K).<ref name=skyandtel /> Pluto's atmosphere is divided into roughly 20 regularly spaced haze layers up to 150&nbsp;km high,<ref name=Stern2015 /> thought to be the result of pressure waves created by airflow across Pluto's mountains.<ref name=skyandtel />

== Natural satellites ==
{{main|Moons of Pluto}}
[[File:Pluto-Charon system-new.gif|alt=|thumb|upright=1.28|An oblique view of the Pluto–Charon system, showing that Pluto orbits a point outside itself. The two bodies are mutually [[tidally locked]].]]
[[File:Nh-pluto moons family portrait.png|thumb|Five known moons of Pluto to scale]]
Pluto has five known [[natural satellite]]s. The largest and closest to Pluto is [[Charon (moon)|Charon]]. First identified in 1978 by astronomer [[James Christy]], Charon is the only moon of Pluto that may be in [[hydrostatic equilibrium]]. Charon's mass is sufficient to cause the barycenter of the Pluto–Charon system to be outside Pluto. Beyond Charon there are four much smaller [[circumbinary]] moons. In order of distance from Pluto they are Styx, Nix, Kerberos, and Hydra. [[Nix (moon)|Nix]] and [[Hydra (moon)|Hydra]] were both discovered in 2005,<ref name="Gugliotta2005" /> [[Kerberos (moon)|Kerberos]] was discovered in 2011,<ref name="P4" /> and [[Styx (moon)|Styx]] was discovered in 2012.<ref>{{cite news |url=http://www.space.com/16531-pluto-fifth-moon-hubble-discovery.html |title=Pluto Has a Fifth Moon, Hubble Telescope Reveals |last=Wall |first=Mike |date=July 11, 2012 |work=Space.com |access-date=July 11, 2012 |archive-date=May 14, 2020 |archive-url=https://web.archive.org/web/20200514184955/https://www.space.com/16531-pluto-fifth-moon-hubble-discovery.html |url-status=live }}</ref> The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°),<ref name="Buie2012">{{cite journal |journal=The Astronomical Journal |last1=Buie |first1=M. |last2=Tholen |first2=D. |last3=Grundy |first3=W. |title=The Orbit of Charon is Circular |year=2012 |volume=144 |issue=1 |pages=15 |doi=10.1088/0004-6256/144/1/15 |bibcode=2012AJ....144...15B|s2cid=15009477 |url=http://pdfs.semanticscholar.org/bfb8/1eb1887c28df5f5348a491cff7d4870e8c77.pdf |archive-url=https://web.archive.org/web/20200412141438/http://pdfs.semanticscholar.org/bfb8/1eb1887c28df5f5348a491cff7d4870e8c77.pdf |url-status=dead |archive-date=April 12, 2020}}</ref><ref name="ShowalterHamilton2015" /> and therefore tilted approximately 120° relative to Pluto's orbit. The Plutonian system is highly compact: the five known satellites orbit within the inner 3% of the region where [[prograde orbit]]s would be stable.<ref name="Sternetal 2005" />

The orbital periods of all Pluto's moons are linked in a system of [[orbital resonance]]s and [[Orbital resonance#Coincidental 'near' ratios of mean motion|near-resonances]].<ref name="ShowalterHamilton2015">{{cite journal |last1=Showalter |first1=M.R. |author1-link=Mark R. Showalter |last2=Hamilton |first2=D.P. |title=Resonant interactions and chaotic rotation of Pluto's small moons |journal=Nature |volume=522 |issue=7554 |date=June 3, 2015 |pages=45–49 |doi=10.1038/nature14469 |bibcode=2015Natur.522...45S |pmid=26040889 |s2cid=205243819}}</ref><ref name="Witze2015">{{cite journal |last=Witze |first=Alexandra |title=Pluto's moons move in synchrony |journal=Nature |year=2015 |doi=10.1038/nature.2015.17681 |s2cid=134519717}}</ref> When [[Apsidal precession|precession]] is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio.<ref name="ShowalterHamilton2015" /> There is a sequence of approximate ratios, 3:4:5:6, between the periods of Styx, Nix, Kerberos, and Hydra with that of Charon; the ratios become closer to being exact the further out the moons are.<ref name="ShowalterHamilton2015" /><ref name="Matson">{{cite web |last=Matson |first=J. |date=July 11, 2012 |title=New Moon for Pluto: Hubble Telescope Spots a 5th Plutonian Satellite |work=[[Scientific American]] web site |url=http://www.scientificamerican.com/article.cfm?id=pluto-moon-p5 |access-date=July 12, 2012 |archive-date=August 31, 2016 |archive-url=https://web.archive.org/web/20160831015135/http://www.scientificamerican.com/article.cfm?id=pluto-moon-p5 |url-status=live }}</ref>

The Pluto–Charon system is one of the few in the Solar System whose barycenter lies outside the primary body; the [[617 Patroclus|Patroclus–Menoetius]] system is a smaller example, and the [[Jupiter#Size and mass|Sun–Jupiter]] system is the only larger one.<ref name="RichardsonWalsh2005" /> The similarity in size of Charon and Pluto has prompted some astronomers to call it a [[Double planet|double dwarf planet]].<ref name="Sicardyetal2006nature" /> The system is also unusual among planetary systems in that each is [[tidally locked]] to the other, which means that Pluto and Charon always have the same hemisphere facing each other — a property shared by only one other known system, [[Eris (dwarf planet)|Eris]] and [[Dysnomia (moon)|Dysnomia]].<ref name="Szakats2022">{{cite journal |display-authors=etal |last1=Szakáts |first1=R. |last2=Kiss |first2=Cs. |last3=Ortiz |first3=J.L. |last4=Morales |first4=N. |last5=Pál |first5=A. |last6=Müller |first6=T.G. |title=Tidally locked rotation of the dwarf planet (136199) Eris discovered from long-term ground based and space photometry |journal=Astronomy & Astrophysics |volume=L3 |page=669 |year=2023 |arxiv=2211.07987 |bibcode=2023A&A...669L...3S |s2cid=253522934 |doi=10.1051/0004-6361/202245234}}</ref> From any position on either body, the other is always at the same position in the sky, or always obscured.<ref name="Young1997" /> This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its barycenter.<ref name="axis" />

Pluto's moons are hypothesized to have been formed by a collision between Pluto and a similar-sized body, early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.<ref name="nasa.gov">{{cite web |title=NASA's Hubble Finds Pluto's Moons Tumbling in Absolute Chaos |url=http://www.nasa.gov/press-release/nasa-s-hubble-finds-pluto-s-moons-tumbling-in-absolute-chaos |date=June 3, 2015 |access-date=June 3, 2015 |archive-date=April 6, 2020 |archive-url=https://web.archive.org/web/20200406161853/https://www.nasa.gov/press-release/nasa-s-hubble-finds-pluto-s-moons-tumbling-in-absolute-chaos |url-status=live }}</ref><!--Kerberos has a much lower albedo than the other moons of Pluto,<ref name="spaceweirdmoons">{{cite web |title=Pluto's moons are even weirder than thought |url=http://www.space.com/29559-pluto-moons-weird-orbit-chaos.html |access-date=June 20, 2015}}</ref> which is difficult to explain with a giant collision.<ref name="nationalgeorandombeat">{{cite web |title=Pluto's moons dance to a random beat |url=http://news.nationalgeographic.com/2015/06/150603-pluto-moons-charon-styx-nix-kerberos-hydra-new-horizons/ |archive-url=https://web.archive.org/web/20150603215824/http://news.nationalgeographic.com/2015/06/150603-pluto-moons-charon-styx-nix-kerberos-hydra-new-horizons/ |url-status=dead |archive-date=June 3, 2015 |access-date=June 20, 2015}}</ref>-->{{clear}}

=== Quasi-satellite ===
In 2012, it was calculated that [[15810 Arawn]] could be a [[quasi-satellite]] of Pluto, a specific type of co-orbital configuration.<ref name="quasi" /> According to the calculations, the object would be a quasi-satellite of Pluto for about 350,000 years out of every two-million-year period.<ref name="quasi" /><ref name="S&T" /> Measurements made by the ''New Horizons'' spacecraft in 2015 made it possible to calculate the orbit of Arawn more accurately,<ref name="2016maynasa">{{cite web|title=New Horizons Collects First Science on a Post-Pluto Object|url=http://www.nasa.gov/feature/new-horizons-collects-first-science-on-a-post-pluto-object|publisher=NASA|date=May 13, 2016|access-date=June 5, 2016|archive-date=June 7, 2016|archive-url=https://web.archive.org/web/20160607150433/http://www.nasa.gov/feature/new-horizons-collects-first-science-on-a-post-pluto-object/|url-status=dead}}</ref> and confirmed the earlier ones.<ref name="analemma">{{cite journal |title=The analemma criterion: accidental quasi-satellites are indeed true quasi-satellites |first1=Carlos |last1=de la Fuente Marcos |last2=de la Fuente Marcos |first2=Raúl |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=2016 |volume=462 |issue=3 |pages=3344–3349 |arxiv=1607.06686 |doi=10.1093/mnras/stw1833 |doi-access=free |bibcode=2016MNRAS.462.3344D|s2cid=119284843 }}</ref> However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on its orbital dynamics, since its orbit is primarily controlled by Neptune with only occasional perturbations by Pluto.<ref name="porter_et_al_2016">{{cite journal |title=The First High-phase Observations of a KBO: New Horizons Imaging of (15810) 1994 JR1 from the Kuiper Belt |first=Simon B. |last=Porter |display-authors=etal |journal=[[The Astrophysical Journal Letters]] |volume=828 |issue=2 |pages=L15 |date=2016 |arxiv=1605.05376 |bibcode=2016ApJ...828L..15P |doi=10.3847/2041-8205/828/2/L15|s2cid=54507506 |doi-access=free }}</ref><ref name="2016maynasa" /><ref name=analemma />

== Origin <span class="anchor" id="Origins"></span> ==
{{Further|Kuiper belt|Nice model}}
[[File:Outersolarsystem objectpositions labels comp.png|thumb|Plot of the known Kuiper belt objects, set against the four [[giant planet]]s]]

Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune<ref>{{Cite book|last=Kuiper|first=Gerard|title=Planets and Satellites|publisher=University of Chicago Press|year=1961|location=Chicago|pages=576}}</ref> knocked out of orbit by Neptune's largest moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.<ref>{{cite book|title=Pluto and Charon|author1-link=S. Alan Stern|first1=S. Alan|last1=Stern|first2=David J.|last2=Tholen|publisher=[[University of Arizona Press]]|year=1997|page=623|url=https://books.google.com/books?id=VcY7iYJwJZoC&pg=PA623|isbn=978-0-8165-1840-1|access-date=October 23, 2015|archive-date=February 26, 2024|archive-url=https://web.archive.org/web/20240226151142/https://books.google.com/books?id=VcY7iYJwJZoC&pg=PA623#v=onepage&q&f=false|url-status=live}}</ref>

Pluto's true place in the [[Solar System]] began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is thought to be the source of many [[short-period comet]]s. Pluto is the largest member of the [[Kuiper belt]],{{efn|name = wiki-kbo}} a stable belt of objects located between 30 and 50 AU from the Sun. As of 2011, surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto-sized objects are expected to be beyond 100&nbsp;AU from the Sun.<ref name="Sheppard2011" /> Like other Kuiper-belt objects (KBOs), Pluto shares features with [[comet]]s; for example, the [[solar wind]] is gradually blowing Pluto's surface into space.<ref name="pluto.jhuapl cousin" /> It has been claimed that if Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.<ref name="Tyson1999" /> This claim has been disputed with the argument that Pluto's escape velocity is too high for this to happen.<ref>{{cite web |date=April 13, 2015 |url=http://www.philipmetzger.com/blog/nine-reasons-why-pluto-is-a-planet/ |title=Nine Reasons Why Pluto Is a Planet |archive-url=https://web.archive.org/web/20150415183323/http://www.philipmetzger.com/blog/nine-reasons-why-pluto-is-a-planet/ |archive-date=April 15, 2015 |url-status=live |author=Philip Metzger |author-link=Philip T. Metzger |website=Philip Metzger}}</ref> It has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper-belt objects.<ref name="SP-20180524">{{cite web |last=Wall |first=Mike |title=Pluto May Have Formed from 1 Billion Comets |url=https://www.space.com/40687-pluto-formation-1-billion-comets.html |date=May 24, 2018 |work=[[Space.com]] |access-date=May 24, 2018 |archive-date=May 24, 2018 |archive-url=https://web.archive.org/web/20180524135822/https://www.space.com/40687-pluto-formation-1-billion-comets.html |url-status=live }}</ref><ref name="ARX-20180524">{{Cite journal|last1=Glein |first1=Christopher R. |last2=Waite |first2=J. Hunter Jr. |title=Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto |journal=Icarus |volume=313 |issue=2018 |pages=79–92 |date=May 24, 2018 |arxiv=1805.09285|doi=10.1016/j.icarus.2018.05.007 |bibcode=2018Icar..313...79G |s2cid=102343522 }}</ref>

Though Pluto is the largest Kuiper belt object discovered,<ref name="Plutosize" /> Neptune's moon [[Triton (moon)|Triton]], which is larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object.<ref name="PlanetaryOrg Triton" /> [[Eris (dwarf planet)|Eris]] ([[#Classification|see above]]) is about the same size as Pluto (though more massive) but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the [[scattered disc]].<ref name="GallardoBrunini">
{{cite journal
| title = On the origin of the High-Perihelion Scattered Disk: the role of the Kozai mechanism and mean motion resonances
|author1=Gomes R. S. |author2=Gallardo T. |author3=Fernández J. A. |author4=Brunini A. |s2cid=18066500 | journal = Celestial Mechanics and Dynamical Astronomy
| date = 2005
| volume = 91
| issue = 1–2
| pages = 109–129
| doi = 10.1007/s10569-004-4623-y
| bibcode = 2005CeMDA..91..109G
|hdl=11336/38379 | hdl-access = free
}}
</ref>

Like other members of the Kuiper belt, Pluto is thought to be a residual [[planetesimal]]; a component of the original [[protoplanetary disc]] around the [[Sun]] that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its position to a [[planetary migration|sudden migration]] undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself (Triton), locking others into resonances, and knocking others into chaotic orbits. The objects in the [[scattered disc]], a dynamically unstable region overlapping the Kuiper belt, are thought to have been placed in their positions by interactions with Neptune's migrating resonances.<ref name="Hahn2005" /> A computer model created in 2004 by [[Alessandro Morbidelli (astronomer)|Alessandro Morbidelli]] of the [[Observatoire de la Côte d'Azur]] in [[Nice]] suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the [[Late Heavy Bombardment]] 600&nbsp;million years after the Solar System's formation and the origin of the [[Jupiter trojan]]s.<ref name="Levison2007" /> It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration [[Perturbation (astronomy)|perturbed]] it into a resonant capture.<ref name="Malhotra1995" /> The [[Nice model]] requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk, which included Triton and Eris.<ref name="Levison2007" />

== Observation and exploration ==
=== Observation ===
[[File:Pluto animiert 200px.gif|thumb|upright=1|Computer-generated rotating image of Pluto based on observations by the [[Hubble Space Telescope]] in 2002–2003]]
Pluto's distance from Earth makes its in-depth study and [[Space exploration|exploration]] difficult. Pluto's visual [[apparent magnitude]] averages 15.1, brightening to 13.65 at perihelion.<ref name="Pluto Fact Sheet" /> To see it, a telescope is required; around 30&nbsp;cm (12&nbsp;in) aperture being desirable.<ref name="SSC2002" /> It looks star-like and without a visible disk even in large telescopes,<ref>{{cite web |title=How to Scope Out Pluto in the Night Sky Friday |url=https://www.space.com/26426-pluto-telescope-skywatching-friday.html |website=Space.com |access-date=April 6, 2022 |language=en |date=July 3, 2014 |archive-date=April 6, 2022 |archive-url=https://web.archive.org/web/20220406093459/https://www.space.com/26426-pluto-telescope-skywatching-friday.html |url-status=live }}</ref> because its [[angular diameter]] is maximum 0.11".<ref name="Pluto Fact Sheet" />

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.<ref name="YoungBinzelCrane2001" /><ref name="BuieTholenHorne1992" />

Better maps were produced from images taken by the [[Hubble Space Telescope]] (HST), which offered higher [[angular resolution|resolution]], and showed considerably more detail,<ref name="Buie_web_map" /> resolving variations several hundred kilometers across, including polar regions and large bright spots.<ref name="Buie_2010 surface-maps" /> These maps were produced by complex computer processing, which finds the best-fit projected maps for the few pixels of the Hubble images.<ref name="Buie_mapmaking" /> These remained the most detailed maps of Pluto until the flyby of ''New Horizons'' in July 2015, because the two cameras on the HST used for these maps were no longer in service.<ref name="Buie_mapmaking" />

=== Exploration ===
{{Main|Exploration of Pluto|New Horizons|}}
[[File:Barycentric view of Pluto and Charon 29 May-3 June by Ralph in near-true colours.gif|thumb|Pluto and Charon seen orbiting each other by ''New Horizons'']]

The ''New Horizons'' spacecraft, which [[planetary flyby|flew by]] Pluto in July 2015, is the first and so far only attempt to explore Pluto directly. Launched in 2006, it captured its first (distant) images of Pluto in late September 2006 during a test of the Long Range Reconnaissance Imager.<ref name="pluto.jhuapl First Pluto Sighting" /> The images, taken from a distance of approximately 4.2&nbsp;billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a [[gravity assist]] from [[Jupiter]].

''New Horizons'' made its closest approach to Pluto on July 14, 2015, after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. Observations were conducted using a [[remote sensing]] package that included [[digital imaging|imaging]] instruments and a radio science investigation tool, as well as [[spectroscopic]] and other experiments. The scientific goals of ''New Horizons'' were to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition, and analyze Pluto's neutral atmosphere and its escape rate. On October 25, 2016, at 05:48 pm ET, the last bit of data (of a total of 50&nbsp;billion bits of data; or 6.25 gigabytes) was received from ''New Horizons'' from its close encounter with Pluto.<ref name="NYT-20161028">{{cite news |last=Chang |first=Kenneth |title=No More Data From Pluto |url=https://www.nytimes.com/2016/10/29/science/pluto-nasa-new-horizons.html |date=October 28, 2016 |work=[[New York Times]] |access-date=October 28, 2016 |archive-date=March 29, 2019 |archive-url=https://web.archive.org/web/20190329002642/https://www.nytimes.com/2016/10/29/science/pluto-nasa-new-horizons.html |url-status=live }}</ref><ref>{{cite web
 | url=http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20161027
 | title=Pluto Exploration Complete: New Horizons Returns Last Bits of 2015 Flyby Data to Earth
 | date=October 27, 2016
| publisher=Johns Hopkins Applied Research Laboratory
 | access-date=October 28, 2016
| archive-date=October 28, 2016
| archive-url=https://web.archive.org/web/20161028100437/http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20161027
 | url-status=live
 }}</ref><ref name="NASA-20150115(b)">{{cite web |last1=Brown |first1=Dwayne |last2=Buckley |first2=Michael |last3=Stothoff |first3=Maria |title=Release 15-011 – NASA's New Horizons Spacecraft Begins First Stages of Pluto Encounter |url=http://www.nasa.gov/press/2015/january/nasa-s-new-horizons-spacecraft-begins-first-stages-of-pluto-encounter |date=January 15, 2015 |work=NASA |access-date=January 15, 2015 |archive-date=April 7, 2020 |archive-url=https://web.archive.org/web/20200407083332/https://www.nasa.gov/press/2015/january/nasa-s-new-horizons-spacecraft-begins-first-stages-of-pluto-encounter |url-status=live }}</ref><ref>{{Cite web|url=http://pluto.jhuapl.edu/Mission/Spacecraft/Data-Collection.php|title=New Horizons|website=pluto.jhuapl.edu|access-date=May 15, 2016|archive-date=October 8, 2016|archive-url=https://web.archive.org/web/20161008104223/http://pluto.jhuapl.edu/Mission/Spacecraft/Data-Collection.php|url-status=live}}</ref>

Since the ''New Horizons'' flyby, scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives.<ref>{{Cite news|url=https://www.theverge.com/2017/4/26/15424770/nasa-spacecraft-new-horizons-flyby-pluto-moons-orbiter-mission|title=Why a group of scientists think we need another mission to Pluto|work=The Verge|access-date=July 14, 2018|archive-date=July 8, 2018|archive-url=https://web.archive.org/web/20180708104340/https://www.theverge.com/2017/4/26/15424770/nasa-spacecraft-new-horizons-flyby-pluto-moons-orbiter-mission|url-status=live}}</ref><ref>{{Cite web|url=https://www.technologyreview.com/2021/07/19/1029680/nasa-pluto-mission-persephone/|title=Why NASA should visit Pluto again|website=MIT Technology Review|access-date=January 18, 2022|archive-date=January 18, 2022|archive-url=https://web.archive.org/web/20220118115746/https://www.technologyreview.com/2021/07/19/1029680/nasa-pluto-mission-persephone/|url-status=live}}</ref><ref>{{Cite web|url = https://www.spaceflightinsider.com/missions/solar-system/videos-simulate-pluto-charon-flyby-follow-up-mission-proposed|title = New videos simulate Pluto and Charon flyby; return mission to Pluto proposed|date = August 2021|access-date = September 4, 2021|archive-date = September 4, 2021|archive-url = https://web.archive.org/web/20210904132010/https://www.spaceflightinsider.com/missions/solar-system/videos-simulate-pluto-charon-flyby-follow-up-mission-proposed/|url-status = dead}}</ref> They include mapping the surface at {{cvt|30|ft|m|sigfig=2|order=flip}} per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, investigations of a possible subsurface ocean, and topographic mapping of Pluto's regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using laser pulses to generate a complete topographic map of Pluto. ''New Horizons'' principal investigator Alan Stern has advocated for a [[Cassini–Huygens|''Cassini'']]-style orbiter that would launch around 2030 (the 100th anniversary of Pluto's discovery) and use Charon's gravity to adjust its orbit as needed to fulfill science objectives after arriving at the Pluto system.<ref>{{Cite news|url=https://www.space.com/36697-pluto-orbiter-mission-after-new-horizons.html|title=Going Back to Pluto? Scientists to Push for Orbiter Mission|work=Space.com|access-date=July 14, 2018|archive-date=July 14, 2018|archive-url=https://web.archive.org/web/20180714110958/https://www.space.com/36697-pluto-orbiter-mission-after-new-horizons.html|url-status=live}}</ref> The orbiter could then use Charon's gravity to leave the Pluto system and study more KBOs after all Pluto science objectives are completed. A conceptual study funded by the NASA Innovative Advanced Concepts ([[NASA Institute for Advanced Concepts|NIAC]]) program describes a fusion-enabled Pluto orbiter and lander based on the [[Princeton field-reversed configuration experiment|Princeton field-reversed configuration reactor]].<ref>{{Cite news|url=https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_Lander|title=Fusion-Enabled Pluto Orbiter and Lander|last=Hall|first=Loura|date=April 5, 2017|work=NASA|access-date=July 14, 2018|language=en|archive-date=April 21, 2017|archive-url=https://web.archive.org/web/20170421033505/https://www.nasa.gov/directorates/spacetech/niac/2017_Phase_I_Phase_II/Fusion_Enabled_Pluto_Orbiter_and_Lander/|url-status=live}}</ref><ref name='PSS'>[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003126.pdf Fusion-Enabled Pluto Orbiter and Lander – Phase I Final Report] {{Webarchive|url=https://web.archive.org/web/20190429071941/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170003126.pdf |date=April 29, 2019 }}. (PDF) Stephanie Thomas, Princeton Satellite Systems. 2017.</ref>

''New Horizons'' imaged all of Pluto's northern hemisphere, and the equatorial regions down to about 30° South. Higher southern latitudes have only been observed, at very low resolution, from Earth.<ref>{{cite web |access-date=August 19, 2021 |title=5 Amazing Things We've Learned a Year After Visiting Pluto |work=[[National Geographic]] |author=Nadia Drake |date=July 14, 2016 |url=https://www.nationalgeographic.com/science/article/pluto-planets-new-horizons-one-year-anniversary-nasa-space-science|archive-url=https://web.archive.org/web/20210307022014/https://www.nationalgeographic.com/science/article/pluto-planets-new-horizons-one-year-anniversary-nasa-space-science|url-status=dead|archive-date=March 7, 2021}}</ref> Images from the Hubble Space Telescope in 1996 cover 85% of Pluto and show large albedo features down to about 75° South.<ref>{{cite web |url=https://hubblesite.org/contents/news-releases/1996/news-1996-09.html |title=HUBBLE REVEALS SURFACE OF PLUTO FOR FIRST TIME |date=March 7, 1996 |work=HubbleSite.org |publisher=Space Telescope Science Institute |access-date=October 18, 2021 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819132618/https://hubblesite.org/contents/news-releases/1996/news-1996-09.html |url-status=live }}</ref><ref>{{cite web |url=https://hubblesite.org/contents/media/images/1996/09/401-Image.html?news=true |title=MAP OF PLUTO'S SURFACE |date=March 7, 1996 |work=HubbleSite.org |publisher=Space Telescope Science Institute |access-date=October 18, 2021 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819132616/https://hubblesite.org/contents/media/images/1996/09/401-Image.html?news=true |url-status=live }}</ref> This is enough to show the extent of the temperate-zone maculae. Later images had slightly better resolution, due to minor improvements in Hubble instrumentation.<ref name="theh_Seei">{{Cite news |title=Seeing Pluto like never before |author=A.S.Ganesh |work=The Hindu |date=March 7, 2021 |access-date=August 19, 2021 |url=https://www.thehindu.com/children/seeing-pluto-like-never-before/article33941881.ece |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819132745/https://www.thehindu.com/children/seeing-pluto-like-never-before/article33941881.ece |url-status=live }}</ref> The equatorial region of the sub-Charon hemisphere of Pluto has only been imaged at low resolution, as ''New Horizons'' made its closest approach to the anti-Charon hemisphere.<ref>{{cite journal |last1=Rothery |first1=David A |date=October 2015 |title=Pluto and Charon from New Horizons |journal=Astronomy & Geophysics |volume=56 |issue=5 |pages=5.19–5.22 |doi=10.1093/astrogeo/atv168 |doi-access=free}}</ref>

Some albedo variations in the higher southern latitudes could be detected by ''New Horizons'' using [[planetshine|Charon-shine]] (light reflected off Charon). The south polar region seems to be darker than the north polar region, but there is a high-albedo region in the southern hemisphere that may be a regional nitrogen or methane ice deposit.<ref name="Charonshine">{{cite journal |last1=Lauer |first1=Todd R. |last2=Spencer |first2=John R. |first3=Tanguy |last3=Bertrand |first4=Ross A. |last4=Beyer |first5=Kirby D. |last5=Runyon |first6=Oliver L. |last6=White |first7=Leslie A. |last7=Young |first8=Kimberly |last8=Ennico |first9=William B. |last9=MacKinnon |first10=Jeffrey M. |last10=Moore |first11=Catherine B. |last11=Olkin |first12=S. Alan |last12=Stern |first13=Harold A. |last13=Weaver |date=October 20, 2021 |title=The Dark Side of Pluto |journal=The Planetary Science Journal |volume=2 |issue=214 |page=214 |doi=10.3847/PSJ/ac2743 |arxiv=2110.11976 |bibcode=2021PSJ.....2..214L |s2cid=239047659 |doi-access=free }}</ref>

[[File:MVIC sunset scan of Pluto.jpg|thumb|center|upright=1.28|Panoramic view of Pluto's icy mountains and flat ice plains, imaged by ''New Horizons'' 15 minutes after its closest approach to Pluto. Distinct haze layers in Pluto's atmosphere can be seen backlit by the Sun.]]

== See also ==
{{div col|colwidth=20em}}
* ''[[How I Killed Pluto and Why It Had It Coming]]''
* [[List of geological features on Pluto]]
* [[Planets in astrology#Pluto|Pluto in astrology]]
* [[Pluto in fiction]]
* [[List of gravitationally rounded objects of the Solar System#Planets|Stats of planets in the Solar System]]
{{div col end}}

== Notes ==
{{notelist| notes =

{{efn| name = Surface area| Surface area derived from the radius ''r'': <math>4\pi r^2</math>.}}

{{efn| name = Volume| Volume ''v'' derived from the radius ''r'': <math>4\pi r^3/3</math>.}}

{{efn| name = Surface gravity| Surface gravity derived from the mass ''M'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>GM/r^2</math>.}}

{{efn| name = Escape velocity| Escape velocity derived from the mass ''M'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>\sqrt{2GM/r}</math>.}}

{{efn| name = Angular size| Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet}}

{{efn| name = wiki-kbo| The dwarf planet [[Eris (dwarf planet)|Eris]] is roughly the same size as Pluto, about 2330&nbsp;km; Eris is 28% more massive than Pluto. Eris is a [[scattered-disc object]], often considered a distinct population from Kuiper-belt objects like Pluto; Pluto is the largest body in the Kuiper belt proper, which excludes the scattered-disc objects.}}

{{efn| name = Perihelion| 1 = The discovery of Charon in 1978 allowed astronomers to accurately calculate the mass of the Plutonian system. But it did not indicate the two bodies' individual masses, which could only be estimated after other moons of Pluto were discovered in late 2005. As a result, because Pluto came to perihelion in 1989, most Pluto perihelion date estimates are based on the Pluto–Charon [[barycenter]]. Charon came to perihelion [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=901 4 September 1989.] The Pluto–Charon barycenter came to perihelion [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=9 5 September 1989.] Pluto came to perihelion [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=999 8 September 1989.]}}

<!--{{efn| name = TNOs| Astronomers do not expect to find an object larger than Pluto closer than 100 AU from the Sun (see [[#Origins|Origins]]).<ref name="Sheppard2011" /> Of the 1687 [[TNOs]] known, {{Plain link|url = http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;ast_orbit_class=TNO;OBJ_field=0;ORB_field=0;c1_group=ORB;c1_item=Bi;c1_op=%3E;c1_value = 30.1;table_format=HTML;max_rows=100;format_option = comp;c_fields=AcBhBgBjBiBnBsCjCpAi;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query = 1&c_sort=AiA 1471}} of them have [[Apsis|perihelion]] further out than Neptune (30.1 AU).}}-->
}}

== References ==
{{Reflist
|30em
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<ref name=Grundy_2013>{{cite journal|title=Near-infrared spectral monitoring of Pluto's ices: Spatial distribution and secular evolution |author1=Grundy, W.M. |author2=Olkin, C.B. |author3=Young, L.A. |author4=Buie, M.W. |author5=Young, E.F. |year=2013 |journal=Icarus |volume=223 |issue=2 |pages=710–721 |doi=10.1016/j.icarus.2013.01.019 |bibcode=2013Icar..223..710G |arxiv=1301.6284 |s2cid=26293543 |url=http://www2.lowell.edu/~grundy/abstracts/preprints/2013.Pluto_SpeX.pdf |url-status=dead |archive-url=https://web.archive.org/web/20151108210124/http://www2.lowell.edu/~grundy/abstracts/preprints/2013.Pluto_SpeX.pdf |archive-date=November 8, 2015}}</ref>

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| access-date = July 20, 2011
| archive-date = May 12, 2020
| archive-url = https://web.archive.org/web/20200512195029/https://www.nasa.gov/mission_pages/hubble/science/pluto-moon.html
| url-status = live
}}</ref>

<ref name="Sternetal 2005">{{cite arXiv
| first1 = S. Alan | last1 = Stern
| first2 = Harold A. Jr. | last2 = Weaver
| first3 = Andrew J. | last3 = Steffl
| display-authors = 3
| first4 = Max J. | last4 = Mutchler
| first5 = William J. | last5 = Merline
| first6 = Marc W. | last6 = Buie
| first7 = Eliot F. | last7 = Young
| first8 = Leslie A. | last8 = Young
| first9 = John R. | last9 = Spencer
| title = Characteristics and Origin of the Quadruple System at Pluto
| date = 2005
| eprint = astro-ph/0512599
}}</ref>

<ref name="RichardsonWalsh2005">{{cite journal
| title = Binary Minor Planets
| first1 = Derek C.
| last1 = Richardson
| first2 = Kevin J.
| last2 = Walsh
| doi = 10.1146/annurev.earth.32.101802.120208
| date = 2005
| journal=Annual Review of Earth and Planetary Sciences
| volume = 34
| issue = 1
| pages = 47–81
| bibcode = 2006AREPS..34...47R
| s2cid = 1692921
}}</ref>

<ref name="Sicardyetal2006nature">{{cite journal
| title = Charon's size and an upper limit on its atmosphere from a stellar occultation
| doi = 10.1038/nature04351
| pmid = 16397493
| date = 2006
| volume = 439
| issue = 7072
| journal = Nature
| bibcode = 2006Natur.439...52S
| first1 = Bruno | last1 = Sicardy
| first2 = Aurélie | last2 = Bellucci
| first3 = Éric | last3 = Gendron
| display-authors = 3
| first4 = François | last4 = Lacombe
| first5 = Sylvestre | last5 = Lacour
| first6 = Jean | last6 = Lecacheux
| first7 = Emmanuel | last7 = Lellouch
| first8 = Stefan | last8 = Renner
| first9 = Sylvain | last9 = Pau
| first10 = Françoise | last10 = Roques
| first11 = Thomas | last11 = Widemann
| first12 = François | last12 = Colas
| first13 = Frédéric | last13 = Vachier
| first14 = Roberto | last14 = Vieira-Martins
| first15 = Nancy | last15 = Ageorges
| first16 = Olivier R. | last16 = Hainaut
| first17 = Orsola | last17 = De Marco
| first18 = Wolfgang | last18 = Beisker
| first19 = Evi | last19 = Hummel
| first20 = Carlos | last20 = Feinstein
| first21 = Hugo | last21 = Levato
| first22 = Alain | last22 = Maury
| first23 = Éric | last23 = Frappa
| first24 = Boris | last24 = Gaillard
| first25 = Maylis | last25 = Lavayssière
| first26 = Mario | last26 = Di Sora
| first27 = Franco | last27 = Mallia
| first28 = Gianluca | last28 = Masi
| first29 = Raoul | last29 = Behrend
| first30 = Fabien | last30 = Carrier <!--
Olivier Mousis; Philippe Rousselot; Alvaro Alvarez-Candal; D. Lazzaro; C. Veiga; A. H. Andrei; M. Assafin; D. N. da Silva Neto; Cristóvão Jacques; E. Pimentel; D. Weaver; J.-F. Lecampion; F. Doncel; T. Momiyama; Gonzalo Tancredi
-->
| pages = 52–54
| hdl = 11336/39754
| s2cid = 4411478
| hdl-access = free
}}</ref>

<ref name="Young1997">{{cite web
| title = The Once and Future Pluto
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| last = Young
| work = Southwest Research Institute, Boulder, Colorado
| url = http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt
| date = 1997
| access-date = March 26, 2007
| archive-date = March 30, 2004
| archive-url = https://web.archive.org/web/20040330212503/http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt
| url-status = live
}}</ref>

<!--- <ref name="spaceflightnow2007 Ice machine">{{cite web
| title = Charon: An ice machine in the ultimate deep freeze
| work = Gemini Observatory News Release
| date = 2007
| url = http://www.spaceflightnow.com/news/n0707/17charon/
| access-date = July 18, 2007
| archive-date = June 7, 2011
| archive-url = https://web.archive.org/web/20110607020940/http://www.spaceflightnow.com/news/n0707/17charon/
| url-status = live
}}</ref> --->

<ref name="pluto.jhuapl cousin">{{cite web
| title = Colossal Cousin to a Comet?
| publisher = Johns Hopkins University Applied Physics Laboratory
| work = pluto.jhuapl.edu&nbsp;– NASA New Horizons mission site
| url = http://pluto.jhuapl.edu/science/everything_pluto/8_cousin.php
| access-date = February 15, 2014
| archive-url = https://web.archive.org/web/20141113225441/http://pluto.jhuapl.edu/science/everything_pluto/8_cousin.php
| archive-date = November 13, 2014
}}</ref>

<ref name=TOP2013>{{cite journal |title=New analytical planetary theories VSOP2013 and TOP2013 |journal=Astronomy and Astrophysics |volume=557 |issue=2 |pages=A49 |date=September 2013 |last1=Simon |first1=J.L. |last2=Francou |first2=G. |last3=Fienga |first3=A. |last4=Manche |first4=H. |bibcode=2013A&A...557A..49S |doi=10.1051/0004-6361/201321843 |s2cid=56344625 |doi-access=free |url=https://hal.archives-ouvertes.fr/hal-01856071/file/aa21843-13.pdf |access-date=February 26, 2024 |archive-date=May 27, 2022 |archive-url=https://web.archive.org/web/20220527015437/https://hal.archives-ouvertes.fr/hal-01856071/file/aa21843-13.pdf |url-status=live }} The elements in the clearer and usual format [https://drive.google.com/file/d/0B7EgQjYURXMTSGE5LVMyMUMwa00/view?usp=sharing is in the spreadsheet] {{Webarchive|url=https://web.archive.org/web/20160515075130/https://drive.google.com/file/d/0B7EgQjYURXMTSGE5LVMyMUMwa00/view?usp=sharing |date=May 15, 2016 }} and the original TOP2013 [http://ftp.imcce.fr/pub/ephem/planets/top2013/TOP2013-secular.txt elements here.] {{Webarchive|url=https://web.archive.org/web/20211019232104/https://ftp.imcce.fr/pub/ephem/planets/top2013/TOP2013-secular.txt |date=October 19, 2021 }}</ref>

<ref name="Tyson1999">{{cite web
| first = Neil deGrasse
| last = Tyson
| author-link = Neil deGrasse Tyson
| date = 1999
| title = Pluto Is Not a Planet
| work = The Planetary Society
| url = http://www.planetary.org/explore/topics/topten/tyson_pluto_is_not.html
| archive-url = https://web.archive.org/web/20110927042714/http://www.planetary.org/explore/topics/topten/tyson_pluto_is_not.html
| archive-date = September 27, 2011
| access-date = November 30, 2011
}}</ref>

<ref name="PlanetaryOrg Triton">{{cite web
| title = Neptune's Moon Triton
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| url = http://www.planetary.org/explore/topics/neptune/triton.html
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| archive-date = December 10, 2011
| access-date = November 30, 2011
}}</ref>

<!--<ref name="Jewitt2004">{{cite web
| title = The Plutinos
| first = David C.
| last = Jewitt
| author-link = David C. Jewitt
| work = University of Hawaiʻi
| url = http://www2.ess.ucla.edu/~jewitt/kb/plutino.html
| date = 2004
| access-date = March 26, 2007
| archive-url = https://web.archive.org/web/20070419234021/http://www.ifa.hawaii.edu/~jewitt/kb/plutino.html
| archive-date = April 19, 2007
}}</ref>-->

<ref name="Hahn2005">{{cite journal
| title = Neptune's Migration into a Stirred-up Kuiper Belt: A Detailed Comparison of Simulations to Observations
| journal = The Astronomical Journal
| volume = 130
| issue = 5
| pages = 2392–2414
| first = Joseph M.
| last = Hahn
| date = 2005
| url = http://gemelli.colorado.edu/~hahnjm/pubs/migrate.pdf
| access-date = March 5, 2008
| bibcode = 2005AJ....130.2392H
| arxiv = astro-ph/0507319
| doi = 10.1086/452638
| s2cid = 14153557
| archive-date = July 23, 2011
| archive-url = https://web.archive.org/web/20110723170440/http://gemelli.colorado.edu/~hahnjm/pubs/migrate.pdf
| url-status = live
}}</ref>

<ref name="Levison2007">{{cite journal
| first1 = Harold F. | last1 = Levison
| first2 = Alessandro | last2 = Morbidelli
| first3 = Christa | last3 = Van Laerhoven
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| first5 = Kleomenis | last5 = Tsiganis
| title = Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune
| date = 2007
| bibcode = 2008Icar..196..258L
| doi = 10.1016/j.icarus.2007.11.035
| journal = Icarus
| volume = 196
| issue = 1
| pages = 258–273
| arxiv = 0712.0553
| s2cid = 7035885
}}</ref>

<ref name="Malhotra1995">{{cite journal
| first = Renu
| last = Malhotra
| title = The Origin of Pluto's Orbit: Implications for the Solar System Beyond Neptune
| journal = Astronomical Journal
| date = 1995
| volume = 110
| page = 420
| doi = 10.1086/117532
| bibcode = 1995AJ....110..420M
| arxiv=astro-ph/9504036
| s2cid = 10622344
}}</ref>

<ref name=Olkin_2015>{{cite journal
 |title           = Evidence that Pluto's atmosphere does not collapse from occultations including the 2013 May 04 event
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 |author2         = Young, L.A.
 |author3         = Borncamp, D.
 |date            = January 2015
 |journal         = Icarus
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 |pages           = 220–225
 |bibcode         = 2015Icar..246..220O
 |doi             = 10.1016/j.icarus.2014.03.026
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|archive-date    = September 29, 2021
|archive-url     = https://web.archive.org/web/20210929080858/https://www.researchgate.net/publication/262937542_Evidence_that_Pluto%27s_atmosphere_does_not_collapse_from_occultations_including_the_2013_May_04_event
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}}</ref>

<ref name="pluto.jhuapl First Pluto Sighting">{{cite web
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|access-date = November 29, 2011
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|df = mdy-all
}}</ref>

<ref name="Tyson2001">{{cite web
| first = Neil deGrasse
| last = Tyson
| author-link = Neil deGrasse Tyson
| title = Astronomer Responds to Pluto-Not-a-Planet Claim
| date = February 2, 2001
| publisher = Space.com
| url = http://www.space.com/1925-astronomer-responds-pluto-planet-claim.html
| access-date = November 30, 2011
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| url-status = live
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<ref name="NASA-JPL press release 07-29-2005">{{cite web
| title = NASA-Funded Scientists Discover Tenth Planet
| work = NASA press releases
| url = http://www.nasa.gov/home/hqnews/2005/jul/HQ_05209_10th_Planet.html
| date = July 29, 2005
| access-date = February 22, 2007
| archive-date = May 12, 2020
| archive-url = https://web.archive.org/web/20200512194615/https://www.nasa.gov/home/hqnews/2005/jul/HQ_05209_10th_Planet.html
| url-status = live
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<ref name="what">{{cite journal
| title = What is a Planet?
| first = Steven
| last = Soter
| place = Department of Astrophysics, American Museum of Natural History
| url = http://www.scientificamerican.com/article.cfm?id=what-is-a-planet&page=2
| date = 2007
| doi = 10.1086/508861
| journal = The Astronomical Journal
| volume = 132
| issue = 6
| pages = 2513–2519
| arxiv = astro-ph/0608359
| bibcode = 2006AJ....132.2513S
| s2cid = 14676169
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| archive-date = November 6, 2013
| archive-url = https://web.archive.org/web/20131106013415/http://www.scientificamerican.com/article.cfm?id=what-is-a-planet&page=2
| url-status = live
}}</ref>

<ref name="IAU2006 GA26-5-6">{{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
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| access-date = June 15, 2008
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| url-status = live
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<ref name="IAU0603">{{cite web
| date = August 24, 2006
| publisher = International Astronomical Union (News Release – IAU0603)
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| url = http://www.iau.org/news/pressreleases/detail/iau0603/
| access-date = June 15, 2008
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| url-status = live
}}</ref>

<ref name="IAU0804">{{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 – IAU0804)
| url = http://www.iau.org/news/pressreleases/detail/iau0804/
| access-date = December 1, 2011
| archive-date = August 10, 2011
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| url-status = live
}}</ref>

<ref name="IAUC 8747">{{cite journal
| first = Daniel W. E.
| last = Green
| title = (134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia)
| journal = IAU Circular
| volume = 8747
| pages = 1
| date = September 13, 2006
| url = http://www.cbat.eps.harvard.edu/iauc/08700/08747.html#Item1
| access-date = December 1, 2011
| archive-url = https://web.archive.org/web/20070205035336/http://www.cfa.harvard.edu/iau/special/08747.pdf
| archive-date = February 5, 2007
| bibcode = 2006IAUC.8747....1G
}}</ref>

<ref name="geoff2006c">{{cite news
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|access-date = September 8, 2006
|url-status = dead
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|df = mdy-all
}}</ref>

<ref name="Ruibal-1999">{{cite news
| first = Sal
| last = Ruibal
| title = Astronomers question if Pluto is real planet
| work = [[USA Today]]
| date = January 6, 1999
}}</ref>

<ref name="Britt-2006">{{cite news
| first = Robert Roy
| last = Britt
| url = http://www.space.com/scienceastronomy/061121_exoplanet_definition.html
| title = Why Planets Will Never Be Defined
| publisher = Space.com
| date = November 21, 2006
| access-date = December 1, 2006
| archive-date = May 24, 2009
| archive-url = https://web.archive.org/web/20090524010320/http://www.space.com/scienceastronomy/061121_exoplanet_definition.html
| url-status = live
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<ref name="geoff2006a">{{cite news
| url = https://www.nbcnews.com/id/wbna14489259
| title = Scientists decide Pluto's no longer a planet
| first = Robert Roy
| last = Britt
| date = August 24, 2006
| publisher = NBC News
| access-date = September 8, 2006
| archive-date = February 11, 2013
| archive-url = https://web.archive.org/web/20130211191638/http://www.nbcnews.com/id/14489259
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<ref name="newscientistspace">{{cite news
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| title = New planet definition sparks furore
| date = August 25, 2006
| publisher = NewScientist.com
| first = David
| last = Shiga
| access-date = September 8, 2006
| archive-date = October 3, 2010
| archive-url = https://web.archive.org/web/20101003074328/http://www.newscientist.com/article/dn9846-new-planet-definition-sparks-furore.html
| url-status = live
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<ref name="Buie2006 IAU response">{{cite web
| first = Marc W.
| last = Buie
| date = September 2006
| title = My response to 2006 IAU Resolutions 5a and 6a
| publisher = Southwest Research Institute
| url = http://www.boulder.swri.edu/~buie/pluto/iauresponse.html
| access-date = December 1, 2011
| archive-url = https://web.archive.org/web/20070603104622/http://www.lowell.edu/users/buie/pluto/iauresponse.html
| archive-date = June 3, 2007
}}</ref>

<ref name="Overbye2006">{{cite news
| first = Dennis
| last = Overbye
| date = August 24, 2006
| title = Pluto Is Demoted to 'Dwarf Planet'
| newspaper = The New York Times
| url = https://www.nytimes.com/2006/08/24/science/space/25pluto.html
| access-date = December 1, 2011
| archive-date = June 22, 2022
| archive-url = https://web.archive.org/web/20220622164604/https://www.nytimes.com/2006/08/24/science/space/25pluto.html?ei=5087
| url-status = live
}}</ref>

<ref name="Minkel2008">{{cite journal
| date = April 10, 2008
| title = Is Rekindling the Pluto Planet Debate a Good Idea?
| journal = Scientific American
| first = J. R.
| last = Minkel
| url = http://www.scientificamerican.com/article.cfm?id=rekindling-the-pluto-planet-debate
| access-date = December 1, 2011
| archive-date = August 11, 2011
| archive-url = https://web.archive.org/web/20110811035447/http://www.scientificamerican.com/article.cfm?id=rekindling-the-pluto-planet-debate
| url-status = live
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<ref name="The Great Planet Debate">{{cite web
| title = The Great Planet Debate: Science as Process. A Scientific Conference and Educator Workshop
| publisher = Johns Hopkins University Applied Physics Laboratory
| work = gpd.jhuapl.edu
| date = June 27, 2008
| url = http://gpd.jhuapl.edu/
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| archive-date = August 17, 2011
| archive-url = https://web.archive.org/web/20110817104308/http://gpd.jhuapl.edu/
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<ref name="PSIedu press release 2008-09-19">"Scientists Debate Planet Definition and Agree to Disagree", Planetary Science Institute press release of September 19, 2008, [http://www.psi.edu/press/archive/20080919planetdebate/ PSI.edu] {{Webarchive|url=https://web.archive.org/web/20110715221348/http://www.psi.edu/press/archive/20080919planetdebate/ |date=July 15, 2011 }}</ref>

<ref name="Discover 2009-JANp76">"Plutoids Join the Solar Family", Discover Magazine, January 2009, p. 76</ref>

<ref name="Science News, July 5, 2008 p. 7">Science News, July 5, 2008, p. 7</ref>

<ref name="DeVore2006">{{cite news
| first = Edna
| last = DeVore
| date = September 7, 2006
| title = Planetary Politics: Protecting Pluto
| publisher = Space.com
| url = http://www.space.com/2855-planetary-politics-protecting-pluto.html
| access-date = December 1, 2011
| archive-date = August 4, 2011
| archive-url = https://web.archive.org/web/20110804224236/http://www.space.com/2855-planetary-politics-protecting-pluto.html
| url-status = live
}}</ref>

<ref name="Holden2007">{{cite journal
| first = Constance
| last = Holden
| date = March 23, 2007
| title = Rehabilitating Pluto
| journal = Science
| volume = 315
| issue = 5819
| page = 1643
| doi = 10.1126/science.315.5819.1643c
| s2cid = 220102037
}}</ref>

<ref name="Gutierrez2007">{{cite web
| first = Joni Marie
| last = Gutierrez
| date = 2007
| title = A joint memorial. Declaring Pluto a planet and declaring March 13, 2007, 'Pluto planet day' at the legislature
| publisher = Legislature of New Mexico
| url = http://www.nmlegis.gov/Sessions/07%20Regular/memorials/house/HJM054.html
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| archive-date = May 11, 2020
| archive-url = https://web.archive.org/web/20200511184914/https://www.nmlegis.gov/Sessions/07%20Regular/memorials/house/HJM054.html
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<ref name="ILGA SR0046">{{cite web
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| url-status = live
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<ref name="Sapa-AP">{{cite news
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| title = Pluto's still the same Pluto
| date = October 21, 2006
| newspaper = Independent Newspapers
| agency = Associated Press
| access-date = November 29, 2011
| quote = Mickey Mouse has a cute dog.
| archive-date = December 1, 2017
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| url-status = live
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<ref name="msnbc">{{cite news
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| access-date = January 10, 2007
| agency = Associated Press
| date = January 8, 2007
| archive-date = March 1, 2013
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<!--<ref name="horizons">{{cite web
| url = http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=9
| title = HORIZONS Web-Interface for Pluto Barycenter (Major Body = 9)
| publisher = [[JPL Horizons On-Line Ephemeris System]]
| access-date = October 11, 2012
}} Select "Ephemeris Type: Elements", "Target Body: Pluto Barycenter" and "Center: @Sun".</ref>-->

<ref name = Sheppard2011>{{cite journal
| first1 = Scott S. | last1 = Sheppard | author-link1 = Scott S. Sheppard
| first2 = Chadwick A. | last2 = Trujillo | author-link2 = Chad Trujillo
| first3 = Andrzej | last3 = Udalski | author-link3 = Andrzej Udalski
| display-authors = 3
| first4 = Marcin | last4 = Kubiak | author-link4 = Marcin Kubiak
| first5 = Grzegorz | last5 = Pietrzynski
| first6 = Radoslaw | last6 = Poleski
| first7 = Igor | last7 = Soszynski
| first8 = Michal | last8 = Szyma
| first9 = Krzysztof | last9 = Ulaczyk
| title = A Southern Sky and Galactic Plane Survey for Bright Kuiper Belt Objects
| journal=Astronomical Journal
| volume = 142 | issue = 4 | date = 2011
| doi=10.1088/0004-6256/142/4/98
| arxiv=1107.5309
| bibcode = 2011AJ....142...98S | pages=98| s2cid = 53552519 }}</ref>

<ref name = Robbins2023>{{cite journal
| first1 = Stuart J. | last1 = Robbins
| first2 = Luke | last2 = Dones
| title = Impact Crater Databases for Pluto and Charon, Version 2
| journal=The Planetary Science Journal
| volume = 4 | issue = 12 | date = December 2023
| doi=10.3847/PSJ/acf7be
| arxiv=
| bibcode = 2023PSJ.....4..233R | pages=6 | doi-access = free
| s2cid = 266147862 | id = 233 }}</ref>

<ref name="Stern2015">{{cite journal |last=Stern |first=S.A. |display-authors=etal |title=The Pluto system: Initial results from its exploration by New Horizons |journal=[[Science (journal)|Science]] |date=2015 |volume=350 |issue=6258 |pages=249–352 |doi=10.1126/science.aad1815 |bibcode=2015Sci...350.1815S |pmid=26472913 |arxiv=1510.07704 |s2cid=1220226}}</ref>

<ref name="Brozovic2024">{{cite journal |last1=Brozović |first1=Marina |last2=Jacobson |first2=Robert A. |title=Post-new-horizons Orbits and Masses for the Satellites of Pluto |date=May 8, 2024 |journal=The Astronomical Journal |volume=167 |issue=256 |page=256 |doi=10.3847/1538-3881/ad39f0 |doi-access=free |bibcode=2024AJ....167..256B }}</ref>
}}

== Further reading ==
{{refbegin}}
* {{Cite book |author=Codex Regius |url=https://books.google.com/books?id=dTs2DQAAQBAJ |title=Pluto & Charon: the new horizons spacecraft at the farthest worldly shores |date=2016 |publisher=Codex Regius |isbn=978-1-5349-6074-9 |location=Wiesbaden}}
* {{Cite book |author-link=Alan Stern |url=https://books.google.com/books?id=VcY7iYJwJZoC |title=Pluto and Charon |author-link2=David J. Tholen |date=1997 |publisher=[[University of Arizona Press]] |isbn=978-0-8165-1840-1 |editor-last=Stern |editor-first=Alan |series=Space science series |location=Tucson |editor-last2=Tholen |editor-first2=David J.}}
* {{Cite book |last=Stern |first=Alan |author-link=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 (imprint)|Picador]] |isbn=978-1-250-09896-2 |location=New York}}
* {{Cite book |url=https://books.google.com/books?id=bi01EAAAQBAJ |title=The Pluto system after New Horizons |date=2021 |publisher=[[University of Arizona Press]] |isbn=978-0-8165-4094-5 |editor-last=Stern |editor-first=Alan |editor-link=Alan Stern |series=The University of Arizona Space science series |location=Tucson |pages=688 |editor-last2=Moore |editor-first2=J. |editor-last3=Grundy |editor-first3=William M. |editor-last4=Young |editor-first4=Leslie A. |editor-last5=Binzel |editor-first5=Richard P. |editor-link5=Richard P. Binzel}}
{{refend}}

== External links ==
{{Sister project links|Pluto|b=no}}
{{refbegin}}
* [http://www.nasa.gov/mission_pages/newhorizons/main/index.html ''New Horizons'' homepage] {{Webarchive|url=https://web.archive.org/web/20150726155211/http://www.nasa.gov/mission_pages/newhorizons/main/index.html |date=July 26, 2015 }}
* [https://web.archive.org/web/20120729075223/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto Pluto Profile] at [http://solarsystem.nasa.gov/ NASA's Solar System Exploration site]
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html NASA Pluto factsheet] {{Webarchive|url=https://archive.today/20151119095810/http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html |date=November 19, 2015 }}
* [http://www.lowell.edu/ Website of the observatory that discovered Pluto] {{Webarchive|url=https://web.archive.org/web/20110302233420/http://www.lowell.edu/ |date=March 2, 2011 }}
* [https://web.archive.org/web/20121004232224/http://www.astrobio.net/pressrelease/5055/sharpest-ever-views-of-pluto-and-charon Earth telescope image of Pluto system]
* [http://www.ifa.hawaii.edu/info/press-releases/PlutoPictures/Pluto-Tholen-10-07.html Keck infrared with AO of Pluto system] {{Webarchive|url=https://web.archive.org/web/20201109033030/http://www.ifa.hawaii.edu/info/press-releases/PlutoPictures/Pluto-Tholen-10-07.html |date=November 9, 2020 }}
* [http://www.nasa.gov/image-feature/goddard/views-of-pluto-through-the-years Video – Pluto – viewed through the years (GIF)] {{Webarchive|url=https://web.archive.org/web/20150726170002/http://www.nasa.gov/image-feature/goddard/views-of-pluto-through-the-years |date=July 26, 2015 }} (NASA; animation; July 15, 2015).
* [http://photojournal.jpl.nasa.gov/archive/PIA19873_FLYTHROUGH_ANIMATION_V5.mp4 Video – Pluto – "FlyThrough" (00:22; MP4)] {{Webarchive|url=https://web.archive.org/web/20210929080856/http://photojournal.jpl.nasa.gov/archive/PIA19873_FLYTHROUGH_ANIMATION_V5.mp4 |date=September 29, 2021 }} [https://www.youtube.com/watch?v=ds_OlZnV9qk (YouTube)] {{Webarchive|url=https://web.archive.org/web/20201202025129/https://www.youtube.com/watch?v=ds_OlZnV9qk |date=December 2, 2020 }} (NASA; animation; August 31, 2015).
* [http://www.scientificamerican.com/video/a-day-on-pluto-reconstructed-from-new-horizons-images/ "A Day on Pluto Video made from July 2015 New Horizon Images"] {{Webarchive|url=https://web.archive.org/web/20160223013923/http://www.scientificamerican.com/video/a-day-on-pluto-reconstructed-from-new-horizons-images/ |date=February 23, 2016 }} [[Scientific American]]
* NASA CGI [http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20170714-2 video] {{Webarchive|url=https://web.archive.org/web/20170801114952/http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20170714-2 |date=August 1, 2017 }} of Pluto flyover (July 14, 2017)
* [https://www.flickr.com/photos/136797589@N04/35260746413/in/photostream/ CGI video] {{Webarchive|url=https://web.archive.org/web/20201003081511/https://www.flickr.com/photos/136797589@N04/35260746413/in/photostream/ |date=October 3, 2020 }} simulation of rotating Pluto by Seán Doran (see [https://www.flickr.com/photos/136797589@N04/albums/72157686474817595 album] {{Webarchive|url=https://web.archive.org/web/20200727235247/https://www.flickr.com/photos/136797589@N04/albums/72157686474817595 |date=July 27, 2020 }} for more)
* [https://www.google.com/maps/space/pluto/@26.4828801,-19.6602925,11338964m/data=!3m1!1e3 Google Pluto 3D] {{Webarchive|url=https://web.archive.org/web/20200806100353/https://www.google.com/maps/space/pluto/@26.4828801,-19.6602925,11338964m/data=!3m1!1e3 |date=August 6, 2020 }}, interactive map of the dwarf planet
* {{cite web |url=https://gravitysimulator.org/solar-system/the-plutonian-system |title=Interactive 3D gravity simulation of the Plutonian system |archive-url=https://web.archive.org/web/20200611221454/https://gravitysimulator.org/solar-system/the-plutonian-system/ |archive-date=June 11, 2020 |url-status=dead }}
{{refend}}

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