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{{About|the dwarf planet}}
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{{Infobox planet
| name = Pluto
| symbol = [[File:Pluto symbol.svg|25px|Astronomical symbol of Pluto]]
| image = [[File:Pluto animiert 200px.gif]]
| caption = Computer-generated map of Pluto<ref>Drawn from [[Hubble space telescope|Hubble]] images, synthesized true color{{efn|name=HST-SynColours}} and among the highest resolutions possible with current technology.</ref>
| discovery = yes
| discoverer = [[Clyde Tombaugh|Clyde W. Tombaugh]]
| discovered = February 18, 1930
| mp_name = '''{{val|134340|u=Pluto}}'''
| named_after = [[Pluto (mythology)|Pluto]]
| mp_category =
  {{plainlist |
* [[dwarf planet]]
* [[trans-Neptunian object|TNO]]
* [[plutoid]]
* [[Kuiper belt|KBO]]
* [[plutino]]
  }}
| orbit_ref =<ref name="horizons"/>{{efn|name=barycentre|Orbital elements refer to the barycentre of the Pluto system, and are the instantaneous [[osculating orbit|osculating]] values at the precise [[J2000]] epoch. Barycentre quantities are given because, in contrast to the planetary centre, they do not experience appreciable changes on a day-to-day basis from the motion of the moons. The orbital period of Pluto is listed as 248 years because most references use the more stable barycentre of the Solar System (Sun+Jupiter) to list the orbital period of the Pluto-Charon system. A J2000 heliocentric solution would give a value of 246 years.}}
| epoch = [[J2000]]
| aphelion =
  {{plainlist |
* {{val|7311000000|u=km}}
* {{val|48.871|ul=AU}}
  }}
| perihelion =
  {{plainlist |
* {{val|4437000000|u=km}}
* {{val|29.657|u=AU}}
* (1989 Sep 05)<ref name="jpl-ssd-horizons" />
  }}
| semimajor =
  {{plainlist |
* {{val|5874000000|u=km}}
* {{val|39.264|u=AU}}
  }}
| eccentricity = {{val|0.244671664}} (J2000)<br>0.248&nbsp;807&nbsp;66 (mean)<ref name="Pluto Fact Sheet" />
| inclination =
  {{plainlist |
* {{val|17.151394|s=°}}
* 11.88° to Sun's equator
  }}
| asc_node = {{val|110.28683|s=°}}
| arg_peri = {{val|113.76349|s=°}}
| period =
  {{plainlist |
* {{val|90465|u=days}}<ref name="Pluto Fact Sheet" /><ref name="planet_years" />
* {{val|247.68}} [[julian year (astronomy)|years]]<ref name="Pluto Fact Sheet" />
* {{val|14164.4|u=Plutonian [[solar day]]s}}<ref name="planet_years" />
  }}
| synodic_period = 366.73 days<ref name="Pluto Fact Sheet" />
| avg_speed = 4.7&nbsp;km/s<ref name="Pluto Fact Sheet" />
| mean_anomaly = {{val|14.86012204|s=°}}<ref name="mean_anomaly" />
| satellites = [[Moons of Pluto|5]]
| physical_characteristics = yes
| mean_radius =
  {{plainlist |
* {{val|1153|10|u=km}}<ref name="BuieGrundyYoung_2006" />
* 0.18 [[Earth mass|Earths]]
* 1161 km<ref name="Young2007" /> (solid)
  }}
| surface_area =
  {{plainlist |
* {{val|1.665|e=7|u=km2}}{{efn|name=Surface area}}
* 0.033 Earths
  }}
| volume =
  {{plainlist |
* {{val|6.39|e=9|u=km3}}{{efn|name=Volume}}
* {{val|0.0059|u=Earths}}
  }}
| mass =
  {{plainlist |
* {{val|1.305|0.007|e=22|u=kg}}<ref name="BuieGrundyYoung_2006" />
* {{val|0.00218|u=[[Earth mass|Earths]]}}
* 0.178 [[Moon mass|Moons]]
  }}
| density = {{val|2.03|0.06|u=g/cm3}}<ref name="BuieGrundyYoung_2006" />
| surface_grav =
  {{plainlist |
* {{Gr|13.05|1153}} [[Acceleration|m/s<sup>2</sup>]]{{efn|name=Surface gravity}}
* 0.067 [[g-force|g]]
  }}
| escape_velocity = {{V2|13.05|1153}}&nbsp;km/s{{efn|name=Escape velocity}}
| sidereal_day =
  {{plainlist |
* [[retrograde motion|{{val|-6.387230|u=day}}]]
* 6 d, 9 h, 17 m, 36 s
  }}
| rot_velocity = 47.18&nbsp;km/h
| axial_tilt = {{val|119.591|0.014|s=°}} (to orbit)<ref name="BuieGrundyYoung_2006" />{{efn|name=Axial tilt}}
| right_asc_north_pole = 312.993°<ref name="SeidelmannArchinalA'hearn_2007" />
| declination = 6.163°<ref name="SeidelmannArchinalA'hearn_2007" />
| albedo = 0.49 to 0.66 ([[Geometric albedo|geometric]], varies by 35%)<ref name="Pluto Fact Sheet" /><ref name="Hamilton" />
| 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 = −0.7<ref name="jpldata" />
| angular_size = 0.065″ to 0.115″<ref name="Pluto Fact Sheet" />{{efn|name=Angular size}}
| pronounced = {{IPAc-en|audio=en-us-Pluto.ogg|ˈ|p|l|uː|t|oʊ}}
| adjectives = Plutonian
| atmosphere = yes
| temperatures = yes
| temp_name1 = [[Kelvin]]
| min_temp_1 = 33 K
| mean_temp_1 = 44 K
| max_temp_1 = 55 K
| surface_pressure = 0.30 [[pascal (unit)|Pa]] (summer maximum)
| atmosphere_composition = [[nitrogen]], [[methane]], [[carbon monoxide]]<ref name="Physorg April 19, 2011" />
| note = no
}}
 
'''Pluto''' ([[minor-planet designation]] '''{{val|134340|u=Pluto}}''') is the largest object in the [[Kuiper belt]], and the tenth-most-massive body observed directly orbiting the [[Sun]]. It is the second-most-massive known [[dwarf planet]], after [[Eris (dwarf planet)|Eris]]. Like other Kuiper-belt objects, Pluto is composed primarily of rock and ice<ref name="Wiley-2005">{{cite web |last1=Stern |first1=Alan |last2=Mitton |first2=Jacqueline |title=Pluto and Charon: ice worlds on the ragged edge of the solar system |url=http://www.worldcat.org/search?q=isbn%3A3527405569 |date=2005 |publisher=Weinheim:[[Wiley-VCH]] |accessdate=July 3, 2013 }}</ref> and is relatively small, approximately one-sixth the mass of the [[Moon]] and one-third its volume. It has an [[orbital eccentricity|eccentric]] and highly inclined orbit that takes it from 30 to 49&nbsp;[[Astronomical unit|AU]] (4.4–7.4 billion&nbsp;km) from the Sun. This causes Pluto to periodically come closer to the Sun than [[Neptune]]. {{As of|2014}}, it is 32.6 AU from the Sun.
 
Discovered in 1930, Pluto was originally classified as the [[Planets beyond Neptune|ninth planet]] from the Sun. However, its status as a major [[planet]] fell into question following further study of it and the outer Solar System over the ensuing 75 years. Starting in 1977 with discovery of minor planet [[2060 Chiron]], numerous icy objects similar to Pluto with eccentric orbits were found.<ref name="ridpath" /> The most notable of these was the [[scattered disc object]] [[Eris (dwarf planet)|Eris]]—discovered in 2005, which is 27% more massive than Pluto.<ref name="hubblesite2007/24" /> The understanding that Pluto is only one of several large icy bodies in the outer Solar System prompted the [[International Astronomical Union]] (IAU) to [[IAU definition of planet|formally define what it means to be a "planet"]] in 2006. This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a [[plutoid]]).<ref name="BBC-Akwagyiram 2005-08-02" /> A number of scientists hold that Pluto should have remained classified as a planet, and that other dwarf planets should be added to the roster of planets along with Pluto.<ref name="Gray 2008-08-10" /><ref>Adam Gorwyn, [http://earthsky.org/space/alan-stern-%E2%80%98a-chihuahua-is-still-a-dog-and-pluto-is-still-a-planet-2 ''Alan Stern: ‘A Chihuahua is still a dog, and Pluto is still a planet’'']. EarthSky interview, 2010 Feb 18</ref>
 
Pluto has five known moons: [[Charon (moon)|Charon]] (the largest, with a diameter just over half that of Pluto), [[Nix (moon)|Nix]], [[Hydra (moon)|Hydra]], [[Kerberos (moon)|Kerberos]], and [[Styx (moon)|Styx]].<ref name="Showalter">{{cite news|last=Showalter|first=M.|title=Hubble Discovers a Fifth Moon Orbiting Pluto (News Release STScI-2012-32)|url=http://hubblesite.org/newscenter/archive/releases/2012/32/|accessdate=11 July 2012|newspaper=HubbleSite NewsCenter|date=11 July 2012<!--, 11:30 AM EDT-->}}</ref> Pluto and Charon are sometimes described as a [[Binary system (astronomy)|binary system]] because the [[barycenter]] of their orbits does not lie within either body.<ref name="Olkin_2003" /> However, the IAU has yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a [[natural satellite|moon]] of Pluto.<ref name="IAU Pluto" />
 
In 2015, the Pluto system is due to be visited by spacecraft for the first time. The ''[[New Horizons]]'' probe will perform a flyby during which it will attempt to take detailed measurements and images of the plutoid and its moons.
 
== Discovery ==
{{main|Planets beyond Neptune}}
[[File:Pluto discovery plates.png|thumb|left|300px|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, using [[Classical mechanics|Newtonian mechanics]], [[Urbain Le Verrier]] predicted the position of the then-undiscovered planet [[Neptune]] after analysing perturbations in the orbit of [[Uranus]].{{sfn|Croswell|1997|p=43}} Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune.
 
In 1906, [[Percival Lowell]], a wealthy Bostonian who had founded the [[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 until his death in 1916, but to no avail. Unknown to Lowell, on March 19, 1915, surveys had captured two faint images of Pluto, but they were not recognised for what they were.<ref name="Hoyt" /><ref name="Littman1990" /> There are fifteen other known [[precovery|prediscoveries]], with the oldest made by the [[Yerkes Observatory]] on August 20, 1909.<ref name="BuchwaldDimarioWild2000" />
 
Because of a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,{{sfn|Croswell|1997|p=50}} when its director, [[Vesto Melvin Slipher]], summarily handed the job of locating Planet X to [[Clyde Tombaugh]], a 23-year-old [[Kansas|Kansan]] who had just arrived at the Lowell 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 taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called 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 January 29 of that year. 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" />
 
=== Name ===
{{main|Venetia Burney}}
The discovery made headlines across the globe. The [[Lowell Observatory]], which had the right to name the new object, received over 1,000 suggestions from all over the world, ranging from Atlas to Zymal.<ref name="pluto guide" /> Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.<ref name="pluto guide" /> Constance Lowell proposed ''[[Zeus]]'', then ''Percival'' and finally ''Constance''. These suggestions were disregarded.<ref name="Mager" /><!-- self-published source -->
 
The name Pluto was proposed by [[Venetia Burney]] (1918–2009), an eleven-year-old schoolgirl in [[Oxford]], England.<ref name="Venetia" /> Burney was interested in [[classical mythology]] as well as astronomy, and considered the name, a name for the [[Pluto (mythology)|god of the underworld]], appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather [[Falconer Madan]], a former librarian at the [[University of Oxford]]'s [[Bodleian Library]]. Madan passed the name to Professor [[Herbert Hall Turner]], who then cabled it to colleagues in the United States.{{fact|date=November 2013}}
 
The object was officially named on March 24, 1930.<ref name="The Times 27 May 1930" /><ref name="NYTimes 25 May 1930" /> Each member of the Lowell Observatory was allowed to vote on a short-list of three: [[Minerva]] (which was already the name for an asteroid), [[Cronus]] (which had lost reputation through being proposed by the unpopular astronomer [[Thomas Jefferson Jackson See]]), and Pluto. Pluto received every vote.{{sfn|Croswell|1997|pp=54–55}} The name was announced on May 1, 1930.<ref name="Venetia" /> Upon the announcement, Madan gave Venetia {{currency|5|GBP}} (£{{formatnum:{{Inflation|UK|5|1930|{{CURRENTYEAR}}|r=0}}}} as of {{CURRENTYEAR}}),{{Inflation-fn|UK}} as a reward.<ref name="Venetia" />
 
The choice of name was partly inspired by the fact that the first two letters of ''Pluto'' are the initials of Percival Lowell, and Pluto's [[astronomical symbol]] ([[File:Pluto symbol.svg|20px|{{Unicode|♇}}]], [[unicode]] ♇) is a [[monogram]] constructed from the letters 'PL'.<ref name="JPL/NASA Pluto's Symbol" /> Pluto's [[astrological symbol]] resembles that of [[Neptune]] ([[File:Neptune symbol.svg|20px]]), but has a circle in place of the middle prong of the trident ([[File:Pluto's astrological symbol.svg|20px]]).
 
The name 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 [[The Walt Disney Company|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|Though if a language does not support something like the Greek pronunciation, these can be rather divergent, such as Somali ''Buluuto'' and Navaho ''Tłóotoo''.}} In [[Japanese language|Japanese]], [[Houei Nojiri]] suggested the translation ''Meiou Sei'' 冥王星 (King of the Underworld star/planet), and this was borrowed into [[Chinese language|Chinese]], [[Korean language|Korean]], and [[Mongolian language|Mongolian]].<ref name="RenshawIhara2000" /><ref name="nineplan" /><ref name="Bathrobe" /> Some [[Languages of India|Indian languages]] use the name Pluto, but others such as [[Hindi]] use the name of ''[[Yama (Buddhism and Chinese mythology)|Yama]],'' the Guardian of Hell in [[Hindu]] mythology, as does [[Vietnamese language|Vietnamese]].<ref name="nineplan" /> [[Polynesian languages]] also tend to use the indigenous god of the underworld, as in [[Maori language|Maori]] ''[[Whiro]]''.<ref name="nineplan" />
 
=== Demise of Planet X ===
[[File:ClydeTombaugh2.gif|upright|thumb|left|alt=A young man in his mid-twenties, wearing glasses, a white shirt, tie and long trousers, stands in an open field, next to a Newtonian telescope resting on the ground and tilted towards the sky. The telescope is taller than him, and is about eight inches in diameter. His right hand is resting up on the barrel, and he looks slightly past the telescope, out to the left.|[[Clyde Tombaugh|Clyde W. Tombaugh]], the discoverer of Pluto]]
 
{| class="wikitable plainrowheaders" style="float: right; margin-right: 0; margin-left: 1em; text-align: center;"
|+ Mass estimates for Pluto
! scope="col" | Year
! scope="col" | Mass
! scope="col" | Notes
|-
! scope="row" | 1931
| 1 Earth
| [[Seth Barnes Nicholson|Nicholson]] & [[Nicholas U. Mayall|Mayall]]<ref name="RAS1931.91" /><ref name="Nicholson et al 1930" /><ref name="Nicholson et al 1931" />
|-
! scope="row" | 1948
| 0.1 (1/10) Earth
| Kuiper<ref name="Kuiper 10.1086/126255" />
|-
! scope="row" | 1976
| 0.01 (1/100) Earth
| Cruikshank, Pilcher, & Morrison{{sfn|Croswell|1997|p=57}}
|-
! scope="row" | 1978
| 0.002 (1/500) Earth
| Christy & Harrington<ref name="ChristyHarrington1978" />
|}
 
Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's [[Planet X]]. Estimates of Pluto's mass were revised downward throughout the 20th century.
 
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 Hawaii]] 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 1.3–2.0 times greater than 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. Its mass, roughly 0.2% that of Earth, was 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 1989 flyby of [[Neptune]], which had revised the planet's total mass downward by 0.5%, 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" /> Today, 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 position in 1915 that was fairly close to Pluto's position at that time; [[Ernest W. Brown]] concluded almost immediately that this was a coincidence,<ref name="Tenn1994" /> a view still held today.<ref name="Standish1993" />
 
== Orbit and rotation ==
[[File:Pluto Orbit.gif|thumb|Pluto's orbit and the ecliptic.]]
[[File:Plutoorbit1.5sideview.gif|thumb|Orbit of Pluto—ecliptic view. This "side view" of Pluto's orbit (in red) shows its large inclination to Earth's ecliptic orbital plane.]]
[[File:TheKuiperBelt Orbits Pluto Neptune2.svg|thumb|left|This diagram shows the relative positions of Pluto (red) and Neptune (blue) on selected dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance between them to emphasise the closest approach in 1896.]]
 
Pluto's orbital period is 248 Earth 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 highly [[orbital inclination|inclined]] relative to the ecliptic (over 17°) and highly [[orbital eccentricity|eccentric]] ([[elliptical]]). This high eccentricity means a small region of Pluto's orbit lies nearer the Sun than [[Neptune]]'s. The Pluto–Charon [[Barycentric coordinates (astronomy)|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. Pluto and Neptune make their closest approach when it is at 27.960 AU<ref name="pluto990209" />
 
In the long term, Pluto's orbit is in fact [[chaos theory|chaotic]]. Although computer simulations can be used to predict its position for several million years (both [[time reversibility|forward and backward]] in time), after intervals longer than the [[Lyapunov time]] of 10–20 million years, calculations become speculative: Pluto is sensitive to unmeasurably small details of the Solar System, hard-to-predict factors that will gradually disrupt its orbit.<ref name="sussman88" /><ref name="wisdom91" /> Millions of years from now, Pluto may well be at [[aphelion]], at [[perihelion]] or anywhere in between, with no way for us to predict which. This does not mean Pluto's orbit itself is unstable, but its position ''on'' that orbit is impossible to determine so far ahead. Several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering.
 
=== Relationship with Neptune ===
[[File:TheKuiperBelt Orbits Pluto Polar.svg|thumb|Orbit of Pluto—polar view. This "view from above" shows how Pluto's orbit (in red) is less circular than Neptune's (in blue), and how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below the [[plane of the ecliptic]].]]
 
Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely. There are several reasons why.
 
At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 [[astronomical unit|AU]] above that of Neptune, preventing a collision.<ref name="huainn01" /><ref name="Hunter2004" /><ref name="malhotra-9planets" /> Pluto's [[orbital node|ascending and descending nodes]], the points at which its orbit crosses the ecliptic, are currently separated from Neptune's by over 21°.<ref name="Williams2010" />
 
This alone is not enough to protect Pluto; [[perturbation (astronomy)|perturbations]] from the planets (especially Neptune) could alter aspects of Pluto's orbit (such as its [[apsidal precession|orbital precession]]) over millions of years so that a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of these is that Pluto lies in the 2:3 [[orbital resonance|mean motion resonance]] with [[Neptune]]: for every two orbits that Pluto makes around the Sun, Neptune makes three. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is such that, in each 500-year cycle, the first time Pluto is near [[perihelion]] Neptune is over 50° ''behind'' Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance ''ahead'' of Pluto. Pluto and Neptune's minimum separation is over 17&nbsp;AU. Pluto comes closer to [[Uranus]] (11&nbsp;AU) than it does to Neptune.<ref name="malhotra-9planets" />
 
The 2:3 resonance between the two bodies is highly stable, and is preserved over millions of years.<ref name="sp-345" /> This prevents their orbits from changing relative to one another; the cycle always repeats in the same way, and so the two bodies can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could never collide.<ref name="malhotra-9planets" />
 
=== Other factors ===
Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto and Neptune's orbits does not change.<ref name=huainn01/><ref name="williams71"/> There are several other resonances and interactions that govern the details of their relative motion, and 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 and the point where it is closest to the Sun, [[libration|librates]] around 90°.<ref name=williams71/> This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a direct 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° (= 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 ecliptic—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 at its maximally above Neptune's orbit. This is known as the ''1:1 superresonance''. All the [[Jovian planets]], particularly Jupiter, play a role in the creation of the superresonance.<ref name=huainn01/>
 
To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit [[counterclockwise]]. After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes [[angular momentum]] to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, according to [[Kepler's third law]]. As its orbit changes, this has the gradual effect of changing the perihelion and longitude of Pluto's orbit (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch up with Pluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original node. The whole process takes about 20,000 years to complete.<ref name="malhotra-9planets"/><ref name="sp-345"/>
 
=== Rotation ===
Pluto's [[rotation period]], its day, is equal to 6.39 [[Earth|Earth day]]s.<ref name="axis" /> Like [[Uranus]], Pluto rotates on its "side" on 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" />
 
== Physical characteristics ==
[[File:Pluto map.jpg|thumb|300px|right|Map of Pluto's surface by [[NASA]], [[ESA]] and Marc W. Buie]]
[[File:PlutoColorMap HST2002-2003.jpg|thumb|Hubble map of Pluto's surface, showing great variations in color and [[albedo]]]]
[[File:Pluto-map-hs-2010-06-a-faces.jpg|thumb|Three views of Pluto from different orientations]]
 
Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the [[New Horizons]] spacecraft is expected to arrive there.<ref name="newhorizons" />
 
=== Appearance and surface ===
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, because its [[angular diameter]] is only 0.11".
 
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="YoungBinzelCrane2000" /><ref name="BuieTholenHorne1992" />
 
Current maps have been produced from images from the [[Hubble Space Telescope]] (HST), which offers the highest [[angular resolution|resolution]] currently available, and show considerably more detail,<ref name="Buie_web_map" /> resolving variations several hundred kilometres across, including polar regions and large bright spots.<ref name="Buie_2010 surface-maps" /> The maps are produced by complex computer processing, which find the best-fit projected maps for the few pixels of the Hubble images.<ref name="Buie_mapmaking" /> The two cameras on the HST used for these maps are no longer in service, so these will likely remain the most detailed maps of Pluto until the 2015 flyby of [[New Horizons]].<ref name="Buie_mapmaking" />
 
These maps, together with Pluto's lightcurve and the periodic variations in its infrared spectra, reveal that Pluto's surface is remarkably varied, with large changes in both brightness and colour.<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 colour varies between charcoal black, dark orange and white:<ref name="Hubble2010" /> Buie et al. term it "significantly less red than [[Mars]] and much more similar to the hues seen on [[Io (moon)|Io]] with a slightly more orange cast".<ref name="Buie_2010 surface-maps" />
 
Pluto's surface has changed between 1994 and 2002–3: the northern polar region has brightened and the southern hemisphere darkened.<ref name="Hubble2010" /> Pluto's overall redness has also increased substantially between 2000 and 2002.<ref name="Hubble2010" /> These rapid changes are probably related to seasonal condensation and [[sublimation (phase transition)|sublimation]] of portions of Pluto's [[Atmosphere of Pluto|atmosphere]], amplified by Pluto's extreme [[obliquity|axial tilt]] and high [[orbital eccentricity]].<ref name="Hubble2010" />
 
[[Spectroscopic]] analysis of Pluto's surface reveals it to be composed of more than 98 percent [[nitrogen]] ice, with traces of methane and carbon monoxide.<ref name="tobias" /> The face of Pluto oriented toward Charon contains more [[methane]] ice, whereas the opposite face contains more nitrogen and [[carbon monoxide]] ice.<ref name="Boyle1999-MSNBC" />
 
=== Structure ===
[[File:Pluto-cutaway.svg|thumb|Theoretical structure of Pluto (2006)<ref name="Hussmann2006" /> <br /> '''1.''' Frozen nitrogen<ref name="tobias" /> <br /> '''2.''' Water ice <br /> '''3.''' Rock]]
 
Observations by the [[Hubble Space Telescope]] place Pluto's density at between 1.8 and 2.1&nbsp;g/cm<sup>3</sup>, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass.<ref name="Solstation" /> 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 ice. The diameter of the core should be around 1,700&nbsp;km, 70% of Pluto's diameter.<ref name="Hussmann2006" /> It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some {{nowrap|100 to 180 km}} thick at the core–mantle boundary.<ref name="Hussmann2006" /><ref name="pluto.jhuapl Inside Story" /> The [[German Aerospace Center|DLR]] ''Institute of Planetary Research'' calculated that Pluto's density-to-radius ratio lies in a transition zone, along with Neptune's moon [[Triton (moon)|Triton]], between icy satellites like the mid-sized [[moons of Uranus]] and [[moons of Saturn|Saturn]], and rocky satellites such as Jupiter's [[Io (moon)|Io]].<ref name="three-up-mess" />
 
=== Mass and size ===
[[File:Pluto, Earth size comparison.jpg|thumb|left|upright|Pluto's volume is about 0.6% that of Earth]]
 
Pluto's mass is 1.31×10<sup>22</sup>&nbsp;kg, less than 0.24 percent that of Earth,<ref name="Davies2001" /> and its diameter is {{val|2306|20|u=km}}, or roughly 66% that of the Moon.<ref name="BuieGrundyYoung_2006" /> Pluto's atmosphere complicates determining its true solid size within a certain margin.<ref name="Young2007" /> Pluto's albedo varies from 0.49–0.66.
 
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#Deriving Kepler's third law|Newton's formulation of Kepler's third law]]. Once Charon's gravitational effect was measured, Pluto's true mass could be determined. 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" />
 
{| class="wikitable plainrowheaders" style="float: right; margin-right: 0; margin-left: 1em; text-align: center;"
|+ Selected size estimates for Pluto
! scope="col" | Year
! scope="col" | Radius (diameter)
! scope="col" | Notes
|-
! scope="row" | 1993
| 1195 (2390)&nbsp;km
| Millis, et al.<ref name="Millis 10.1006/icar.1993.1126" /> (If no haze)<ref name="Plutosize" />
|-
! scope="row" | 1993
| 1180 (2360)&nbsp;km
| Millis, et al. (surface & haze)<ref name="Plutosize" />
|-
! scope="row" | 1994
| 1164 (2328)&nbsp;km
| Young & Binzel<ref name="YoungBinzel 10.1006/icar.1994.1056" />
|-
! scope="row" | 2006
| 1153 (2306)&nbsp;km
| Buie, et al.<ref name="BuieGrundyYoung_2006" />
|-
! scope="row" | 2007
| 1161 (2322)&nbsp;km
| Young, Young, & Buie<ref name="Young2007" />
|}
 
Among the objects of the Solar System, Pluto is much less massive than the [[terrestrial planet]]s, and at less than 0.2 lunar masses, it is also less massive than seven [[natural satellite|moons]]: [[Ganymede (moon)|Ganymede]], [[Titan (moon)|Titan]], [[Callisto (moon)|Callisto]], [[Io (moon)|Io]], the [[Moon]], [[Europa (moon)|Europa]] and [[Triton (moon)|Triton]]. Pluto is so small relative to Earth that South America would not even fit on Pluto's surface.
 
Pluto is more than twice the diameter and a dozen times the mass of the [[dwarf planet]] [[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. Given the [[error bar]]s in the different size estimates, it is currently unknown whether Eris or Pluto has the larger diameter.<ref name="Plutosize" /> Both Pluto and Eris are estimated to have solid-body diameters of about 2330&nbsp;km.<ref name="Plutosize" /> Determinations of Pluto's size are complicated by its atmosphere, and possible hydrocarbon haze.<ref name="Plutosize" />
 
=== Atmosphere ===
[[File:ESO-L. Calçada - Pluto (by).jpg|thumb|left|CRIRES model-based computer-generated impression of the Plutonian surface, with atmospheric haze, and Charon and the Sun in the sky.]]
 
Pluto's [[atmosphere]] consists of a thin envelope of [[nitrogen]], [[methane]], and [[carbon monoxide]] gases, which are derived from the ices of these substances on its surface.<ref name="Croswell1992" /> Its surface pressure ranges from 6.5 to 24 μbar.<ref name="atmosphere2009" /> Pluto's elongated orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to [[Sublimation (phase transition)|sublimate]] into gas. This creates an [[anti-greenhouse effect]]; much as [[sweat]] cools the body as it evaporates from the surface of the skin, this sublimation cools the surface of Pluto. Scientists using the [[Submillimeter Array]] have recently discovered that Pluto's temperature is about {{convert|43|K|C|0|abbr=on}}, 10 K colder than would otherwise be expected.<ref name="KerThan2006-CNN" />
 
The presence of methane, a powerful [[greenhouse gas]], in Pluto's atmosphere creates a [[Inversion (meteorology)|temperature inversion]], with average temperatures 36&nbsp;K warmer 10&nbsp;km above the surface.<ref name="atmtemp" /> The lower atmosphere contains a higher concentration of methane than its upper atmosphere.<ref name="atmtemp" />
 
Evidence of Pluto's atmosphere was first suggested by [[Noah Brosch]] and Haim Mendelson of the Wise Observatory in Israel in 1985,<ref name="IAUC 4097" /> and then definitively detected by the [[Kuiper Airborne Observatory]] in 1988, from observations of [[occultation]]s of stars by Pluto.<ref>{{cite web|title=NOFS Contributes to SOFIA’S Successful Observation of Challenging Pluto Occultation|publisher=US Naval Observatory|url=http://www.usno.navy.mil/USNO/tours-events/NOFS_SOFIA_Release_110711.pdf|accessdate=2012-02-11}}</ref> When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.<ref name="IAUC 4097" /> From the rate of dimming, the atmospheric pressure was determined to be 0.15&nbsp;[[Pascal (unit)|pascal]], roughly 1/700,000 that of Earth.<ref name="Johnston2006" />
 
In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the [[Paris Observatory]],<ref name="Sicardy 10.1038/nature01766" /> [[James L. Elliot]] of [[MIT]],<ref name="MIT2002-Pluto global warming" /> and [[Jay Pasachoff]] of [[Williams College]].<ref name="WilliamsEdu2003" /> Surprisingly, the atmospheric pressure was estimated to be 0.3&nbsp;pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the north pole's now continuously dark ice cap.<ref name="britty" /> Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere.<ref name="britty" /> Another stellar occultation was observed by the MIT-Williams College team of James Elliot, [[Jay Pasachoff]], and a [[Southwest Research Institute]] team led by Leslie Young on June 12, 2006 from sites in Australia.<ref name="Elliot_2006" />
 
In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of [[ethane]] on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.<ref name="Stern2006" />
 
== Satellites ==
{{main|Moons of Pluto}}
Pluto has five known [[natural satellite]]s: [[Charon (moon)|Charon]], first identified in 1978 by astronomer [[James W. Christy|James Christy]]; [[Nix (moon)|Nix]] and [[Hydra (moon)|Hydra]], both discovered in 2005,<ref name="Gugliotta2005" /> [[Kerberos (moon)|Kerberos]], identified by the [[Hubble Space Telescope]] in 2011,<ref name="P4" /> and [[Styx (moon)|Styx]], 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=11 July 2012|work=Space.com|accessdate=11 July 2012}}</ref>
 
The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto at up to 53% (or 69%, if retrograde) of the [[Hill sphere|Hill]] radius, the stable gravitational zone of Pluto's influence. For example, [[Psamathe (moon)|Psamathe]] orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers' terms, the Plutonian system appears to be "highly compact and largely empty",<ref name="Sternetal 2006" /> although others have pointed out the possibility of additional objects, including a small ring system.<ref name="Steffl 2006" /><ref>{{cite news|url=http://news.discovery.com/space/astronomy/pluto-could-have-ten-moons.htm|title=Pluto Could Have Ten Moons|date=18 March 2013|work=Discovery.com|accessdate=25 March 2013}}</ref>
 
=== Charon ===
{{main|Charon (moon)}}
[[File:Pluto-Charon double planet.png|thumb|left|300px|An oblique view of the Pluto–Charon system showing that Pluto orbits a point outside itself. Pluto's orbit is shown in red and Charon's orbit is shown in green.]]
[[File:Charon.png|thumb|180px|The surface of [[Charon (moon)|Charon]]]]
The Pluto–Charon system is noteworthy for being one of the Solar System's few binary systems, defined as those whose [[Earth-Moon barycenter|barycentre]] lies above the primary's surface ([[617 Patroclus]] is a smaller example, the [[Mass of Jupiter|Sun and Jupiter]] the only larger one).<ref name="RichardsonWalsh2005" /> This and the large size of Charon relative to Pluto has led some astronomers to call it a [[double planet|dwarf double planet]].<ref name="Sicardy 10.1038/nature04351" /> The system is also unusual among planetary systems in that each is [[Tidal locking|tidally locked]] to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured.<ref name="Young1997" /> Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity.<ref name="axis" /> Just as Pluto revolves on its side relative to the orbital plane, so the Pluto–Charon system does also.<ref name="oregon" /> In 2007, observations by the [[Gemini Observatory]] of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.<ref name="spaceflightnow2007 Ice machine" />
 
=== Small moons ===
[[File:Pluto moon P5 discovery with moons' orbits.jpg|thumb|The Pluto system: Pluto, Charon, Nix, Hydra, Kerberos, and Styx, taken by the Hubble Space Telescope in July 2012]]
Two additional moons were imaged by astronomers working with the [[Hubble Space Telescope]] on May 15, 2005, and received [[Provisional designation in astronomy|provisional designations]] of S/2005&nbsp;P&nbsp;1 and S/2005&nbsp;P&nbsp;2. The International Astronomical Union officially named Pluto's newest moons [[Nix (moon)|Nix]] (or Pluto&nbsp;II, the inner of the two moons, formerly P&nbsp;2) and [[Hydra (moon)|Hydra]] (Pluto&nbsp;III, the outer moon, formerly P&nbsp;1), on June 21, 2006.<ref name="IAUC 8723"/>
 
These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular [[direct motion|prograde]] orbits in the same orbital plane as Charon.
 
Observations of Nix and Hydra to determine individual characteristics are ongoing. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Their sizes are estimated from albedos. If their albedo is similar to that of Charon (0.35), then their diameters are 46 kilometres for Nix and 61 kilometres for Hydra. Upper limits on their diameters can be estimated by using the albedo of the darkest Kuiper-belt objects (0.04); these bounds are 137&nbsp;±&nbsp;11&nbsp;km and 167&nbsp;±&nbsp;10&nbsp;km, respectively. At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% of that Pluto.<ref name="Weaver 2006"/>
 
The discovery of Nix and Hydra suggests that Pluto may possess a variable [[ring system]]. Small-body impacts can create debris that can form into planetary rings. Data from a deep-optical survey by the [[Advanced Camera for Surveys]] on the [[Hubble Space Telescope]] suggest that no ring system is present. If such a system exists, it is either tenuous like the [[rings of Jupiter]] or is tightly confined to less than 1,000&nbsp;km in width.<ref name="Steffl 2006"/> Similar conclusions have been made from occultation studies.<ref name="Pasachoff 2006"/>
 
A fourth moon, [[Kerberos (moon)|Kerberos]], was announced on July 20, 2011. It was detected using [[NASA]]'s Hubble Space Telescope during a survey searching for [[Planetary ring|rings]] around Pluto. It has an estimated diameter of 13 to 34&nbsp;km and is located between the orbits of Nix and Hydra.<ref name="P4"/> Kerberos was first seen in a photo taken with Hubble's Wide Field Camera 3 on June 28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18.<ref name="P4"/>
 
A fifth moon, [[Styx (moon)|Styx]], was announced on July 7, 2012 while looking for potential hazards for [[New Horizons]].<ref name="P5">[http://www.nasa.gov/mission_pages/hubble/science/new-pluto-moon.html Hubble Discovers a Fifth Moon Orbiting Pluto]</ref> Styx is believed to have a diameter of between 10 and 25 km and to orbit Pluto at a distance between Charon and Nix.<ref name="Sanders 2012-07-11">
{{cite web
| last = Sanders
| first = Ray
| date = 11 July 2012
| title = Hubble Space Telescope detects fifth moon of Pluto
| publisher = Phys.org
| url = http://phys.org/news/2012-07-hubble-space-telescope-moon-pluto.html
| accessdate = 11 July 2012
}}
</ref>
 
=== Near resonances ===
Styx, Nix, Kerberos and Hydra are fairly close to 3:1, 4:1, 5:1 and 6:1 mean-motion [[orbital resonance]]s with Charon, respectively<ref name="WardCanup2006" /><ref name="Matson">{{cite web
| last = Matson | first = J. | date = 11 July 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
| accessdate = 12 July 2012}}</ref> (the ratios approach integral [[Commensurability (astronomy)|commensurabilities]] more closely going outward from Pluto). Determining how near any of these orbital period ratios actually is to a true resonance requires accurate knowledge of the satellites' [[Apsidal precession|precessions]].
 
{| class="wikitable plainrowheaders" style="text-align: center;"
|+ Pluto and its satellites, with the [[Moon]] comparison<ref name="BuieGrundyYoung_2006">{{cite journal | author = Marc W. Buie, William M. Grundy, Eliot F. Young, Leslie A. Young, S. Alan Stern | title = ''Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2'' | journal = Astronomical Journal | year = 2006 | volume = 132 | issue = 1 | page = 290 | bibcode = 2006AJ....132..290B | doi = 10.1086/504422 | arxiv=astro-ph/0512491 }}</ref><ref name="Lakdawalla" />
|-
! scope="col" colspan="2" | Name <br> ([[Help:IPA for English|Pronunciation]])
! scope="col" | Discovery <br> Year
! scope="col" | Diameter <br> (km)
! scope="col" | Mass <br> (kg)
! scope="col" | Orbital radius (km) <br> (barycentric)
! scope="col" | Orbital period (d)
! scope="col" | Period ratio
! scope="col" | Magnitude (mag)
|-
! scope="row" | Pluto
| {{IPAc-en|ˈ|p|l|uː|t|oʊ}}
| 1930
| 2,306 <br> (66%&nbsp;Moon)
| 1.305 {{e|22}} <br> (18%&nbsp;Moon)
| 2,035
| 6.3872 <br> (25%&nbsp;Moon)
| 1.000
| 15.1
|-
! scope="row" | [[Charon (moon)|Charon]]
| {{IPAc-en||ˈ|ʃ|ær|ən}}, <br> {{IPAc-en|ˈ|k|ɛər|ən}}
| 1978
| 1,205 <br> (35%&nbsp;Moon)
| 1.52 {{e|21}} <br> (2%&nbsp;Moon)
| 17,536 <br> (5%&nbsp;Moon)
| 6.3872 <br> (25%&nbsp;Moon)
| 1.000
| 16.8
|-
! scope="row" | [[Styx (moon)|Styx]]
| {{IPAc-en|ˈ|s|t|ɪ|k|s}}
| 2012
| 10–25
| ?
| ~42,000 +/- 2,000
| 20.2 +/- 0.1
| 3.16
| 27
|-
! scope="row" | [[Nix (moon)|Nix]]
| {{IPAc-en|ˈ|n|ɪ|k|s}}
| 2005
| 91
| 4 {{e|17}}
| 48,708
| 24.856
| 3.892
| 23.7
|-
! scope="row" | [[Kerberos (moon)|Kerberos]]
| {{IPAc-en|ˈ|k|ɛər|b|ər|ə|s}}
| 2011
| 13–34
| ?
| ~59,000
| 32.1
| 5.03
| 26
|-
! scope="row" | [[Hydra (moon)|Hydra]]
| {{IPAc-en|ˈ|h|aɪ|d|r|ə}}
| 2005
| 114
| 8 {{e|17}}
| 64,749
| 38.206
| 5.982
| 23.3
|-
|}
 
{{clear}}
 
<small>Mass of Nix and Hydra assumes icy/porous density of 1.0 g/cm<sup>3</sup></small>
 
=== Quasi-satellite ===
At least one minor body is trapped in the 1:1 commensurability with Pluto, {{mpl|(15810) 1994 JR|1}}, specifically in the [[quasi-satellite]] dynamical state.<ref name="quasi">{{cite journal |last=de la Fuente Marcos & de la Fuente Marcos |title = Plutino 15810 (1994 JR1), an accidental quasi-satellite of Pluto |journal=Monthly Notices of the Royal Astronomical Society Letters |volume= 427|issue= |pages= L85|year=2012 |doi= 10.1111/j.1745-3933.2012.01350.x|arxiv=1209.1577 |bibcode= 2012MNRAS.427L..85D |last2=De La Fuente Marcos |first2=R. |first1=C.}}</ref> The object has been a quasi-satellite of Pluto for about 100,000 years and it will remain in that dynamical state for perhaps another 250,000 years. Its quasi-satellite behavior is recurrent with a periodicity of 2 million years.<ref name=quasi/><ref name=S&T>{{cite web | url = http://www.skyandtelescope.com/news/Plutos-Fake-Moon-170998771.html |
title = Pluto's fake moon | accessdate = 2012-09-24}}</ref> There may be additional Pluto co-orbitals.
 
== Origins ==
{{main|Kuiper belt|Nice model}}
[[File:Outersolarsystem objectpositions labels comp.png|thumb|300px|Plot of known Kuiper belt objects, set against the four [[gas giants]].]]
 
Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, [[Triton (moon)|Triton]]. This notion has been heavily criticised because Pluto never comes near Neptune in its orbit.<ref name="pluto.jhuapl plutoOrbit" />
 
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 believed to be the source of many [[short-period comet]]s. Astronomers now believe Pluto to be the largest{{efn|name=wiki-kbo}} member of the [[Kuiper belt]], a somewhat stable ring of objects located between 30 and 50 AU from the Sun. Like other Kuiper-belt objects (KBOs), Pluto shares features with [[comets]]; for example, the [[solar wind]] is gradually blowing Pluto's surface into space, in the manner of a comet.<ref name="pluto.jhuapl cousin" /> If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.<ref name="Tyson1999" />
 
Though Pluto is the largest of the Kuiper belt objects discovered so far, Neptune's moon [[Triton (moon)|Triton]], which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt object.<ref name="PlanetaryOrg Triton" /> Eris ([[#Classification|see below]]) 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]].
 
A large number of Kuiper belt objects, like Pluto, possess a 2:3 orbital resonance with Neptune. KBOs with this orbital resonance are called "[[plutino]]s", after Pluto.<ref name="Jewitt2004" />
 
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 current 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 believed to have been placed in their current positions by interactions with Neptune's migrating resonances.<ref name="Hahn2005" /> A computer model created in 2004 by Alessandro Morbidelli of the [[Côte d'Azur Observatory|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 Jupiter's [[Trojan asteroid]]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="Malhorta1995" /> The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk; these may have included the early Triton and Eris.<ref name="Levison2007" />
 
== Exploration ==
{{main|New Horizons}}
[[File:New Horizons Jan19 06.jpg|thumb|upright|''[[New Horizons]]'', launched on January 19, 2006]]
<!---[[Pluto Fast Flyby]]--->
Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. ''[[Voyager 1]]'' could have visited Pluto, but controllers opted instead for a close flyby of [[Saturn|Saturn's]] moon Titan, resulting in a trajectory incompatible with a Pluto flyby. ''[[Voyager 2]]'' never had a plausible trajectory for reaching Pluto.<ref name="JPL Voyager FAQ" /> No serious attempt to explore Pluto by spacecraft occurred until the last decade of the 20th century. In August 1992, [[JPL]] scientist [[Robert Staehle]] telephoned Pluto's discoverer, [[Clyde Tombaugh]], requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip."<ref name="Sobel1993" /> Despite this early momentum, in 2000, NASA cancelled the ''[[Pluto Kuiper Express]]'' mission, citing increasing costs and launch vehicle delays.<ref name="Williams2005" />
 
[[File:First Pluto sighting from New Horizons.gif|thumb|left|First Pluto sighting from ''New Horizons'']]
After an intense political battle, a revised mission to Pluto, dubbed ''[[New Horizons]]'', was granted funding from the US government in 2003.<ref name="Britt2003" /> ''New Horizons'' was launched successfully on January 19, 2006. The mission leader, [[S. Alan Stern]], confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.<ref name="Stern2006 Tombaugh 100th" />
 
In early 2007 the craft made use of a [[gravity assist]] from [[Jupiter]]. Its closest approach to Pluto will be on July 14, 2015; scientific observations of Pluto will begin 5 months before closest approach and will continue for at least a month after the encounter. ''New Horizons'' captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI).<ref name="pluto.jhuapl First Pluto Sighting" /> The images, taken from a distance of approximately 4.2 billion kilometres, confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.
 
''New Horizons'' will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterise the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyse Pluto's neutral atmosphere and its escape rate. ''New Horizons'' will also photograph the surfaces of Pluto and Charon.
 
Pluto's small moons, discovered shortly before or after the probes's launch, may present it with unforeseen challenges. Debris from collisions between Kuiper belt objects and the smaller moons, with their relatively low escape velocities, may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for {{dp|micrometeoroid}} damage that could disable the probe.<ref name="Steffl 2006" />
 
=== Concepts ===
A Pluto orbiter/lander/sample return mission was proposed in 2003. The plan included a twelve-year trip from Earth to Pluto, mapping from orbit, multiple landings, a warm water probe, and possible ''in situ'' propellant production for another twelve-year trip back to Earth with samples. Power and propulsion would come from the bimodal MITEE nuclear reactor system.<ref name="powell" />
 
{{clear}}
 
== Classification ==
{{main|Definition of planet}}
{{TNO imagemap}}
 
After Pluto's place within the Kuiper belt was determined, its official status as a planet became 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. The [[Hayden Planetarium]] reopened after renovation in 2000 with a model of only eight planets. The controversy made headlines at the time.<ref name="Tyson2001" />
 
In 2002, the KBO [[50000 Quaoar]] was discovered, with a diameter then thought to be roughly 1280 kilometres, about half that of Pluto.<ref name="BrownTrujillo2004" /> In 2004, the discoverers of [[90377 Sedna]] placed an upper limit of 1800&nbsp;km on its diameter, nearer to Pluto's diameter of 2320&nbsp;km,<ref name="GrundyNollStephens2005" /> although Sedna's diameter was revised downward to less than 1600&nbsp;km by 2007.<ref name="Stansberry_2007" /> Just as [[Ceres (dwarf planet)|Ceres]], [[2 Pallas|Pallas]], [[3 Juno|Juno]] and [[4 Vesta|Vesta]] eventually lost their planet status after the discovery of many other [[asteroid]]s, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects.
 
On July 29, 2005, the discovery of a new [[trans-Neptunian object]] was announced. Named [[Eris (dwarf planet)|Eris]], it is now known to be approximately the same size as Pluto.<ref name="Plutosize" /> This was the largest 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 name="what" />
 
=== 2006: IAU classification ===
{{main|IAU definition of planet}}
 
The debate came to a head in 2006 with an [[2006 definition of planet|IAU resolution]] that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':
# The object must be in orbit around the [[Sun]].
# The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of [[hydrostatic equilibrium]].
# It must have [[Clearing the neighbourhood|cleared the neighbourhood]] around its orbit.<ref name="IAU2006 GA26-5-6" /><ref name="IAU0603" />
 
Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit).<ref name="what" /><ref name="IAU0603" /> The IAU further resolved that Pluto be classified in the simultaneously created [[dwarf planet]] category, and that it act as the prototype for the [[plutoid]] category of [[trans-Neptunian object]]s, in which it would be separately, but concurrently, classified.<ref name="IAU0804" />
 
On September 13, 2006, the IAU included Pluto, [[Eris (dwarf planet)|Eris]], and the Eridian moon [[Dysnomia (moon)|Dysnomia]] in their [[Minor Planet Catalogue]], giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia".<ref name="IAUC 8747" /> If Pluto had been given a [[minor planet name]] upon its discovery, the number would have been about 1,164 rather than 134,340.
 
There has been some resistance within the astronomical community toward the reclassification.<ref name="geoff2006c"/><ref name="Ruibal-1999" /><ref name="Britt-2006" /> [[Alan Stern]], principal investigator with [[NASA]]'s ''[[New Horizons]]'' mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons".<ref name="geoff2006a"/> Stern's contention is that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded.<ref name="newscientistspace"/> His other claim is 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]] of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitioners against the definition.<ref name="Buie2006 IAU response"/> Others have supported the IAU. Mike Brown, the astronomer who discovered [[Eris (dwarf planet)|Eris]], said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It's been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."<ref name="Overbye2006" />
 
Researchers on both sides of the debate gathered on August 14–16, 2008, at [[The Johns Hopkins University Applied Physics Laboratory]] for a conference that included back-to-back talks on the current 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" /> Just before the conference, on June 11, 2008, the IAU announced in a press release that the term "[[plutoid]]" would henceforth be used to describe Pluto and other objects similar to Pluto which have an orbital [[semimajor axis]] greater than that of Neptune and enough mass to be of near-spherical shape.<ref name="IAU0804" /><ref name="Discover 2009-JANp76" /><ref name="Science News, July 5, 2008 p. 7"/>
 
==== Reaction ====
[[File:Pluto Protest and Counter Protest.jpg|thumb|A promotional event with a staged Pluto "protest". Members playing protesters of the reclassification of Pluto on the left, with those playing counter-protesters on the right]]
 
Reception to the IAU decision was mixed. Although many accepted the reclassification, some seek to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the [[California State Assembly]] light-heartedly denounces the IAU for "scientific heresy", among other crimes.<ref name="DeVore2006" /> The U.S. state of [[New Mexico]]'s [[New Mexico House of Representatives|House of Representatives]] passed a resolution in honor of Tombaugh, a longtime resident of that state, which 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]] [[Illinois Senate|State Senate]] passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, 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" />
 
== See also ==
{{Portal|Solar System}}
{{Wikipedia books
|1=Dwarf Planets Of The Solar System & Their Satellites
|3=Solar System
}}
* ''[[How I Killed Pluto and Why It Had It Coming]]''
* [[Planets in astrology#Pluto|Pluto in astrology]]
* [[Pluto in fiction]]
* [[Solar eclipses on Pluto]]
 
== Notes ==
{{notes
| notes =
{{efn
| name = HST-SynColours
| The HST observations were made in two wavelengths, which is insufficient to directly make a true-colour image. The surface maps at each wavelength limit the shape of the [[spectrum]] that could be produced by the materials that are potentially on Pluto's surface. These spectra, generated for each resolved point on the surface, are then converted to the [[RGB]] colour values seen here. See Buie et al, 2010.
}}
 
{{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 = Axial tilt
| Based on the orientation of Charon's orbit, which is assumed the same as Pluto's spin axis due to the mutual [[tidal locking]].
}}
 
{{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,[http://www.mikebrownsplanets.com/2010/11/how-big-is-pluto-anyway.html] but 28% more massive. [[Eris (dwarf planet)|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 if the scattered objects are excluded.
}}
 
{{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, until the discovery of Pluto's outer moons in late 2005. As a result, because Pluto came to perihelion in 1989, most Pluto perihelion date estimates are based on the Pluto–Charon [[Earth-Moon barycenter|barycentre]]. Charon came to perihelion [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=901 1989-Sep-04.] The Pluto–Charon barycenter came to perihelion [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=9 1989-Sep-05.] Pluto came to perihelion [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=mb&sstr=999 1989-Sep-08.]
}}
 
}}
 
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{{cite web
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{{cite web
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{{cite web
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{{cite journal
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{{cite news
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{{cite news
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{{cite news
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{{cite news
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<ref name="Buie2006 IAU response">
{{cite web
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|date=September 2006
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| publisher = Southwest Research Institute
| url = http://www.boulder.swri.edu/~buie/pluto/iauresponse.html
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<ref name="Overbye2006">
{{cite news
| first = Dennis
| last = Overbye
| date = 2006-08-24
| title = Pluto Is Demoted to 'Dwarf Planet'
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<ref name="Minkel2008">
{{cite journal
| date = April 10, 2008
| title = Is Rekindling the Pluto Planet Debate a Good Idea?
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{{cite web
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{{cite news
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{{cite journal
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{{cite web
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| year = 2007
| title = A joint memorial. Declaring Pluto a planet and declaring March 13, 2007, 'Pluto planet day' at the legislature
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{{cite web
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{{cite news
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{{cite web
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}}
 
== Bibliography ==
* {{cite book
  | last = Croswell
  | first = Ken
  | year = 1997
  | title = Planet Quest: The Epic Discovery of Alien Solar Systems
  | publisher = The Free Press
  | location = New York
  | isbn = 978-0-684-83252-4
  | ref = harv
  }}
* {{cite book
  | title = How I Killed Pluto and Why It Had It Coming
  | last = Brown
  | first = Mike
  | year = 2010
  | isbn = 0-385-53108-7
  }}
* Stern, Allen & Jaqueline Mitton (2005) ''Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System'', [[Wiley-VCH]] ISBN 3-527-40556-9 .
 
== External links ==
{{Sisterlinks|Pluto}}
* [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]
* [http://www.lowell.edu/ Website of the observatory that discovered Pluto]
* [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]
* {{cite web|last=Gray|first=Meghan|title=Pluto|url=http://www.sixtysymbols.com/videos/pluto.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]|year=2009}}
 
{{Pluto}}
{{Trans-Neptunian dwarf planets}}
{{Trans-Neptunian objects}}
{{Solar System}}
{{MinorPlanets Navigator|(134339) 5628 T-3|(134341) 1979 MA|PageName=(134340) Pluto|state=autocollapse}}
{{Featured article}}
 
[[Category:Pluto| ]]
[[Category:Astronomical objects discovered in 1930]]
[[Category:Plutinos]]
 
{{Link FA|cs}}
{{Link FA|de}}
{{Link FA|fr}}
{{Link FA|mr}}
{{Link FA|pt}}
{{Link FA|ru}}

Latest revision as of 16:58, 20 December 2014

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