Pluto 1

PlutoThis article is about the dwarf planet. For other uses, see Pluto (disambiguation).

Pluto

Computer-generated map of Pluto[1] and among the highest resolutions possible with current technology.</ref>

Discovery

Discovered by Clyde W. Tombaugh

Discovery date February 18, 1930

Designations

MPC designation 134340 Pluto

Pronunciation i/ˈpluːtoʊ/

Named after Pluto

Minor planet category •• dwarf planet•• TNO•• plutoid•• KBO•• plutino

Adjectives Plutonian

Orbital characteristics[2]

Epoch J2000

Aphelion •• 7311000000 km• 48.871 AU

Perihelion •• 4437000000 km•• 29.657 AU•• (1989 Sep 05)

Semi-major axis •• 5874000000 km•• 39.264 AU

Eccentricity 0.244671664 (J2000)0.248 807 66 (mean)

Orbital period •• 90465 d• 247.68 years• 14164.4 Plutonian solar days

Synodic period 366.73 days

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Average orbital speed 4.7 km/s

Mean anomaly 14.86012204°

Inclination •• 17.151394°•• (11.88° to Sun's equator)

Longitude of ascending node 110.28683°

Argument of perihelion 113.76349°

Known satellites 5

Physical characteristics

Mean radius •• 1184±10 km• 0.18 Earths•• 1161 km (solid)

Surface area • 1.665×107 km2

•• 0.033 Earths

Volume • 6.39×109 km3

•• 0.0059 Earths

Mass • (1.305±0.007)×1022 kg• 0.00218 Earths• 0.178 Moons

Mean density 2.03±0.06 g/cm3

Surface gravity • 0.658 m/s2

• 0.067 g

Escape velocity 1.229 km/s

Sidereal rotation period • −6.387230 d•• 6 d, 9 h, 17 m, 36 s

Equatorial rotation velocity 47.18 km/h

Axial tilt 119.591°±0.014° (to orbit)

North pole right ascension 312.993°

North pole declination 6.163°

Albedo 0.49 to 0.66 (geometric, varies by 35%)

Surface temp. min mean max

Kelvin 33 K 44 K (−229 °C) 55 K

Apparent magnitude 13.65 to 16.3(mean is 15.1)

Absolute magnitude (H) −0.7

Angular diameter 0.065″ to 0.115″

Atmosphere

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Surface pressure 0.30 Pa (summer maximum)

Composition nitrogen, methane, carbon monoxide

Pluto (minor-planet designation 134340 Pluto) is the largest object in the Kuiper belt, and the tenth-most-massivebody observed directly orbiting the Sun. It is the second-most-massive known dwarf planet, after Eris. Like otherKuiper-belt objects, Pluto is composed primarily of rock and ice and is relatively small, approximately one-sixth themass of the Moon and one-third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune, butan orbital resonance with Neptune prevents the bodies from colliding. In 2014 it was 32.6 AU from the Sun.Discovered in 1930, Pluto was originally classified as the ninth planet from the Sun. Its status as a major planet fellinto question following further study of it and the outer Solar System over the ensuing 75 years. Starting in 1977with the discovery of the minor planet 2060 Chiron, numerous icy objects similar to Pluto with eccentric orbits werefound. The most notable of these was the scattered disc object Eris, discovered in 2005, which is 27% more massivethan Pluto. The understanding that Pluto is only one of several large icy bodies in the outer Solar System promptedthe International Astronomical Union (IAU) to define formally in 2006 what it means to be a "planet". Thisdefinition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as aplutoid). Astronomers who oppose this decision hold that Pluto should have remained classified as a planet, and thatother dwarf planets and even moons should be added to the roster of planets along with Pluto.[3]

Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Nix, Hydra, Kerberos,and Styx. Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits doesnot lie within either body. The IAU has yet to formalise a definition for binary dwarf planets, and Charon isofficially classified as a moon of Pluto.On July 14, 2015, the Pluto system is due to be visited by spacecraft for the first time. The New Horizons probe willperform a flyby during which it will attempt to take detailed measurements and images of the plutoid and its moons.

DiscoveryMain article: Planets beyond NeptuneIn the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planetNeptune after analysing perturbations in the orbit of Uranus.[4] Subsequent observations of Neptune in the late 19thcentury 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, in1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X". By 1909, Lowelland William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and hisobservatory 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 recognized for what they were. There are fifteenother known prediscoveries, with the oldest made by the Yerkes Observatory on August 20, 1909.Because of a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest theobservatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,[5]

when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a23-year-old Kansan who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample ofhis astronomical drawings.[5]

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 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,

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Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of thatyear. A lesser-quality photograph taken on January 21 helped confirm the movement.[6] After the observatoryobtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard CollegeObservatory on March 13, 1930.

NameMain article: Venetia BurneyThe discovery made headlines across the globe. The Lowell Observatory, which had the right to name the newobject, received over 1,000 suggestions from all over the world, ranging from Atlas to Zymal. Tombaugh urgedSlipher to suggest a name for the new object quickly before someone else did. Constance Lowell proposed Zeus,then Percival and finally Constance. These suggestions were disregarded.The name Pluto, after the god of the underworld, was proposed by Venetia Burney (1918–2009), an eleven-year-oldschoolgirl in Oxford, England, who was interested in classical mythology. She suggested it in a conversation withher grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library, who passed thename to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States.The object was officially named on March 24, 1930. Each member of the Lowell Observatory was allowed to voteon a short-list of three: Minerva (which was already the name for an asteroid), Cronus (which had lost reputationthrough being proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto receivedevery vote.[7] The name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia GB£5 (£276as of 2014),[8] as a reward.The choice of name was partly inspired by the fact that the first two letters of Pluto are the initials of PercivalLowell, and Pluto's astronomical symbol ( , unicode U+2647, ♇) is a monogram constructed from the letters 'PL'.Pluto's astrological symbol resembles that of Neptune ( ), but has a circle in place of the middle prong of thetrident ( ).The name was soon embraced by wider culture. In 1930, Walt Disney was apparently inspired by it when heintroduced for Mickey Mouse a canine companion named Pluto, although Disney animator Ben Sharpsteen could notconfirm why the name was given. In 1941, Glenn T. Seaborg named the newly created element plutonium afterPluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, whichwas named after Uranus, and neptunium, which was named after Neptune.Most languages use the name "Pluto" in various transliterations.[9] In Japanese, Houei Nojiri suggested thetranslation Meiōsei (冥 王 星?, "King of the Underworld Star") , and this was borrowed into Chinese, Korean, andVietnamese. Some Indian languages use the name Pluto, but others such as Hindi use the name of Yama, theGuardian of Hell in Hindu and Buddhist mythology, as does Vietnamese. Polynesian languages also tend to use theindigenous god of the underworld, as in Maori Whiro.

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Demise of Planet X

Clyde W. Tombaugh, the discovererof Pluto

Mass estimates for Pluto

Year Mass Notes

1931 1 Earth Nicholson & Mayall

1948 0.1 (1/10) Earth Kuiper

1976 0.01 (1/100) Earth Cruikshank, Pilcher, & Morrison[10]

1978 0.002 (1/500) Earth Christy & Harrington

2006 0.00218 (1/459) Earth Buie et al.

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 wascalculated to be roughly the mass of Earth, with further calculations in 1948 bringing the mass down to roughly thatof Mars. In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto'salbedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionallyluminous for its size and therefore could not be more than 1 percent the mass of Earth.[10] (Pluto's albedo is 1.3–2.0times greater than that of Earth.)In 1978, the discovery of Pluto's 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. Subsequentsearches for an alternative Planet X, notably by Robert Sutton Harrington, failed. In 1992, Myles Standish used datafrom Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5%, to recalculateits gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for aPlanet X, vanished. Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist. Lowellhad 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, a view still held today.

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Orbit and rotation

Pluto's orbit and the ecliptic

Orbit of Pluto—ecliptic view. This "side view" ofPluto's orbit (in red) shows its large inclination to

Earth's ecliptic orbital plane.

This diagram shows the relative positions ofPluto (red) and Neptune (blue) on selected dates.

The size of Neptune and Pluto is depicted asinversely proportional to the distance between

them to emphasise the closest approach in 1896.

Pluto's orbital period is 248 Earth years. Its orbital characteristics aresubstantially different from those of the planets, which follow nearlycircular orbits around the Sun close to a flat reference plane called theecliptic. In contrast, Pluto's orbit is highly inclined relative to theecliptic (over 17°) and highly eccentric (elliptical). This higheccentricity means a small region of Pluto's orbit lies nearer the Sunthan Neptune's. The Pluto–Charon barycenter came to perihelion onSeptember 5, 1989, and was last closer to the Sun than Neptunebetween February 7, 1979, and February 11, 1999. Pluto and Neptunemake their closest approach when it is at 27.960 AU.

In the long term, Pluto's orbit is in fact chaotic. Although computersimulations can be used to predict its position for several million years(both forward and backward in time), after intervals longer than theLyapunov time of 10–20 million years, calculations becomespeculative: Pluto is sensitive to unmeasurably small details of theSolar System, hard-to-predict factors that will gradually disrupt itsorbit. Millions of years from now, Pluto may well be at aphelion, atperihelion or anywhere in between, with no way for us to predictwhich. This does not mean Pluto's orbit itself is unstable, but itsposition on that orbit is impossible to determine so far ahead. Severalresonances and other dynamical effects keep Pluto's orbit stable, safefrom planetary collision or scattering.

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Relationship with Neptune

Orbit of Pluto—polar view. This "view fromabove" shows how Pluto's orbit (in red) is less

circular than Neptune's (in blue), and how Plutois sometimes closer to the Sun than Neptune. Thedarker 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 viewedfrom directly above, the two objects' orbits are aligned so that they cannever collide or even approach closely. There are several reasons why.At the simplest level, one can examine the two orbits and see that theydo not intersect. When Pluto is closest to the Sun, and hence closest toNeptune's orbit as viewed from above, it is also the farthest aboveNeptune's path. Pluto's orbit passes about 8 AU above that of Neptune,preventing a collision. Pluto's ascending and descending nodes, thepoints at which its orbit crosses the ecliptic, are currently separatedfrom Neptune's by over 21°.

This alone is not enough to protect Pluto; perturbations from theplanets (especially Neptune) could alter aspects of Pluto's orbit (suchas its orbital precession) over millions of years so that a collision couldbe possible. Some other mechanism or mechanisms must therefore beat work. The most significant of these is that Pluto lies in the 2:3 meanmotion resonance with Neptune: for every two orbits that Pluto makesaround the Sun, Neptune makes three. The two objects then return totheir initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is such that, in each500-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 ofPluto. Pluto and Neptune's minimum separation is over 17 AU. Pluto comes closer to Uranus (11 AU) than it does toNeptune.

The 2:3 resonance between the two bodies is highly stable, and is preserved over millions of years. This preventstheir orbits from changing relative to one another; the cycle always repeats in the same way, and so the two bodiescan never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could nevercollide.

Other factorsNumerical studies have shown that over periods of millions of years, the general nature of the alignment betweenPluto and Neptune's orbits does not change. There are several other resonances and interactions that govern thedetails 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 itis closest to the Sun, librates around 90°. This means that when Pluto is nearest the Sun, it is at its farthest above theplane of the Solar System, preventing encounters with Neptune. This is a direct consequence of the Kozaimechanism, which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this caseNeptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion tothe orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000years.Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—are innear-resonance with the above libration. When the two longitudes are the same—that is, when one could draw astraight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and hence it comes closest to theSun 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.

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To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distantvantage point where the planets orbit counterclockwise. After passing the ascending node, Pluto is interior toNeptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the twocauses angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly largerorbit, where it travels slightly slower, according to Kepler's third law. As its orbit changes, this has the gradual effectof changing the perihelion and longitude of Pluto's orbit (and, to a lesser degree, of Neptune). After many suchrepetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch up withPluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, andPluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptuneagain at the original node. The whole process takes about 20,000 years to complete.

RotationPluto's rotation period, its day, is equal to 6.39 Earth days. Like Uranus, Pluto rotates on its "side" on its orbitalplane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its solstices, one-fourth of its surface is incontinuous daylight, whereas another fourth is in continuous darkness.

Physical characteristics

Map of Pluto's surface by NASA, ESA and Marc W. Buie

Pluto's distance from Earth makes in-depthinvestigation difficult. Many details aboutPluto will remain unknown until 2015, whenthe New Horizons spacecraft is expected toarrive there.

Appearance and surface

Pluto's visual apparent magnitude averages15.1, brightening to 13.65 at perihelion. Tosee it, a telescope is required; around 30 cm(12 in) aperture being desirable. It looksstar-like and without a visible disk even inlarge telescopes, because its angulardiameter is only 0.11".

The earliest maps of Pluto, made in the late1980s, were brightness maps created fromclose observations of eclipses by its largestmoon, Charon. Observations were made ofthe change in the total average brightness ofthe Pluto–Charon system during theeclipses. For example, eclipsing a brightspot on Pluto makes a bigger totalbrightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create abrightness map. This method can also track changes in brightness over time.

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Hubble map of Pluto's surface, showing greatvariations in color and albedo

Three views of Pluto from different orientations

Current maps have been produced from images from the Hubble SpaceTelescope (HST), which offers the highest resolution currentlyavailable, and show considerably more detail, resolving variationsseveral hundred kilometres across, including polar regions and largebright spots. The maps are produced by complex computer processing,which find the best-fit projected maps for the few pixels of the Hubbleimages. The two cameras on the HST used for these maps are nolonger in service, so these will likely remain the most detailed maps ofPluto until the 2015 flyby of New Horizons.

These maps, together with Pluto's lightcurve and the periodicvariations in its infrared spectra, reveal that Pluto's surface isremarkably varied, with large changes in both brightness and color.Pluto is one of the most contrastive bodies in the Solar System, with asmuch contrast as Saturn's moon Iapetus. The color varies betweencharcoal black, dark orange and white: Buie et al. term it "significantlyless red than Mars and much more similar to the hues seen on Io with aslightly more orange cast".

Pluto's surface has changed between 1994 and 2002–3: the northern polar region has brightened and the southernhemisphere darkened. Pluto's overall redness has also increased substantially between 2000 and 2002. These rapidchanges are probably related to seasonal condensation and sublimation of portions of Pluto's atmosphere, amplifiedby Pluto's extreme axial tilt and high orbital eccentricity.Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen ice, with tracesof methane and carbon monoxide. The face of Pluto oriented toward Charon contains more methane ice, whereas theopposite face contains more nitrogen and carbon monoxide ice.

Structure

Theoretical structure of Pluto (2006)1. Frozen nitrogen

2. Water ice3. Rock

Observations by the Hubble Space Telescope place Pluto's density atbetween 1.8 and 2.1 g/cm3, suggesting its internal composition consistsof roughly 50–70 percent rock and 30–50 percent ice by mass.Because the decay of radioactive elements would eventually heat theices enough for the rock to separate from them, scientists expect thatPluto's internal structure is differentiated, with the rocky materialhaving settled into a dense core surrounded by a mantle of ice. Thediameter of the core is hypothesized to be approximately 1700 km,70% of Pluto's diameter. It is possible that such heating continuestoday, creating a subsurface ocean layer of liquid water some 100 to180 km thick at the core–mantle boundary. The DLR Institute ofPlanetary Research calculated that Pluto's density-to-radius ratio liesin a transition zone, along with Neptune's moon Triton, between icysatellites like the mid-sized moons of Uranus and Saturn, and rockysatellites such as Jupiter's Io.

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Mass and size

Pluto's volume is about 0.6% that ofEarth

Pluto's mass is 1.31×1022 kg, less than 0.24 percent that of Earth, and itsdiameter is 2306±20 km, or roughly 66% that of the Moon. Its surface area(1.665×107 km2) is about 10% smaller than that of South America. Pluto'satmosphere complicates determining its true solid size within a certain margin.Pluto's albedo varies from 0.49–0.66.

The discovery of Pluto's satellite Charon in 1978 enabled a determination of themass of the Pluto–Charon system by application of Newton's formulation ofKepler's third law. Once Charon's gravitational effect was measured, Pluto's truemass could be determined. Observations of Pluto in occultation with Charonallowed scientists to establish Pluto's diameter more accurately, whereas theinvention of adaptive optics allowed them to determine its shape more

accurately.

Selected size estimates for Pluto

Year Radius (diameter) Notes

1993 1195 (2390) km Millis, et al. (If no haze)

1993 1180 (2360) km Millis, et al. (surface & haze)

1994 1164 (2328) km Young & Binzel

2006 1153 (2306) km Buie, et al.

2007 1161 (2322) km Young, Young, & Buie

2011 1180 (2360) km Zalucha, et al.

2014 1184 (2368) km Lellouch, et al.

Among the objects of the Solar System, Pluto is much less massive than the terrestrial planets, and at less than 0.2lunar masses, it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton.Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in theasteroid belt. It is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005. Given theerror bars in the different size estimates, it is currently unknown whether Eris or Pluto has the larger diameter. BothPluto and Eris are estimated to have solid-body diameters of about 2330 km.Determinations of Pluto's size are complicated by its atmosphere, and possible hydrocarbon haze. In March, 2014,Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with aPlutonian diameter greater than 2360 km, with a "best guess" of 2368 km, which would make it slightly larger thanEris.

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Atmosphere

CRIRES model-based computer-generatedimpression of the Plutonian surface, with

atmospheric haze, and Charon and the Sun in thesky.

Pluto's atmosphere consists of a thin envelope of nitrogen, methane,and carbon monoxide gases, which are derived from the ices of thesesubstances on its surface. Its surface pressure ranges from 6.5 to 24μbar. Pluto's elongated orbit is predicted to have a major effect on itsatmosphere: as Pluto moves away from the Sun, its atmosphere shouldgradually freeze out, and fall to the ground. When Pluto is closer to theSun, the temperature of Pluto's solid surface increases, causing the icesto sublimate into gas. This creates an anti-greenhouse effect; much assweat cools the body as it evaporates from the surface of the skin, thissublimation cools the surface of Pluto. Scientists using theSubmillimeter Array have recently discovered that Pluto's temperature

is about 43 K (−230 °C), 10 K colder than would otherwise be expected.

The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, withaverage temperatures 36 K warmer 10 km above the surface. The lower atmosphere contains a higher concentrationof methane than its upper atmosphere.Evidence of Pluto's atmosphere was first suggested by Noah Brosch and Haim Mendelson of the Wise Observatoryin Israel in 1985, and then definitively detected by the Kuiper Airborne Observatory in 1988, from observations ofoccultations of stars by Pluto. When an object with no atmosphere moves in front of a star, the star abruptlydisappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmospheric pressurewas determined to be 0.15 pascal, roughly 1/700,000 that of Earth.In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the ParisObservatory, James L. Elliot of MIT, and Jay Pasachoff of Williams College. Surprisingly, the atmospheric pressurewas estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have beencolder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole ofPluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. Itwill take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the north pole's nowcontinuously dark ice cap. Spikes in the data from the same study revealed what may be the first evidence of wind inPluto's atmosphere. Another stellar occultation was observed by the MIT-Williams College team of James L. Elliot,Jay Pasachoff, and a Southwest Research Institute team led by Leslie A. Young on June 12, 2006 from sites inAustralia.In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and hiscolleagues announced the spectroscopic discovery of ethane on Pluto's surface. This ethane is produced from thephotolysis or radiolysis (i.e. the chemical conversion driven by sunlight and charged particles) of frozen methane onPluto's surface and suspended in its atmosphere.

SatellitesMain article: Moons of PlutoPluto has five known natural satellites: Charon, first identified in 1978 by astronomer James Christy; Nix and Hydra,both discovered in 2005, Kerberos, identified by the Hubble Space Telescope in 2011, and Styx, discovered in 2012.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 radius, the stable gravitational zone of Pluto's influence. For example, 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", although others have pointed out the possibility of additional objects, including a small ring

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system.

CharonMain article: Charon (moon)

An oblique view of the Pluto–Charon system showing that Pluto orbits a pointoutside itself. Pluto's orbit is shown in red and Charon's orbit is shown in green.

The surface of Charon

The Pluto–Charon system is noteworthy forbeing one of the Solar System's few binarysystems, defined as those whose barycenterlies above the primary's surface (617Patroclus is a smaller example, the Sun andJupiter the only larger one). This and thelarge size of Charon relative to Pluto has ledsome astronomers to call it a dwarf doubleplanet. The system is also unusual amongplanetary systems in that each is tidallylocked to the other: Charon always presentsthe same face to Pluto, and Pluto alwayspresents the same face to Charon: from anyposition on either body, the other is alwaysat the same position in the sky, or alwaysobscured. This also means that the rotationperiod of each is equal to the time it takesthe entire system to rotate around itscommon center of gravity. Just as Plutorevolves on its side relative to the orbitalplane, so the Pluto–Charon system doesalso. In 2007, observations by the GeminiObservatory of patches of ammoniahydrates and water crystals on the surface ofCharon suggested the presence of activecryo-geysers.

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Small moons

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 theHubble Space Telescope on May 15, 2005, and received provisionaldesignations of S/2005 P 1 and S/2005 P 2. The InternationalAstronomical Union officially named Pluto's newest moons Nix (orPluto II, the inner of the two moons, formerly P 2) and Hydra(Pluto III, the outer moon, formerly P 1), on June 21, 2006.

These small moons orbit Pluto at approximately two and three timesthe distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800kilometres from the barycenter of the system. They have nearlycircular prograde orbits in the same orbital plane as Charon.

Observations of Nix and Hydra to determine individual characteristicsare ongoing. Hydra is sometimes brighter than Nix, suggesting eitherthat it is larger or that different parts of its surface may vary inbrightness. Their sizes are estimated from albedos. If their albedo issimilar to that of Charon (0.35), then their diameters are 46 kilometresfor Nix and 61 kilometres for Hydra. Upper limits on their diameters can be estimated by using the albedo of thedarkest Kuiper-belt objects (0.04); these bounds are 137 ± 11 km and 167 ± 10 km, respectively. At the larger end ofthis range, the inferred masses are less than 0.3% that of Charon, or 0.03% that of Pluto.The discovery of Nix and Hydra suggests that Pluto may possess a variable ring system. Small-body impacts cancreate debris that can form into planetary rings. Data from a deep-optical survey by the Advanced Camera forSurveys on the Hubble Space Telescope suggest that no ring system is present. If such a system exists, it is eithertenuous like the rings of Jupiter or is tightly confined to less than 1,000 km in width. Similar conclusions have beenmade from occultation studies.A fourth moon, Kerberos, was announced on July 20, 2011. It was detected using NASA's Hubble Space Telescopeduring a survey searching for rings around Pluto. It has an estimated diameter of 13 to 34 km and is located betweenthe orbits of Nix and Hydra. Kerberos was first seen in a photo taken with Hubble's Wide Field Camera 3 on June28. It was confirmed in subsequent Hubble pictures taken on July 3 and July 18.A fifth moon, Styx, was announced on July 7, 2012 while looking for potential hazards for New Horizons.[11] Styx isbelieved to have a diameter of between 10 and 25 km and to orbit Pluto at a distance between Charon and Nix.

Near resonancesStyx, Nix, Kerberos and Hydra are fairly close to 3:1, 4:1, 5:1 and 6:1 mean-motion orbital resonances with Charon,respectively (the ratios approach integral commensurabilities more closely going outward from Pluto). Determininghow near any of these orbital period ratios actually is to a true resonance requires accurate knowledge of thesatellites' precessions.

Pluto 14

Pluto and its satellites, with the Moon comparison

Name(Pronunciation)

DiscoveryYear

Diameter(km)

Mass(kg)

Orbital radius(km)

(barycentric)

Orbital period(d)

Periodratio

Magnitude(mag)

Pluto /ˈpluːtoʊ/ 1930 2,306(66% Moon)

1.305×1022

(18% Moon)

2,035 6.3872(25% Moon)

1.000 15.1

Charon /Help:IPA forEnglish#Keyˈʃærən/,

/ˈkɛərən/

1978 1,205(35% Moon)

1.52×1021

(2% Moon)

17,536(5% Moon)

6.3872(25% Moon)

1.000 16.8

Styx /ˈstɪks/ 2012 10–25 ? ~42,000 ± 2,000 20.2 ± 0.1 3.16 27

Nix /ˈnɪks/ 2005 91 4×1017 48,708 24.856 3.892 23.7

Kerberos /ˈkɛərbərəs/ 2011 13–34 ? ~59,000 32.1 5.03 26

Hydra /ˈhaɪdrə/ 2005 114 8×1017 64,749 38.206 5.982 23.3

Mass of Nix and Hydra assumes icy/porous density of 1.0 g/cm3

Quasi-satelliteAt least one minor body is trapped in the 1:1 commensurability with Pluto, (15810) 1994 JR1, specifically in thequasi-satellite dynamical state. The object has been a quasi-satellite of Pluto for about 100,000 years and it willremain in that dynamical state for perhaps another 250,000 years. Its quasi-satellite behavior is recurrent with aperiodicity of 2 million years. There may be additional Pluto co-orbitals.

OriginsMain articles: Kuiper belt and Nice model

Plot of known Kuiper belt objects, set against the four gas giants.

Pluto's origin and identity had long puzzledastronomers. One early hypothesis was thatPluto was an escaped moon of Neptune,knocked out of orbit by its largest currentmoon, Triton. This notion has been heavilycriticized because Pluto never comes nearNeptune in its orbit.

Pluto's true place in the Solar System beganto reveal itself only in 1992, whenastronomers began to find small icy objectsbeyond Neptune that were similar to Plutonot only in orbit but also in size andcomposition. This trans-Neptunianpopulation is believed to be the source ofmany short-period comets. Astronomersnow believe Pluto to be the largest memberof the Kuiper belt, a somewhat stable ring ofobjects located between 30 and 50 AU from

Pluto 15

the Sun. As of 2011, surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto-sizedobjects are expected to be beyond 100 AU from the Sun. Like other Kuiper-belt objects (KBOs), Pluto sharesfeatures with comets; for example, the solar wind is gradually blowing Pluto's surface into space, in the manner of acomet. If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.Though Pluto is the largest of the Kuiper belt objects discovered, Neptune's moon Triton, which is slightly largerthan Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt object.Eris (see below) is about the same size as Pluto (though more massive) but is not strictly considered a member of theKuiper 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 thisorbital resonance are called "plutinos", after Pluto.Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the originalprotoplanetary disc around the Sun that failed to fully coalesce into a full-fledged planet. Most astronomers agreethat Pluto owes its current position to a sudden migration undergone by Neptune early in the Solar System'sformation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbitaround itself (Triton), locking others into resonances, and knocking others into chaotic orbits. The objects in thescattered disc, a dynamically unstable region overlapping the Kuiper belt, are believed to have been placed in theircurrent positions by interactions with Neptune's migrating resonances. A computer model created in 2004 byAlessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice suggested that the migration of Neptune into theKuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created agravitational 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 beltcould also explain the Late Heavy Bombardment 600 million years after the Solar System's formation and the originof the Jupiter trojans. It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune'smigration perturbed it into a resonant capture. The Nice model requires that there were about a thousand Pluto-sizedbodies in the original planetesimal disk; these may have included the early Triton and Eris.

ExplorationMain article: New Horizons

New Horizons, launched on January19, 2006

Pluto presents significant challenges for spacecraft because of its small mass andgreat distance from Earth. Voyager 1 could have visited Pluto, but controllersopted instead for a close flyby of Saturn's moon Titan, resulting in a trajectoryincompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory forreaching Pluto. No serious attempt to explore Pluto by spacecraft occurred untilthe last decade of the 20th century. In August 1992, JPL scientist Robert Staehletelephoned Pluto's discoverer, Clyde Tombaugh, requesting permission to visithis planet. "I told him he was welcome to it," Tombaugh later remembered,"though he's got to go one long, cold trip." Despite this early momentum, in2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costsand launch vehicle delays.

Pluto 16

First Pluto sighting from New Horizons

After an intense political battle, a revised mission to Pluto, dubbedNew Horizons, was granted funding from the US government in 2003.New Horizons was launched successfully on January 19, 2006. Themission leader, S. Alan Stern, confirmed that some of the ashes ofClyde Tombaugh, who died in 1997, had been placed aboard thespacecraft.

In early 2007 the craft made use of a gravity assist from Jupiter. Itsclosest approach to Pluto will be on July 14, 2015; scientificobservations of Pluto will begin 5 months before closest approach andwill continue for at least a month after the encounter. New Horizonscaptured its first (distant) images of Pluto in late September 2006,during a test of the Long Range Reconnaissance Imager (LORRI). Theimages, 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 Beltobjects.

New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigationtool, as well as spectroscopic and other experiments, to characterise the global geology and morphology of Pluto andits moon Charon, map their surface composition and analyse Pluto's neutral atmosphere and its escape rate. NewHorizons 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 unforeseenchallenges. Debris from collisions between Kuiper belt objects and the smaller moons, with their relatively lowescape velocities, may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, therewould be an increased potential for micrometeoroid damage that could disable the probe.

ConceptsA Pluto orbiter/lander/sample return mission was proposed in 2003. The plan included a twelve-year trip from Earthto Pluto, mapping from orbit, multiple landings, a warm water probe, and possible in situ propellant production foranother twelve-year trip back to Earth with samples. Power and propulsion would come from the bimodal MITEEnuclear reactor system.

ClassificationMain article: Definition of planet

Pluto 17

TNO imagemap}})

After Pluto's place within the Kuiper belt wasdetermined, its official status as a planet becamecontroversial, with many questioning whether Plutoshould be considered together with or separately fromits surrounding population.Museum and planetarium directors occasionallycreated controversy by omitting Pluto from planetarymodels of the Solar System. The Hayden Planetariumreopened after renovation in 2000 with a model of onlyeight planets. The controversy made headlines at thetime.

In 2002, the KBO 50000 Quaoar was discovered, witha diameter then thought to be roughly 1280 kilometres,about half that of Pluto. In 2004, the discoverers of 90377 Sedna placed an upper limit of 1800 km on its diameter,nearer to Pluto's diameter of 2320 km, although Sedna's diameter was revised downward to less than 1600 km by2007. Just as Ceres, Pallas, Juno and Vesta eventually lost their planet status after the discovery of many otherasteroids, 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, it is now known to beapproximately the same size as Pluto. This was the largest object discovered in the Solar System since Triton in1846. Its discoverers and the press initially called it the tenth planet, although there was no official consensus at thetime on whether to call it a planet. Others in the astronomical community considered the discovery the strongestargument for reclassifying Pluto as a minor planet.

2006: IAU classificationMain article: IAU definition of planetThe debate came to a head in 2006 with an 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':1. The object must be in orbit around the Sun.2. 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.3. It must have cleared the neighborhood around its orbit.Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in itsorbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolvedthat Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for theplutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.On September 13, 2006, the IAU included Pluto, Eris, and the Eridian moon Dysnomia in their Minor PlanetCatalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) ErisI Dysnomia". If Pluto had been given a minor planet name upon its discovery, the number would have been about1,164 rather than 134,340.There has been some resistance within the astronomical community toward the reclassification. S. Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons". Stern's contention was that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded. His other claim was that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Marc W. Buie, then at Lowell Observatory, voiced his opinion on the new definition on his

Pluto 18

website and petitioned against the definition. Others have supported the IAU. Mike Brown, the astronomer whodiscovered 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."Researchers on both sides of the debate gathered on August 14–16, 2008, at The Johns Hopkins University AppliedPhysics Laboratory for a conference that included back-to-back talks on the current IAU definition of a planet.Entitled "The Great Planet Debate", the conference published a post-conference press release indicating thatscientists could not come to a consensus about the definition of planet. 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 otherobjects similar to Pluto which have an orbital semimajor axis greater than that of Neptune and enough mass to be ofnear-spherical shape.

Reaction

A promotional event with a staged Pluto"protest". Members playing protesters of the

reclassification of Pluto on the left, with thoseplaying counter-protesters on the right

Reception to the IAU decision was mixed. Although many acceptedthe reclassification, some sought to overturn the decision with onlinepetitions urging the IAU to consider reinstatement. A resolutionintroduced by some members of the California State Assemblylight-heartedly denounced the IAU for "scientific heresy", among othercrimes. The U.S. state of New Mexico's House of Representativespassed a resolution in honor of Tombaugh, a longtime resident of thatstate, which declared that Pluto will always be considered a planetwhile in New Mexican skies and that March 13, 2007, was Pluto PlanetDay. The Illinois State Senate passed a similar resolution in 2009, onthe basis that Clyde Tombaugh, the discoverer of Pluto, was born inIllinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU.

Some members of the public have also rejected the change, citing the disagreement within the scientific communityon the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continueto do so regardless of the IAU decision.In 2006 in its 17th annual words of the year vote, the American Dialect Society voted plutoed as the word of theyear. To "pluto" is to "demote or devalue someone or something".

Notes[1] Drawn from Hubble images, synthesized true color<ref name="HST-SynColours" group="lower-alpha">[2] Orbital elements refer to the barycenter of the Pluto system, and are the instantaneous osculating values at the precise J2000 epoch.

Barycenter quantities are given because, in contrast to the planetary center, they do not experience appreciable changes on a day-to-day basisfrom the motion of the moons. The orbital period of Pluto is listed as 248 years because most references use the more stable barycenter of theSolar System (Sun+Jupiter) to list the orbital period of the Pluto-Charon system. A J2000 heliocentric solution would give a value of 246years.

[3] Gorwyn, Adam; Alan Stern: ‘A Chihuahua is still a dog, and Pluto is still a planet’ (http:/ / earthsky. org/ space/alan-stern-â��a-chihuahua-is-still-a-dog-and-pluto-is-still-a-planet-2), EarthSky interview, 2010 February 18

[4][4] Croswell 1997, p. 43.[5][5] Croswell 1997, p. 50.[6][6] Croswell 1997, p. 52.[7] Croswell 1997, pp. 54–55.[8] UK CPI inflation numbers based on data available from Gregory Clark (2014), " What Were the British Earnings and Prices Then? (New

Series) (http:/ / www. measuringworth. org/ ukearncpi/ )" MeasuringWorth.[9] Though if a language does not support something like the Greek pronunciation, these can be rather divergent, such as Somali Buluuto and

Navaho Tłóotoo.[10][10] Croswell 1997, p. 57.[11] Hubble Discovers a Fifth Moon Orbiting Pluto (http:/ / www. nasa. gov/ mission_pages/ hubble/ science/ new-pluto-moon. html)

Pluto 19

References

Bibliography• Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. New York, NY: The Free Press.

ISBN 978-0-684-83252-4.• Brown, Michael E. (2010). How I Killed Pluto and Why It Had It Coming. ISBN 0-385-53108-7.• Stern, S. Alan; Mitton, Jacqueline (2005). "Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar

System" (http:/ / www. worldcat. org/ search?q=isbn:3527405569). Weinheim:Wiley-VCH.ISBN 3-527-40556-9. Retrieved July 3, 2013.

External links• Pluto Profile (http:/ / solarsystem. nasa. gov/ planets/ profile. cfm?Object=Pluto) at NASA's Solar System

Exploration site (http:/ / solarsystem. nasa. gov/ )• NASA Pluto factsheet (http:/ / nssdc. gsfc. nasa. gov/ planetary/ factsheet/ plutofact. html)• Website of the observatory that discovered Pluto (http:/ / www. lowell. edu/ )• Earth telescope image of Pluto system (http:/ / www. astrobio. net/ pressrelease/ 5055/

sharpest-ever-views-of-pluto-and-charon)• Keck infrared with AO of Pluto system (http:/ / www. ifa. hawaii. edu/ info/ press-releases/ PlutoPictures/

Pluto-Tholen-10-07. html)• Gray, Meghan (2009). "Pluto" (http:/ / www. sixtysymbols. com/ videos/ pluto. htm). Sixty Symbols. Brady Haran

for the University of Nottingham.

Article Sources and Contributors 20

Article Sources and ContributorsPluto  Source: http://en.wikipedia.org/w/index.php?oldid=626500070  Contributors: (jarbarf), -- April, 0pulse, 134340Goat, 159753, 208.47.78.xxx, 23skidoo, 281186, 493Titanollante, 500cake, 88wolfmaster, A Train, A bit iffy, ACSE, ADouBTor, ANNAfoxlover, Aaron Schulz, Ab humayun, AbJ32, Aborigine, Absconded Northerner, Abyssoft, Academic Challenger, Acalamari, Ace Class Shadow, Achtung5, Acom, Adam1213, Adashiel, Addshore, Adistinguishedhistorian, Aelffin, Aeon1006, Afogarty, Ageekgal, Ahkilinc, Ahodes1, Ahoerstemeier, Aitias, Ajsteffl, Aksi great, Al1as, Alan Peakall, Alandbrown, Albertod4, Albireo3000, Ale jrb, Alethiareg, Alex mayorga, Alex.muller, Alexf, AlexiusHoratius, Alexkraegen, Ali mcmc, AliveFreeHappy, All Is One, AllHailZeppelin, Alphabetagamma, Alsandro, AltiusBimm, Alumnum, Alvinying, AlwaysUnite, Alxeedo, Amakuru, Amatulic, AmericanLemming, Amorymeltzer, Amos Han, Anaxial, Andattaca2010, Andonic, Andrew Gray, Andrew Norman, Andromeda321, Andronimo, Androstachys, Anetode, Angela, Anika7, Animum, Anna Frodesiak, Anomalocaris, AnonMoos, Anonymous Dissident, Anonymous editor, Ant133, Antandrus, Anthony, Antony-22, Apc123, Aprogressivist, Apyule, Aquirata, Aquzenn, Arce, Arctic.gnome, Are You The Cow Of Pain?, Areeshie, ArglebargleIV, Arjun01, Armaced, ArmadilloFromHell, ArnoldReinhold, Arrenlex, Arsia Mons, Art LaPella, ArthurWeasley, Artman40, Arvind Vyas, Asams10, Asbestos, Asher196, Ashmin2006, Ashmoo, AspaasBekkelund, Asteron, Astrazoic, AstroHurricane001, Astrobhadauria, Astronouth7303, Atakdoug, Atanasio, Attys, Aussiepete, AuthorizeditorA, AvicAWB, Avsa, Awolf002, Axeman89, Axl, Az1568, Aza, AzaToth, Azalea pomp, BIL, Babylone, Bacteria, Bad Astronomer, Badgernet, Bake hi, Banedon, Bangabalunga, Bardnet, Barneca, Bart l, BaseballSport, Baseballdude12321, Basilbrushleo, Bballandrea38, Beao, Bear475, Before My Ken, Beland, Belovedfreak, Ben dude56, Ben-Zin, Benbest, Bender235, Benhocking, Benwildeboer, Bernard the Varanid, Betacommand, BetoCG, Bevo74, Bevo873, Bhadani, BhaiSaab, Bigboy6, Bigdan201, Bigsean0300, Bill shannon 2001, BillCook, Billiamkarr, Bkell, Bkessler23, Blaccjesus, Blehfu, Bletch, Bleubeatle, Blobbie244, Blowski, Bluap, BlueMoonlet, BlytheG, Bmdavll, Bob Man Lot, Bob rulz, Bobathon71, Bobblehead, Bobo192, Bobrayner, Bomazi, Bongwarrior, Boojum, Bookandcoffee, Boombasha, Bornhj, Bosola, Boxjam, Br'er Rabbit, Bradv, Braincricket, Brainster 101, Brandmeister, Brandon 0104, Bravo-Alpha, Brian0918, Brianrusso, Brighterorange, Brion VIBBER, Brisvegas, Bro1990, BrokenSegue, Broocson, Brusselsshrek, Bryan Derksen, BuShWaCkErS, Bubba73, Bubbha, BudMann9, Bugtrio, Bulldogsully, Burmiester, BurnDownBabylon, Burwellian, Burzmali, Byakuya 999, CIS, CKHideki, CTF83!, CWii, Caboose117, Cadiomals, Caerwine, Cal2391, Calculusfreak, Calcwatch, Calltheroofer, Callum859, Caltas, Cam, Can't sleep, clown will eat me, Canadian-Bacon, CanadianCaesar, Cancina5645, Canderson7, Cantus, CapitalLetterBeginning, Caponer, Caracaskid, CardinalDan, Careax, Carlosguitar, Carlwev, Carolc91, Cat-five, Caulde, Cbrodersen, Ccol7280, Centrx, ChadScott, Chal7ds, ChamPro, CharlotteWebb, Chaser, Cherry Cotton, Chesnok, Child of Albion, Chinmaypatel, Chivista, Chnv, Chr.K., ChrisJBenson, ChrisO, Chrisruhr, ChristTrekker, ChristineD95132, Chromaticity, Chun-hian, Chupon, Chèvredansante, CieloEstrellado, Circeus, Ckatz, Cl3rkenwell, Clausewitz01, ClockworkSoul, Cmoney8, Cogiati, Colin, Colinmartin74, Cometstyles, Comicist, Comp25, Concordbandgeek, Conedodger, Conversion script, Cookies1245, Cool3, Coolhandscot, Coredesat, CovenantD, CowAteMe, CowboySpartan, Craigheinke, Crazycomputers, Crust, Cryptic, Crzycheetah, Ctachme, Cuchullain, Curps, Cuspla, Cutejoe, Cybercobra, Cyde, Cyktsui, D.M. from Ukraine, D4S, D6, DARTH SIDIOUS 2, DDima, DERAMOZ, DHN, DOSGuy, DVD R W, Daakun, Daguerio, Dah 144 144, Dan100, DanMS, Daniel J. 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Image Sources, Licenses and ContributorsFile:Pluto symbol.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto_symbol.svg  License: Public Domain  Contributors: LexiconFile:Pluto animiert 200px.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto_animiert_200px.gif  License: Public Domain  Contributors: Aineias, NASA, ESA, and M. Buie(Southwest Research Institute) derivative work: Aineias, Ilmari Karonenfile:speakerlink-new.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Speakerlink-new.svg  License: Creative Commons Zero  Contributors: User:KelvinsongFile:Neptune symbol.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Neptune_symbol.svg  License: Public Domain  Contributors: Amit6File:Pluto's astrological symbol.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto's_astrological_symbol.svg  License: Public Domain  Contributors: LexiconFile:ClydeTombaugh2.gif  Source: http://en.wikipedia.org/w/index.php?title=File:ClydeTombaugh2.gif  License: Public Domain  Contributors: Ds02006, Jan.Kamenicek, Lankiveil, LobStoR, 1anonymous editsFile:Pluto Orbit.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto_Orbit.gif  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:LookangFile:Plutoorbit1.5sideview.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Plutoorbit1.5sideview.gif  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:LookangFile:TheKuiperBelt Orbits Pluto Neptune2.svg  Source: http://en.wikipedia.org/w/index.php?title=File:TheKuiperBelt_Orbits_Pluto_Neptune2.svg  License: Creative CommonsAttribution-ShareAlike 3.0 Unported  Contributors: User:EurocommuterFile:TheKuiperBelt Orbits Pluto Polar.svg  Source: http://en.wikipedia.org/w/index.php?title=File:TheKuiperBelt_Orbits_Pluto_Polar.svg  License: Creative Commons Attribution-ShareAlike3.0 Unported  Contributors: User:EurocommuterFile:Pluto map.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto_map.jpg  License: Public domain  Contributors: NASA/ESA/M. Buie (Southwest Research Institute)File:PlutoColorMap HST2002-2003.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:PlutoColorMap_HST2002-2003.jpg  License: Public Domain  Contributors: Marc Buie (LowellObservatory)/NASA/ESAFile:Pluto-map-hs-2010-06-a-faces.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto-map-hs-2010-06-a-faces.jpg  License: Public domain  Contributors: NASA, ESA, and MarcW. Buie (Southwest Research Institute)File:Pluto-cutaway.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto-cutaway.svg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: RP88 Original uploaderwas RP88 at en.wikipedia Later version(s) were uploaded by Dream out loud at en.wikipedia.File:Pluto, Earth size comparison.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto,_Earth_size_comparison.jpg  License: Public Domain  Contributors: Original uploader wasBrian0918 at en.wikipedia. Later version(s) were uploaded by Ephemeronium, Dream out loud at en.wikipedia.File:ESO-L. Calçada - Pluto (by).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ESO-L._Calçada_-_Pluto_(by).jpg  License: unknown  Contributors: ESO/L. CalçadaFile:Pluto-Charon double planet.png  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto-Charon_double_planet.png  License: Public domain  Contributors: Lanthanum-138 aten.wikipediaFile:Charon.png  Source: http://en.wikipedia.org/w/index.php?title=File:Charon.png  License: Public Domain  Contributors: Double sharp, Huntster, Jan.Kamenicek, Ruslik0, XBrain130File:Pluto moon P5 discovery with moons' orbits.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto_moon_P5_discovery_with_moons'_orbits.jpg  License: Public Domain Contributors: Fotaun, Morio, WolfmanSF, 1 anonymous editsFile:Outersolarsystem objectpositions labels comp.png  Source: http://en.wikipedia.org/w/index.php?title=File:Outersolarsystem_objectpositions_labels_comp.png  License: Free Art License Contributors: 84user, Kaldari, Peteforsyth, Poppy, Venkat.athma, Wikibob, WilyD, 7 ,باسم anonymous editsFile:New Horizons Jan19 06.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:New_Horizons_Jan19_06.jpg  License: Public Domain  Contributors: NASA/Kim Shiflett (cropped byat en.wikipedia)File:First Pluto sighting from New Horizons.gif  Source: http://en.wikipedia.org/w/index.php?title=File:First_Pluto_sighting_from_New_Horizons.gif  License: Public Domain  Contributors:NASAFile:EightTNOs.png  Source: http://en.wikipedia.org/w/index.php?title=File:EightTNOs.png  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: LexiconFile:Pluto Protest and Counter Protest.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pluto_Protest_and_Counter_Protest.jpg  License: Public Domain  Contributors: Fanboy

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