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Pluto

*** Shopping-Tip: Pluto
: ''For more uses of the term Pluto, see Pluto (disambiguation).'' {{Planet Infobox/Pluto}} {{SpecialCharsNote}} ''' '''Pluto''' is the traditional ninth planet of the Solar system, though this status has been disputed in recent years. {{cite web |url=http://news.bbc.co.uk/1/hi/magazine/4737647.stm |title=Farewell Pluto? |first=Alexis |last=Akwagyiram |publisher=BBC News |year=2005 |format=HTML |accessdate=2006-03-05}} It has an eccentric orbit that is highly inclined with respect to the other planets and takes it inside the orbit of Neptune (planet) Neptune. It is also the smallest planet and indeed is smaller than several natural satellite moons. Pluto itself has a large moon named Charon (moon) Charon; two small moons were discovered in 2005. Its astronomical symbol is a P-L monogram, {{Unicode.html">Percival Lowell, who had searched extensively for a ninth planet and who had lent his name to the observatory that eventually discovered Pluto. (A less common symbol resembles that of Neptune.html" title="Meaning of Percival Lowell.html" title="Meaning of Neptune (planet) Neptune">Neptune (planet)|Neptune, â™†, but has a circle in place of the middle spoke of the trident.) The ''New Horizons'' spacecraft, which lifted off from Cape Canaveral, Florida on January 19, 2006, is expected to become the first unmanned spacecraft to fly by Pluto on July 14 2015. Due to its small size and unusual orbit, there has been well publicized #The Pluto debate debate regarding Pluto's classification as a major or as a minor planet, with increasing momentum for recognizing dual status. Pluto is the largest known member of the main Kuiper Belt, though one Scattered disk scattered Kuiper Belt object, {{mpl|2003 UB|313}}, had been found that is larger than Pluto.

Discovery and naming
Image:Pluto, Earth size comparison.jpg thumb|left|200px|Pluto's diameter is about 18% that of Earth. The story of how Pluto was discovered actually begins with the discovery of Neptune. In the 1840s, using Newtonian mechanics, Urbain Le Verrier and John Couch Adams had correctly predicted the position of the then-undiscovered planet Neptune after analysing the perturbations in the orbit of Uranus which could only have been caused by the gravitational pull of another massive planet. Thanks to their calculations, Neptune was discovered by Johann Gottfried Galle on September 23, 1846. By the late 19th century, astronomers started speculating that Neptune's orbit too was being disturbed by another planet. By 1909, William H. Pickering and Percival Lowell had suggested several possible celestial co-ordinates for such a planet. In May 1911, the Bulletin of the Astronomical Society of France published the calculations of the Indian astronomer Ketakar which predicted a location for the undiscovered planet. Though Lowell died in 1916, the search for the elusive planet continued. Pluto was discovered after an extensive search by the astronomer Clyde Tombaugh at the Lowell Observatory in Arizona on February 18, 1930 when he compared photographic plates taken on January 23 and January 29 29. Tombaugh also referenced a lesser-quality photo taken on January 20 to confirm movement. After the observatory obtained confirming photographs, the news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The planet was later found on photographs dating back to March 19, 1915. Its mean distance from Earth and its mean daily motion turned out to be 39.48 A.U and 14.283". Ironically, Pluto is far too small to have the effect on Neptune's orbit that initiated the search. The discrepancies in Neptune's orbit observed by 19th century astronomers were due instead to an inaccurate estimate of Neptune's mass. Tombaugh's discovery is therefore even more surprising, given that the proximity of the predictions of Pickering, Lowell and Ketakar were coincidences. In the matter of Pluto, the discretion of naming the new object belonged to the Lowell Observatory and its director, Vesto Melvin Slipher, who, in the words of Tombaugh, was "urged to suggest a name for the new planet before someone else did." Soon suggestions began to pour in from all over the world. Constance Lowell, Percival Lowell Percival's widow who had delayed the search through her lawsuit, proposed ''Zeus,'' then ''Lowell,'' and finally her own first name, none of which met with any enthusiasm. One young couple even wrote to ask that the planet be named after their newborn child. Mythological names were much to the fore: ''Cronus'' and ''Minerva'' (proposed by the ''New York Times'', unaware that it had been proposed for Uranus (planet) Uranus some 150 years earlier) were high on the list. Also there were ''Artemis, Athene, Atlas (mythology) Atlas, Cosmos, Hera, Hercules, Icarus (mythology) Icarus, Idana, Odin, Pax (mythology) Pax, Persephone, Perseus, Prometheus, Tantalus, Vulcan (god) Vulcan,'' and many more. One complication was that many of the mythological names had already been allotted to the numerous asteroids. The name retained for the planet is that of the Roman god Pluto (god) Pluto, and it is also intended to evoke the initials of the astronomer Percival Lowell, who predicted that a planet would be found beyond Neptune. The name was first suggested by Venetia Burney Venetia Phair (nÃ©e Burney), at the time an eleven-year-old girl from Oxford, England Oxford, England.{{cite web |url=http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm |title=The girl who named a planet |first= Paul |last= Rincon |publisher=BBC News |format=HTML |accessdate=2006-03-05}} Over the breakfast table one morning her grandfather, who worked at Oxford University's Bodleian Library, was reading about the discovery of the new planet in the ''The Times Times'' newspaper. He asked his granddaughter to suggest a good name for it. Venetia, who was quite interested in Greek and Roman myths and legends, suggested the name of the Roman god of the underworld. Professor Herbert Hall Turner cabled his colleagues in America with this suggestion, and after favourable consideration which was almost unanimous, the name Pluto was officially adopted and an announcement made by Slipher on May 1, 1930. In the Chinese language Chinese, Japanese language Japanese, and Korean language Korean languages, the planet's name was translated as ''star of the king of the dead'' å†¥çŽ‹æ˜Ÿ. In Vietnamese language Vietnamese it is named after Yama ''(DiÃªm VÆ°Æ¡ng Tinh),'' the Guardian of Hell in Hinduism Hindu and Buddhism Buddhist traditions.

Orbit
Image:Pluto top orbit 3.jpg thumb|right|250px|The orbits of the outer planets, showing Pluto's eccentricity, which causes it to cross Neptune's orbit Pluto's orbit is different from the orbit of other planets. It is highly orbital inclination inclined above the plane of the ecliptic, and highly orbital eccentricity eccentric (non-circular). The eccentricity of its orbit is such that it crosses the orbit of Neptune, making it only the eighth-most distant planet from the Sun for part of each orbit; the most recent occurrence of this phenomenon lasted from February 7, 1979 through February 11, 1999. Mathematical calculations indicate that the previous occurrence only lasted fourteen years from July 11, 1735 to September 15, 1749. However, the same calculations indicate that Pluto was the eighth-most distant planet between April 30, 1483 and July 23, 1503, which is almost exactly the same length as the 1979 to 1999 period. Recent studies suggest each crossing of Pluto to inside Neptune's orbit lasts alternately for approximately thirteen and twenty years with minor variations. Image:Pluto orbit side.jpg ecliptic.html" title="Meaning of thumb thumb|250px|left|Pluto's orbit seen from the plane of the [[ecliptic, showing its high orbital inclination inclination compared to the other planets.html" title="Meaning of 250px|left|Pluto's orbit seen from the plane of the [[ecliptic">thumb|250px|left|Pluto's orbit seen from the plane of the [[ecliptic, showing its high orbital inclination inclination compared to the other planets">250px|left|Pluto's orbit seen from the plane of the [[ecliptic">thumb|250px|left|Pluto's orbit seen from the plane of the [[ecliptic, showing its high orbital inclination inclination compared to the other planets Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity starts to pull on each other slightly, resulting in an interaction between their positions in orbit of the same sort that produces Trojan points. Since the orbits are eccentric, the 3:2 periodic ratio is favoured because this means Neptune always passes Pluto when they are almost farthest apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it initially seems as if Neptune is about to catch up to Pluto. But Pluto speeds up due to the gravitational acceleration from the Sun, stays ahead of Neptune, and pulls ahead until they meet again on the other side of Pluto's orbit. Beginning in the 1990s, other trans-Neptunian objects (TNOs) were discovered, and a certain number of these also have a 3:2 orbital resonance with Neptune. TNOs with this orbital resonance are named "plutinos", after Pluto.

Physical characteristics
{{Mergefrom|Geology of Pluto}} More than seventy-six years after its discovery, many facts about Pluto remain unknown, mainly due to the fact that it is the only planet that has not been visited by spacecraft and that it is too far away for in-depth investigations with telescopes from Earth. What is known are the few physical properties listed below.

Mass and size
Image:Pluto_compared2.jpg Ganymede_(moon) right|thumb|250px|Pluto (bottom right) compared to [[Ganymede_(moon)|Ganymede, Titan (moon) Titan, Callisto (moon) Callisto, Io (moon) Io, Moon Luna, Europa (moon) Europa, and Triton (moon) Triton..html" title="Meaning of Ganymede.html" title="Meaning of right|thumb|250px|Pluto (bottom right) compared to [[Ganymede_(moon)|Ganymede">right|thumb|250px|Pluto (bottom right) compared to [[Ganymede_(moon)|Ganymede, Titan (moon) Titan, Callisto (moon) Callisto, Io (moon) Io, Moon Luna, Europa (moon) Europa, and Triton (moon) Triton.">Ganymede.html" title="Meaning of right|thumb|250px|Pluto (bottom right) compared to [[Ganymede_(moon)|Ganymede">right|thumb|250px|Pluto (bottom right) compared to [[Ganymede_(moon)|Ganymede, Titan (moon) Titan, Callisto (moon) Callisto, Io (moon) Io, Moon Luna, Europa (moon) Europa, and Triton (moon) Triton. Image:ThePlutinos_Size_Albedo_Color2.svg plutino.html" title="Meaning of right right|thumb|250px|Pluto and its moons (top) compared with the largest [[plutinos 90482 Orcus Orcus and 28978 Ixion Ixion..html" title="Meaning of thumb|250px|Pluto and its moons (top) compared with the largest [[plutino">right|thumb|250px|Pluto and its moons (top) compared with the largest [[plutinos 90482 Orcus Orcus and 28978 Ixion Ixion.">thumb|250px|Pluto and its moons (top) compared with the largest [[plutino">right|thumb|250px|Pluto and its moons (top) compared with the largest [[plutinos 90482 Orcus Orcus and 28978 Ixion Ixion. Pluto is not only smaller and much less massive than every other planet, but at less than 0.2 lunar masses it is also smaller and less massive than seven natural satellite moons: Ganymede (moon) Ganymede, Titan (moon) Titan, Callisto (moon) Callisto, Io (moon) Io, Earth's Moon, Europa (moon) Europa and Triton (moon) Triton. However, Pluto is more than twice the diameter, and a dozen times the mass, of 1 Ceres Ceres, the largest minor planet in the asteroid belt, and it was larger than any other object known in the trans-Neptunian Kuiper belt until {{mpl|2003 UB|313}} was announced in 2005. See List of solar system objects by mass and List of solar system objects by radius. Pluto's mass and diameter could only be estimated for many decades after its discovery. The discovery of its satellite Charon (moon) Charon in 1978 enabled a determination of the mass of the Pluto-Charon system by simple application of Kepler's laws of planetary motion#Kepler's understanding of the laws Newton's formulation of Kepler's third law. Later Pluto's diameter was measured when it was occultation occulted by Charon, and its disk can now be resolved by telescopes using adaptive optics. The last graph illustrates the size, albedo and color index of the largest plutinos.

Atmosphere
Pluto's thin atmosphere is most likely nitrogen and carbon monoxide, in equilibrium with solid nitrogen and carbon monoxide ices on the surface. As Pluto moves away from its perihelion and farther from the Sun, more of its atmosphere freezes. When it returns to a closer proximity to the sun, the temperature of Pluto's solid surface will increase, causing the nitrogen ice to Sublimation (physics) sublimate into gas - creating an Anti-Greenhouse Effect anti-greenhouse effect. Much as sweat evaporating from the surface of human skin, this sublimation has a cooling effect on the planet and scientists have recently discovered,{{cite web |url=http://www.cnn.com/2006/TECH/space/01/03/pluto.temp/index.html |title=Astronomers: Pluto colder than expected |first=Ker |last=Than |format=HTML |publisher=Space.com (via CNN.com) |year=2006 |accessdate=2006-03-05}} by use of the Submillimeter Array, that Pluto's temperature is 10 kelvins less than they expected. Pluto was found to have an atmosphere from an occultation observation in 1988. When an object with no atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmosphere was determined to have a pressure of 0.15 Pascal Pa, roughly 1/700,000 that of Earth. In 2002, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy of the Paris Observatory{{cite web |url=http://calys.obspm.fr/~sicardy/pluton/pr_obs_en.html |title=Drastic expansion of Pluto's atmosphere as revealed by stellar occultations |accessdate=2006-03-05 |format=HTML}} and by Jim Elliot of MIT{{cite web |url=http://web.mit.edu/newsoffice/2002/pluto.html |title=Pluto is undergoing global warming, researchers find |year=2002 |format=HTML |accessdate=2006-03-05}} and Jay Pasachoff of Williams College.{{cite web |url=http://www.williams.edu/admin/news/releases.php?id=162 |title=Williams Scientists Contribute to New Finding About Pluto |year=2002 |format=HTML |accessdate=2006-03-05}} Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pa, even though Pluto was further from the Sun than in 1988, and hence should be colder and have a less dense atmosphere. The current best hypothesis is that the south pole of Pluto came out of shadow for the first time in 120 years in 1987, and extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.

Surface
The surface of Pluto is remarkably heterogeneous, as evidenced by its lightcurve, maps of its surface constructed from Hubble Space Telescope observations, and by periodic variations in its infrared spectra. The face of Pluto oriented toward Charon has more methane ice, while the opposite face has more ices of nitrogen and carbon monoxide.

Appearance
Pluto's apparent magnitude is fainter than 14 ''m'' and therefore a telescope is required for observation. To be easily seen, a telescope of around 30 cm aperture is desirable. It looks star-like even in very big telescopes, because its angular diameter is only 0.15″. The colour of Pluto is light brown with a very slight tint of yellow.

Pluto's moons
{{main|Pluto's natural satellites}} Image:Pluto system.jpg thumb|250px|right|Diagram of the Plutonian system Pluto has three known natural satellites: Charon (moon) Charon, first identified in 1978, by astronomer James Christy, and two smaller, provisionally designated S/2005 P 1 and S/2005 P 2 that were discovered in 2005.

Charon
The Pluto-Charon system is noteworthy for being the only planetary system in the solar system whose barycenter lies above the planet's surface. This and the large size of Charon relative to Pluto have prompted some astronomers to label it a double planet. The Pluto-Charon system is also unusual among planetary systems in that they are Tidal locking tidally locked to each other: Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon. The discovery of Charon allowed astronomers to determine the mass of the Pluto-Charon pair from their observed orbital period and separation by a straightforward application of Kepler's law Kepler's third law of planetary motion. The mass was found to be lower than even the lowest earlier estimates. The discovery also led astronomers to alter their estimate of Pluto's size. Originally, it was believed that Pluto was larger than Mercury (planet) Mercury but smaller than Mars (planet) Mars, but that calculation was based on the premise that a single object was being observed. Once it was realized that there were in fact two objects instead of one, the estimated size of Pluto was revised downward. Today, with modern adaptive optics, Pluto's disc can be resolved and thus its size can be directly determined. Image:Plutoncharon.jpg thumb|250px|right|Pluto and its biggest satellite Charon Charon's discovery also resulted in the calculation of Pluto's albedo being revised upward; since the planet was now seen as being far smaller than originally estimated, by necessity its capacity to reflect light must be greater than what had been formerly believed. Current estimates place Pluto's albedo as marginally less than that of Venus, which is fairly high. {| class="wikitable" |- ! style="background:#ffffff;" colspan="6" | Pluto and Charon, compared to Earth's moon Moon Luna |- |- style="background:#efefef;" ! colspan="2" | Name
(Asteroid pronunciation key Pronunciation key) ! Diameter
(km) ! Mass
(kg) ! Orbital radius (km) ! Orbital period (days) |- style="text-align:center; background:#ccccff" | '''Pluto''' || ''pl'''oo''''-toe''
{{IPA|/ËˆpluË?tÉ”ÊŠ/}} || 2306
(65% Luna) || 13Ã—10²¹
(18% Luna) || 2390
(0.6% Luna) || style="background:#ffffff;" rowspan="2" | 6.3872
(25% Luna) |- style="text-align:center; background:#eeeeff" | '''Charon (moon) Charon''' || ''shair'-É™n''
{{IPA|/ËˆÊƒÉ›É™rÉ™n/}} || 1205
(35% Luna) || 1.5Ã—10²¹
(2% Luna) || 19,570
(5% Luna) |} Previously, some researchers had theorized that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptunian orbit when Triton (moon) Triton was captured. Triton, the largest moon of Neptune, which shares many atmospherical and geological composition similarities with Pluto, may once have been a Kuiper belt object in a solar orbit, and today it is widely accepted that Pluto never orbited Neptune. An occultation of a star by Charon (moon) Charon in 2005, observed in South America by teams from MIT-Williams College, the Paris Observatory, and the Southwest Research Institute has led to improved knowledge of Charon's parameters.

Smaller moons
Image:Plutonianmoons.jpg thumb|225px|Pluto and its three known moons. Pluto and Charon are the bright objects in the center, the two smaller moons are at the right and bottom, farther out. Image:Plutonian system.jpg S/2005 P 1.html" title="Meaning of thumb thumb|230px|Artist's concept of the surface of [[S/2005 P 1. '''Pluto''' & Charon (moon) Charon (right) & S/2005 P 2 (bright dot on left)..html" title="Meaning of 230px|Artist's concept of the surface of [[S/2005 P 1">thumb|230px|Artist's concept of the surface of [[S/2005 P 1. '''Pluto''' & Charon (moon) Charon (right) & S/2005 P 2 (bright dot on left).">230px|Artist's concept of the surface of [[S/2005 P 1">thumb|230px|Artist's concept of the surface of [[S/2005 P 1. '''Pluto''' & Charon (moon) Charon (right) & S/2005 P 2 (bright dot on left). Two additional moons were imaged by astronomers working with the Hubble Space Telescope on May 15 2005, and received provisional designations of S/2005 P 1 and S/2005 P 2. They were confirmed with a series of "precovery" Hubble images from June 2002 through May 2003, which led to their orbits being determined. Additional follow-up observations were made in February and March 2006, confirming the orbit solutions. The small moons orbit Pluto at approximately two and three times the distance of Charon: P2 at 48,700 km and P1 at 64,800 km from the barycenter of the system. They have nearly circular prograde and retrograde motion prograde orbits in the same orbital plane as Charon, and are very close to but not in 4:1 and 6:1 mean motion orbital resonances with Charon. P1 is sometimes brighter than P2 and thus may be larger in some dimensions; alternately different parts of its surface may vary in brightness. Their sizes are not known but can be estimated from likely albedos. The moons' spectral similarity with Charon suggests a similar 35% albedo to Charon; this results in estimates of 46 km for P2 and 61 km for the brighter orientation of P1. Likely upper limits on their diameters can be estimated by assuming the 4% albedo of the dardkest Kuiper Belt objects; these bounds are 137 Â± 11km and 167 Â±10km respectively. At the larger end of this range, the inferred masses are less than 0.3% of Charon's mass, or 0.03% of Pluto's. H. A. Weaver, S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young ''Discovery of two new satellites of Pluto''. Nature '''439''', 943–945 (23 February 2006) [http://arxiv.org/PS_cache/astro-ph/pdf/0601/0601018.pdf Final preprint on ArXiv]

Limits on additional moons
In imaging the Plutonian system, the Hubble observations placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km¹ exist beyond the glare of Pluto 5 arcseconds from the planet, assuming Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 km.Hill sphere radius (stable gravitational zone of influence) of 2.2 million km. For example, Phoebe (moon) Phoebe orbits Saturn at ²/₃ 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 discoverersâ€™ terms, the Plutonian system appears to be "highly compact and largely empty".

Exploration of Pluto
image:New_Horizons_Jan19_06.jpg New Horizons.html" title="Meaning of thumb thumb|right|250px|Photo of ''[[New Horizons'', the world's first probe to Pluto, being launched on January 19, 2006 (it is expected to reach Pluto in July 2015).html" title="Meaning of right|250px|Photo of ''[[New Horizons">thumb|right|250px|Photo of ''[[New Horizons'', the world's first probe to Pluto, being launched on January 19, 2006 (it is expected to reach Pluto in July 2015)">right|250px|Photo of ''[[New Horizons">thumb|right|250px|Photo of ''[[New Horizons'', the world's first probe to Pluto, being launched on January 19, 2006 (it is expected to reach Pluto in July 2015) Little is known about Pluto because of its great distance from Earth and because no exploratory spacecraft have visited Pluto yet. The ''Voyager 1'' probe was originally intended to visit Pluto, but due to budget cuts and lack of interest — before the discovery of Charon or Pluto's size and atmosphere — the flyby was scrapped in order to facilitate a close flyby of Saturn (planet) Saturn's moon Titan (moon) Titan. The first spacecraft to visit Pluto will be NASA's ''New Horizons'', a mission led by the Southwest Research Institute and the Johns Hopkins University Johns Hopkins Applied Physics Laboratory. The mission launched on January 19, 2006. It will benefit from a gravity assist from Jupiter, and arrive at Pluto in July 2015. ''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 characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and characterize Pluto's neutral atmosphere and its escape rate. ''New Horizons'' would also photograph the surfaces of Pluto and Charon. The mission plan also calls for a flyby of one or more Kuiper belt objects by 2022. The ''New Horizons'' mission replaced the ''Pluto Kuiper Express'' mission, which was cancelled in 2000 because of increasing costs and launch vehicle delays.

The Pluto debate

Planet X?
{{main|Planet X}} The planet Pluto was originally discovered in 1930 in the course of a search for a body sufficiently massive to account for supposed anomalies in the orbits of Uranus (planet) Uranus and Neptune (planet) Neptune. Once it was found, its faintness and failure to show a visible disk cast doubt on the idea that it could be Percival Lowell Percival Lowell's Planet X. Lowell had made a prediction of Pluto's position in 1915 which had turned out to be fairly close to its actual position at that time; however Ernest W. Brown concluded almost immediately that this was a coincidence, and this view is retained today. Lowell had also made earlier, different predictions of Planet X's position beginning in 1902.{{cite web |url=http://www.phys-astro.sonoma.edu/people/faculty/tenn/asphistory/1994.html |title=THE ASTRONOMICAL SOCIETY OF THE PACIFIC â€œ106th ANNUAL MEETINGâ€? HISTORY SESSIONS |format=HTML |accessdate=2006-03-05}} In the following decades estimates of the Plutonian mass and diameter were the subject of debate as telescopes and imaging systems improved. The consensus steadily favored smaller masses and diameters as time passed. Indeed, one observer waggishly pointed out that if the trend were extrapolated, the planet seemed to be in danger of vanishing altogether, a remark which proved possibly prophetic in light of later debates over Pluto's status as a "planet". In an attempt to reconcile Pluto's small apparent size with its identification as "Planet X", the theory of specular reflection was proposed. This held that observers were measuring only the diameter of a bright spot on the highly reflective surface of a much larger planet which could thereby be massive without having an exceptionally high density. The uncertainty was conclusively resolved by the discovery of Pluto's satellite Charon in 1978. This made it possible to determine the combined mass of the Pluto-Charon system which turned out to be lower even than that anticipated by skeptics of the specular reflection theory, which was then rendered completely untenable. The accepted figure for Pluto's diameter today makes it considerably smaller than the Moon, with only a fraction of the Moon's mass on account of its being largely composed of ice. More recently, measurements of the path of ''Voyager 2'' have shown that Neptune has a greater mass than previously believed and that when the updated mass is taken into account there is no anomalous movement of Uranus or Neptune. Thus Pluto's discovery and Lowell's 1915 prediction were largely coincidental as Pluto actually has no role in what were believed to be anomalies in Neptune and Uranus' motion. Pluto's discovery was mostly due to the diligence of Tombaugh's search. While Pluto's identification as Planet X began to be doubted soon after its discovery, and for some decades afterwards some considered that a hypothetical tenth planet might be the true Planet X which supposedly caused anomalies in Uranus and Neptune's position, Pluto's identity as the solar system's ninth planet was unquestioned until the 1990s.

Minor planet?
Image:Plutoandcharon.jpg right|thumb|160px|An image of Pluto and Charon taken with a 61" telescope; note the difficulty in resolution despite telescope size. This small size is one of the reasons Pluto's planetary status is debated. Starting in September 1992 scientists began discovering hundreds of other bodies in the area of the solar system beyond the orbit of Neptune, a region now known as the Kuiper belt. The first of these new objects was {{mpl|(15760) 1992 QB|1}}. The continued discovery of these objects, especially of plutinos, rekindled a debate that goes on to this day: is Pluto a major planet or simply one of the largest trans-Neptunian objects? Trans-Neptunian objects are considered to be minor planets, so the question arose whether to consider Pluto to be one also. This planetary sciences debate landed in newspaper headlines, editorials, and on the Internet in the mid- to late-1990s. Thoughts that Pluto might be "demoted" to non-planet status created an emotional response in certain sectors of the public. Such news outlets as the ''BBC News Online'', the ''Boston Globe'', and ''USA Today'' all printed stories noting that the International Astronomical Union was considering dropping Pluto's planetary status. "Save Pluto" websites sprang up, and school children sent letters to astronomers and the IAU. On February 3 1999, Brian Marsden of the Minor Planet Center inadvertently fueled the debate when he issued an editorial in the ''Minor Planet Electronic Circular'' 1999-C03 noting that the 10,000th minor planet was about to be numbered and this called for a large celebration (the IAU celebrates every thousandth numbered minor planet in some way). He suggested that Pluto be honored with the number 10,000, giving it "dual citizenship" of sorts as both a major and a minor planet.{{cite web |url=http://cfa-www.harvard.edu/iau/mpec/J99/J99C03.html |title=Minor Planet Electronic Circular 1999-C03: EDITORIAL NOTICE |format=HTML |accessdate=2006-03-05}} Between the media reports and the Minor Planet Electronic Circulars, IAU General Secretary Joannes Anderson issued a press release that same day, stating there were ''no'' plans to change Pluto's planetary status. Eventually, the number 10,000 was assigned to an "ordinary" asteroid, 10000 Myriostos. The debate centers on whether a "planet", from the Greek language Greek for "wanderer", is an appellation that depends upon an object's particular size, formation, or orbit. Some argue that not only is Pluto a major planet but also some moons like Titan (moon) Titan, Europa (moon) Europa or Triton (moon) Triton, or even the larger asteroids are as well. Some argue that an astronomical object more than about 360 km in diameter, at which point the object has a tendency to become round under its own gravity, should be known as a major planet; this would include several moons and a handful of asteroids. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury, the smallest terrestrial planet with a diameter of 4879.4 km and Ceres (asteroid) Ceres, the largest known asteroid with a mean diameter of 950 km, which would include Pluto but not most moons.

New discoveries
Continuing discoveries in the Trans-Neptunian region keep rekindling the debate. In 2002, 50000 Quaoar was discovered, with a 1,280 km diameter, making it a bit more than half the size of Pluto. Another recent discovery, 90482 Orcus, is probably even larger. In 2004 the discoverers of 90377 Sedna, an extremely distant object well beyond the other known Trans-Neptunian objects, placed an upper limit of 1,800 km on its diameter, close to Pluto's 2,320 km. On July 29, 2005, a Trans-Neptunian object called {{mpl.html">apparent magnitude magnitude and simple albedo considerations is assumed to be larger than Pluto. This caused its discoverers to call it the "10th planet" of the solar system, although there is no consensus yet on whether to call it a planet, and others consider the new discovery to be the strongest argument yet for demoting Pluto to the status of a minor planet. {{mp.html">Neptune (planet) Neptune in 1846. The last remaining distinguishing feature of Pluto is now its large moon, Charon (moon) Charon, and its atmosphere; these characteristics may not, however, be unique to Pluto: several other Trans-Neptunian objects are known to have satellites; and {{mp|2003 UB|313}}'s spectrum suggests that it has a similar surface composition to Pluto, as well as a moon discovered in September 2005. There is some historical precedent for "demoting" a "planet" in the light of subsequent discoveries. The first four asteroids (1 Ceres, 2 Pallas, 3 Juno and 4 Vesta) were considered to be planets for several decades (in part because their sizes were not accurately known at the time). However, in 1845, the first new asteroid in thirty-eight years was discovered (5 Astraea), just one year before Neptune (planet) Neptune, and soon every year brought more asteroid discoveries. It was soon recognized that Ceres and the others were just the most prominent members of a populous asteroid belt, and although asteroids are also known as "minor planets", they are no longer considered "planets". Some see in this a precedent for noting that Pluto is just the most prominent member of the Kuiper belt. On the other hand, it may very well be that regardless of future astronomical discoveries, Pluto will remain grandfather clause grandfathered as a planet in much the same way that Europe is considered a separate continent for historical reasons although geographically it makes more sense, from first principles, to consider both Europe and Asia to comprise the single continent of Eurasia. {{seealso|Definition of planet}}

See also
{{commons|Pluto}} *Pluto in popular culture *Planets in astrology#Pluto Pluto in astrology *Solar eclipses on Pluto *Aspects of Pluto

References
*Henderson, Mark (Oct. 30, 2005). "Pluto may lose status of planet". ''New Straits Times'', p. F17. *Kaufmann III, William J., "Universe", 2nd Edition, pp.302-303 *Chhabra et al., "Prediction of Pluto by Ketakar", Indian Journal of History of Science, 19(1), pp.18-26, 1984

External links

- NASA's Pluto fact sheet
- StudyWorks! Online: Is Pluto a Planet? - summarizes the pros and the cons of classifying Pluto as a planet *David H. Freedman, [http://www.theatlantic.com/issues/98feb/pluto.htm "When is a Planet Not a Planet?"], ''Atlantic Monthly'', February 1998.
- NASA's Pluto-Kuiper Belt Mission - launching in early 2006.
- Pluto in Science Fiction - Bibliography of science fiction which is set on Pluto. *[http://www.windows.ucar.edu/tour/link=/pluto/pluto.html] -Windows to the universe-Pluto
- Pluto/Neptune crossings
- Space.com article on second and third moons
- NASA's Hubble Reveals Possible New Moons Around Pluto â€“ Hubble Space Telescope Hubble Press Release
- Possible New Moons
- Elliot et al., The recent expansion of Pluto's atmosphere
- Pasachoff et al., "The Structure of Pluto's Atmosphere from the 2002 August 21 Stellar Occultation," Astron. J., 129, 1718-1723.
- The Girl who named a Planet - BBC News
- Planets - Pluto A kid's guide to Pluto. {{Pluto Footer}} {{Footer TransNeptunian}} {{Footer_SolarSystem}}

Special characters
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