- For more uses of the term Pluto, see Pluto (disambiguation).
Template:Planet Infobox/Pluto Pluto is a celestial body in the outer solar system. Discovered in 1930 and officially classified as a planet, its status is currently under dispute. Pluto has an eccentric orbit that is highly inclined in respect to the other planets and takes it inside the orbit of Neptune. Its largest moon is Charon, discovered in 1978; two smaller moons were discovered in 2005. Pluto's astronomical symbol is a P-L monogram, ♇. This represents both the first two letters of the name Pluto and the initials of Percival Lowell, the man who lent his name to the observatory that was used to find Pluto. An alternate symbol resembles that of Neptune, but has a circle in place of the middle spoke in the top center.
Due to its size and unusual orbit, there has been debate regarding Pluto's classification as a major or a minor planet, and there is increasing momentum for recognizing "dual status." Pluto is classified as a trans-Neptunian object. As of July 31, 2005, one other trans-Neptunian object, Template:Mpl, had been found that is larger than Pluto.
- 1 Discovery and naming
- 2 Orbit
- 3 Physical characteristics
- 4 Pluto's moons
- 5 Exploration of Pluto
- 6 The Pluto debate
- 7 Pluto in Fiction and Film
- 8 See also
- 9 References
- 10 External links
Discovery and naming
Pluto was discovered 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 29. 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. Tombaugh was searching for a "Planet X" to explain discrepancies in the predicted orbit of Neptune. It is now known these discrepancies were an artifact of the slightly incorrect value then assumed for the mass of Neptune.
In the matter of Pluto the discretion of naming the new object belonged to 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'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 some 150 years earlier) were high on the list. Also there were Artemis, Athene, Atlas, Cosmos, Hera, Hercules, Icarus, Idana, Odin, Pax, Persephone, Perseus, Prometheus, Tantalus, Vulcan, Zymal, and many more. One complication was that many of the mythological names had already been allotted to the numerous asteroids. Virtually all the female names had been used up, and male names were usually reserved for objects with unusual orbits.
The name retained for the planet is that of the Roman 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, at the time an eleven-year-old girl from Oxford, England. 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 Times newspaper. He asked his granddaughter what she thought would be good name for it. Venetia thought that as it was so cold and so distant it should be named after 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.
Because Pluto was nowhere near the asteroid belt (trans-Neptunian objects would not be discovered until much later), it never got an asteroidal provisional designation; had it obtained one, it would probably have been "1930 BD".
Pluto's orbit is unlike those of the other planets. It is highly inclined above the plane of the ecliptic, and highly eccentric (non-circular). The eccentricity of its orbit is such that it crosses the orbit of Neptune, and making Pluto only the eighth-most distant planet from the Sun for part of each orbit; the most recent occurrence of this phemonenon 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.
Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity start 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're 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.
More than 75 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 human 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
Pluto is not only smaller and much less massive than every other planet, at less than 0.2 lunar masses it is also smaller and less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. However, Pluto is more than twice the diameter, and a dozen times the mass, of 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 Template:Mpl 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 in 1978 enabled a determination of the mass of the Pluto-Charon system by simple application of Newton's formulation of Kepler's third law. Later Pluto's diameter was measured when it was occulted by Charon, and its disk can now be resolved by telescopes using adaptive optics.
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. 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 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  and by Jim Elliot. 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.
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 30cm 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.
- Main article: Pluto's natural satellites
The Pluto-Charon system is noteworthy for being the only planet/moon system in the solar system whose barycenter lies above the planet's surface, thus prompting some astronomers to label it a double planet (a term complicated by the discovery of two more Plutonian moons).
The Pluto-Charon system is also unusual among planetary systems in that they are 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 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 but smaller than 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.
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.
Previously, some researchers had theorized that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptune's orbit. Today it is widely accepted that Pluto never orbited Neptune. Instead, 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.
The outer moons
Two additional moons were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005 and have received provisional designations of S/2005 P 1 and S/2005 P 2. They were confirmed with "precovery" Hubble images from June 14, 2002. Observations suggest they orbit Pluto at at least twice the distance Charon does. P2 stays about 49,000km from the planet, P1 lies even further away at 65,000km. The two candidate moons seem to orbit Pluto in an anti-clockwise direction. Preliminary observations are also consistent with the outer moons lying in the same orbital plane as Charon, and orbiting at distances two and three times farther away, with orbital resonances of 4:1 and 6:1 with Charon.
Both objects appear to be on the order of 50-150 km in diameter, compared to Charon's 1,200 km, and are thought to have masses less than 0.3% of Charon's (or 0.03% of Pluto's mass). The discovery team plans follow-up observations with Hubble in February 2006 to work out the precise orbits, but ground-based observatories will attempt to image the moons as well. Once the orbits are confirmed, the moons can be given permanent names.  
Exploration of Pluto
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 Pluto's moon, size, and atmosphere — the flyby was scrapped in order to facilitate a close flyby of Saturn's moon Titan.
The mission's launch window is between January 11 and February 14, 2006. Assuming it launches within the first 23 days of the window, it will benefit from a gravity assist from Jupiter, and arrive at Pluto in July 2015.
New Horizons weighs half a ton and will travel at speeds reaching 43,000 km/h (27,000 mph). It will use a remote sensing package that includes imaging instruments and a radio science investigation, 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. The mission plan also calls for a flyby of one or more Kuiper belt objects by 2022.
The Pluto debate
- Main article: 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 and Neptune. Once it was found, its faintness and failure to show a visible disc cast doubt on the idea that it could be 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. 
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 lower mass than previously believed and that when this lower 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 thoroughness and diligence of Tombaugh's search, which he continued for some time after the discovery and left him satisfied that no other planet of a comparable magnitude existed.
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.
Starting in September of 1992 scientists began discovering hundreds of other bodies in the area of the solar system beyond the orbit of Neptune. The second of these, after Pluto and Pluto's moon Charon, was Template:Mpl. 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 too. 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.
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 how a "planet", from the 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, Europa or Triton, or even the larger asteroids. 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, the largest known asteroid with a mean diameter of 950 km, which would include Pluto but not most moons.
Continuing discoveries in the transneptunian region keep rekindling the debate. In 2002, 50000 Quaoar was discovered, with a 1280 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 transneptunian objects, placed an upper limit of 1800 km on its diameter, close to Pluto's 2320 km.
On July 29, 2005, a Trans-Neptunian object called Template:Mpl was announced, which on the basis of its 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. Template:Mp could be the largest object yet discovered in the solar system since Neptune in 1846. The last remaining distinguishing feature of Pluto is now its large moon, Charon, and its atmosphere; these characteristics may not, however, be unique to Pluto: several other transneptunian objects are known to have satellites; and Template:Mp's spectrum suggests that it has a similar surface composition to Pluto, as well as a moon discovered in September of 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 38 years was discovered (5 Astraea), just one year before 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 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.
- See also: Definition of Planet
Pluto in Fiction and Film
- Donald W. Horner, in Their Winged Destiny (1912), described spaceflight to Alpha Centauri by astronauts who, as they leave the solar system, pass a planet beyond Neptune.
- Mickey Mouse's dog Pluto first appeared in the 1930 cartoon The Chain Gang; it was adapted into Minnie Mouse's dog Rover, and shortly thereafter became Mickey's Pluto, being named for the planet.
- Stanton Coblentz's In Plutonian Depths (Wonder Stories Quarterly, Spring 1931) was the first story to take advantage of the newly discovered and named world.
- In H. P. Lovecraft's Cthulhu mythos stories (1928–), the Mi-go are a fictional alien race with a base on Pluto, called Yuggoth and Iukkoth in the stories. There are some stories, though, that identify Yuggoth with a huge world situated beyond Pluto on an orbit perpendicular to the ecliptic.
- In Robert Heinlein's novel Have Space Suit-Will Travel (1958), Pluto is used by aliens as a remote base for Earth exploration.
- In Larry Niven's novel World of Ptavvs (1966), Pluto was theorized to have been a moon of Neptune until it was knocked out of orbit by an interstellar craft.
- Martinex of the Guardians of the Galaxy is the last survivor of a human colony in Pluto. His ancestors were African. The character first appeared in January, 1969 and operates in the 31st century.
- In the 1974 Japanese anime series Space Battleship Yamato, also known as Star Blazers, the eponymous starship destroys an alien base on Pluto and fights a subsequent battle in an asteroid belt beyond Pluto. Eighteen years later astronomers confirmed the existence of the real-life Kuiper belt.
- In the Doctor Who (1963–) serial The Sun Makers (1977), set far in the future, Pluto is covered with vast cities that are warmed by artificial suns, but access to sunlight is controlled by a sinister ruling elite.
- In John DeChancie's Starrigger series (1983), Pluto is the location of our solar system's dimensional gate to the interstellar Skyway.
- Pluto is featured in Kim Stanley Robinson's novel Icehenge (1985), in which a mysterious artificial structure is found on the planet's north pole.
- In the computer game Star Control II (1990), and consequently in The Ur-Quan Masters, the Spathi Captain Fwiffo can be found on Pluto.
- The final section of Dave Sim's graphic novel Minds (Cerebus the Aardvark, Volume 10, 1996) takes place on Pluto.
- Christine Lavin's song Planet X (1996) is a good-natured protest against suggestions that Pluto is not a planet.
- In the Japanese anime series Cowboy Bebop (1998), it is mentioned that a "supermax" maximum security penitentiary is located on Pluto.
- In the game Starsiege (1999), Pluto is destroyed at the end of the game.
- The television show Futurama (1999-2003) has featured Pluto on occasion, mainly as a habitat for penguins.
- In the television show Aqua Teen Hunger Force (2000), two of the show's "Villains", Oglethorpe and Emory (The Plutonians), are from Pluto.
- In the cartoon Fairly Oddparents (2001–) Cosmo destroys Pluto with a button the president lost.
- In the game Epoch Star (2004), Pluto is the home planet of the Anthropite civilization.
- In the second part of a BBC drama documentary called Space Odyssey: Voyage to the Planets (2004) Pluto is the penultimate destination on a hypothetical human space flight to planets of the Solar System
- Henderson, Mark (Oct. 30, 2005). "Pluto may lose status of planet". New Straits Times, p. F17.
- 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, "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.
- Pluto/Neptune crossings
- Space.com article on second and third moons
- NASA's Hubble Reveals Possible New Moons Around Pluto – Hubble Press Release
- Possible New Moons
ar:بلوتو bg:Плутон (планета) bs:Pluton br:Ploudon ca:Plutó (planeta) cs:Pluto (planeta) cy:Plwto da:Pluto (planet) de:Pluto (Planet) et:Pluuto el:Πλούτωνας (πλανήτης) es:Plutón (planeta) eo:Plutono eu:Plutoi fr:Pluton (planète) ga:Plútón (pláinéad) gl:Plutón (planeta) gu:પ્લૂટો (ગ્રહ) ko:명왕성 hr:Pluton (planet) io:Plutono id:Pluto it:Plutone (astronomia) he:פלוטו (כוכב לכת) kn:ಪ್ಲುಟೊ la:Pluto (planeta) lv:Plutons lt:Plutonas hu:Plútó (bolygó) ms:Pluto nl:Pluto (planeet) ja:冥王星 no:Pluto (planet) nn:Planeten Pluto pl:Pluton (planeta) pt:Plutão ro:Pluto (planetă) ru:Плутон (планета) scn:Plutoni simple:Pluto (planet) sk:Pluto (planéta) sl:Pluton (planet) sr:Плутон (планета) fi:Pluto (planeetta) sv:Pluto (planet) tl:Pluto th:ดาวพลูโต vi:Diêm Vương Tinh tr:Plüton (gezegen) uk:Плутон (планета) zh:冥王星 zh-min-nan:Mê-ông-chheⁿ