Pluto
- For more uses of the term Pluto, see Pluto (disambiguation).
 Click image for description | |||||||
|---|---|---|---|---|---|---|---|
| Discovery | |||||||
| Discovered by | Clyde W. Tombaugh | ||||||
| Discovered on | February 18, 1930 | ||||||
| Orbital characteristics (Epoch J2000) | |||||||
| Semi-major axis | 5,906,376,272 km 39.481 686 77 AU | ||||||
| Orbital circumference | 36.530 Tm 244.186 AU | ||||||
| Eccentricity | 0.248 807 66 | ||||||
| Perihelion | 4,436,824,613 km 29.658 340 67 AU | ||||||
| Aphelion | 7,375,927,931 km 49.305 032 87 AU | ||||||
| Orbital period | 90,613.3058 d (248.09 a) | ||||||
| Synodic period | 366.74 d | ||||||
| Avg. Orbital Speed | 4.666 km/s | ||||||
| Max. Orbital Speed | 6.112 km/s | ||||||
| Min. Orbital Speed | 3.676 km/s | ||||||
| Inclination | 17.141 75° (11.88° to Sun's equator) | ||||||
| Longitude of the ascending node |
110.303 47° | ||||||
| Argument of the perihelion |
113.763 29° | ||||||
| Number of satellites | 1 | ||||||
| Physical characteristics | |||||||
| Diameter | 2390 km (0.180 Earths or 1,485 miles) | ||||||
| Surface area | 1.795×107 km2 (0.033 Earths) | ||||||
| Volume | 7.15×109 km3 (0.0066 Earths) | ||||||
| Mass | 1.25×1022 kg (0.0021 Earths) | ||||||
| Mean density | 1.750 g/cm3 | ||||||
| Equatorial gravity | 0.58 m/s2 (0.059 gee) | ||||||
| Escape velocity | 1.2 km/s | ||||||
| Rotation period | 6.387 d (6 d 9 h 17.6 min) | ||||||
| Rotation velocity | 47.18 km/h (at the equator) | ||||||
| Axial tilt | 122.54° (to orbit) 115.60° (to the ecliptic)[1] | ||||||
| Right ascension of North pole |
133.02° (8 h 52 min 5 s) | ||||||
| Declination | -9.09° | ||||||
| Albedo | 0.30 | ||||||
| Surface temp. |
| ||||||
| Atmospheric characteristics | |||||||
| Atmospheric pressure | 0.15-0.30 pascals | ||||||
| Composition | nitrogen, methane | ||||||
Pluto is the ninth planet from the Sun. Discovered in 1930, Pluto is the smallest planet in our solar system and has a highly eccentric orbit which is highly inclined in respect to the other planets, and takes it inside the orbit of Neptune. It has one moon, Charon. Pluto's symbol is a combination of the name's first two letters, "P-L", which are also Percival Lowell's initials (Unicode: ♇ ), although some prefer a different symbol resembling that of Neptune but with a circle in the top center instead of the middle spoke.
Due to its size and unusual orbit, there has been debate regarding whether Pluto should be classified as a planet. As of July 31, 2005, at least one other trans-Neptunian object (2003 UB313) has been found that is larger.
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 known 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 grandaughter 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.
Physical characteristics
Mass and size
Pluto is not only smaller and much less massive than every other planet, 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 an order of magnitude larger than any minor planet in the asteroid belt, and it was larger than any other object known in the trans-Neptunian Kuiper belt until 2003 UB313 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.
Eccentric orbit
Pluto's highly eccentric orbit makes it the eighth-most distant planet from the Sun for part of each orbit; this most recently occurred from February 7, 1979 through February 11, 1999. 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.
Because of its small size and eccentric orbit, there has been some debate over whether it truly should be classified as a planet. There is mounting evidence that Pluto may in fact be a member of the Kuiper belt, only one of a large number of distant icy bodies. A subclass of such objects have been dubbed plutinos, after Pluto.
Atmosphere
Pluto has an atmosphere when it is close to perihelion; the atmosphere may freeze out as Pluto moves farther from the Sun. It is thought by some that Pluto shares its atmosphere with its moon. Pluto was found to have an atmosphere from an occultation observation in 1988. When a planet or asteroid occults a star, if it has no atmosphere, 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). This thin atmosphere is most likely nitrogen and carbon monoxide, in equilibrium with solid nitrogen and carbon monoxide ices on the surface.
In 2003, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy [2] and by Jim Elliot. Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pa, even though Pluto was farther away 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 in 1987 (for the first time in 120 years), and extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.
Appearance
Pluto's apparent magnitude is fainter than 14 m and therefore a telescope larger than 30 centimetres (12 inches) is required for observation. It looks star-like even in very big telescopes, because its angular diameter is only 0.15″. The colour of Pluto is yellow.
Pluto's moon
Pluto has one natural satellite, Charon, first identified in 1978. Pluto and Charon are noteworthy for being the only planet/moon pair in the solar system whose barycenter lies above the planet's surface. Pluto and Charon are also unusual among planets in that they are tidally locked to each other. This means that Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon. Note that some binary asteroids may also possess both of these traits, and that the Jupiter/Sun barycenter is above the Sun's surface, so neither is unique.
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 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, shares many atmospherical and geological composition similarities with Pluto, suggesting that Triton was once a plutino in a solar orbit.
Exploration of Pluto
Little is known about Pluto because of its great distance from Earth and because no exploratory spacecraft have visited Pluto yet. In 2001, NASA approved preliminary studies for a mission named New Horizons to Pluto, and the mission was formally announced in November 2003. It is led by the Southwest Research Institute and the Johns Hopkins Applied Physics Laboratory.
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.
It will weigh half a ton and will travel at speeds reaching 43,000 km/h (27,000 mph). The spacecraft would 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 New Horizons mission replaces the Pluto Kuiper Express mission, which was cancelled in 2000 because of increasing costs and launch vehicle delays. Earlier, the Voyager 1 probe was 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 redirected for Saturn's moon Titan.
The Pluto debate
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. [3]
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.
Minor planet?
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, was (15760) 1992 QB1. 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 as to 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.
New discoveries
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 2003 UB313 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 as 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. 2003 UB313 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 (not including Sedna) are known to have satellites; and 2003 UB313's spectrum suggests that it has a similar surface composition to Pluto.
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
- In H. P. Lovecraft's Cthulhu mythos stories (1925-1935), the Mi-go are a fictional alien race with a base on Pluto (called Yuggoth in the stories).
- 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 Doctor Who 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.
- 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 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 game Epoch Star (2004), Pluto is the home planet of the Anthropite civilization.
- In John DeChancie's Starrigger series, Pluto is the location of our solar system's dimensional gate to the interstellar Skyway.
- In the Japanese anime series Star Blazers, the Argo stops at Pluto.
- In the Japanese anime series Cowboy Bebop, it is mentioned that a "supermax" maximum security penitentiary is located on Pluto.
- In the cartoon Fairly Oddparents Cosmo destroys Pluto with a button the president lost.
See also
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, "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.
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