User:JohnnyBGood/Pluto

For more uses of the term Pluto, see Pluto (disambiguation).

Pluto is the ninth and smallest of the traditional planets of the Solar system, though its status as a planet has been disputed in recent years. It has an eccentric orbit that is highly inclined with respect to the other planets and takes it closer to the Sun than Neptune during a portion of its orbit. It is also the smallest planet and indeed is smaller than several moons of other planets (see the list of solar system objects by radius). Pluto itself has a large moon named Charon; two small moons named Nix and Hydra were discovered in 2005. The New Horizons spacecraft, which lifted off from Cape Canaveral, Florida on January 19, 2006, is expected to become the first spacecraft to fly by Pluto on July 14 2015.

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, who had searched extensively for a ninth planet and who had lent his name to Lowell Observatory, the place from where, after initiating several earlier searches, Pluto was eventually discovered. (Another symbol sometimes used for Pluto is an astrological symbol and not an astronomical one. It resembles that of Neptune, ♆, but has a circle in place of the middle spoke of the trident.)

Pluto and its satellite Charon have often been considered a binary planet because they are more nearly equal in size than any other planet/moon combination in the Solar System.

Discovery and naming

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 was also being disturbed by another planet. By 1909, William H. Pickering and Percival Lowell had suggested several possible celestial coordinates for such a planet. In May 1911, the Bulletin of the Astronomical Society of France published the calculations of the Indian astronomer, Venkatesh Ketakar, which predicted a location for the undiscovered planet. Although Lowell died in 1916, the search for the elusive planet continued.

Pluto was discovered after an extensive search by the astronomer Clyde Tombaugh at Lowell Observatory in Arizona on February 18, 1930 when he compared photographic plates taken on January 23 and 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 AU and 14.283".

File:Discovery Pluto.png

Clyde Tombaugh found Pluto by systematically taking pictures of the plane of the solar system. He took pictures in pairs, one or two weeks apart, and looked for anything that moved.

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'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, 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, 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 Phair (née 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 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, Japanese, and Korean languages, the planet's name was translated as star of the king of the dead 冥王星. In Vietnamese it is named after Yama (Diêm Vương Tinh), the Guardian of Hell in Buddhist traditions.

Orbit

Orbit of Pluto – ecliptic view.

Orbit of Pluto – polar view.

Pluto’s orbit is exceptional among the planets for its inclination (>17°) and eccentricity (~0.25). Only Mercury's orbit shows a significant inclination (~7°) and eccentricity (~0.2); other planets follow quasi circular, near–ecliptic orbits. High eccentricity makes Pluto's approach to the Sun closer than Neptune, often misleading into thinking that its perihelion is situated 'inside' Neptune’s orbit, Actually, due to the orbit’s inclination, the perihelion is well above (~8.0 AU) the ecliptic. The diagrams show both orbits in an ecliptic view (from the direction of the descending node) and the polar view (from above the ecliptic) with the current¹ positions of the planets. Segments of orbits above the ecliptic are plotted with brighter colours, segments below the ecliptic with darker colours; perihelion and aphelion are marked with q and Q respectively.

¹As of April 16th, 2006. In a year the planets' positions on the diagram will have barely changed by ~ 1 pixel.

Consequences of the orbital resonance

The closest points.

The closest approach.

The impression of Pluto’s orbit crossing Neptune’s orbit given by the polar view is dispelled by the next diagram (a view from around 10° above the ecliptic). Pluto's nodes (points where the orbit crosses the ecliptic) are both situated outside Neptune’s orbit (illustrated by red segments becoming darker). The diagram shows also the closest points of the two orbits, separated there by ~ 6.4 AU. However, Pluto and Neptune never get so close. Due to the orbital resonance Pluto executes 2 full cycles while Neptune makes 3 and their relative positions are locked near these points. As shown on the next diagram, with dates given for the current orbit, when Neptune reached this point Pluto remained behind and when Pluto in turn reaches this point, Neptune will be well (>50°) ahead. During the next (or previous) orbit of Pluto, Neptune will be (was) half an orbit away. Consequently, Pluto never gets closer to Neptune in this point of its orbit than 30 AU.

The actual closest approach takes place in the opposite part of the orbit, some 30 years after Pluto’s aphelion (the last aphelion was in 1866) when Neptune catches up with Pluto i.e. the planets have similar longitudes. The graph shows planets' positions every 10 years. Sphere size is inversely proportional to the distance to facilitate the correlation. The minimal distance was 18.9 AU in June 1896. As result of the resonance, Pluto does not approach Neptune much closer than it approaches Saturn.

Heliocentric distance

Near perihelion Pluto gets closer to the Sun than Neptune, becoming 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.

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

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

Pluto (bottom right) compared to Ganymede, Titan, Callisto, Io, the Moon (Luna), Europa, and Triton.

Pluto and its moons (top) compared with the largest plutinos Orcus and Ixion.

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 disc can now be resolved by telescopes using adaptive optics.

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 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 Eris ((136199) 2003 UB₃₁₃) was announced in 2005. See List of solar system objects by mass and List of solar system objects by radius.

The two figures illustrate the size and look of Pluto compared to the largest moons of the solar systems (first figure) and the size, albedo and color index of Pluto and its moons compared to the largest plutinos (second figure).

Atmosphere

Pluto's thin atmosphere is most likely made up of nitrogen, methane, 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 and falls to the ground. When it returns to a closer proximity to the sun, the temperature of Pluto's solid surface will increase, causing the nitrogen ice to sublimate into gas—creating an 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, 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 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 and by Jim Elliot of MIT and Jay Pasachoff of Williams College. 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.

Composition

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 color of Pluto is light brown with a very slight tint of yellow.

Because of its distance, Pluto is nearly impossible to directly photograph with any surface detail. Hubble telescope images just barely show any surface markings. The best image of Pluto comes from reconstructing a brightness map using the eclipsing of its largest moon, Charon. For example, eclipsing a bright spot on Pluto will make for a bigger total brightness change than eclipsing a gray spot. Using this technique, one only has to measure total average brightness of the Pluto system and track changes in brightness over time. Computer processing is then used to match brightness changes against the known position of the eclipsing moon.

Pluto's moons

Main article: Pluto's natural satellites

Diagram of the Plutonian system. P 1 is Hydra, and P 2 is Nix.

Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, 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 prompted some astronomers to label it a double planet.

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.

Pluto and Charon, compared to Earth's moon Luna
Name (Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km)	Orbital period (days)
Pluto	ploo'-toe /ˈpluːtɔʊ/	2306 (65% Luna)	13×1021 (18% Luna)	2390 (0.6% Luna)	6.3872 (25% Luna)
Charon	shair'-ən /ˈʃɛərən/	1205 (35% Luna)	1.5×1021 (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 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 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.

Nix and Hydra

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.

Artist's concept of the surface of Hydra. Pluto & Charon (right) and Nix (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. The International Astronomical Union has officially christened Pluto's two newest moons Hydra (Pluto III, the outer moon) and Nix (Pluto II, the inner moon) as of June 22, 2006. 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: Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular 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.

Hydra is sometimes brighter than Nix 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 35% albedo similar to Charon's; this results in diameter estimates of 46 kilometres for Nix and 61 kilometres for the brighter orientation of Hydra. Likely upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km 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.

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 kilometres.

¹Or a maximum of 37 kilometres with an albedo of 0.04

This is a striking distribution. Moons could potentially orbit Pluto up to the 53% (or 69%, if retrograde) of the Hill sphere radius (stable gravitational zone of influence) of 6.0 million kilometres. 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".

Exploration of Pluto

Photo of New Horizons, the 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's moon Titan.

In 1989, NASA also considered sending Voyager 2 to Pluto using a gravitational assist from Neptune, but this would not permit the best flyby of Triton for scientific observations. It was eventually decided to calculate the Neptune and Triton visit for the best results around that planet, regardless of the consequences to the craft's final trajectory.

The first spacecraft to visit Pluto will be NASA's New Horizons, a mission led by the Southwest Research Institute and the 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. If launched, Pluto Kuiper Express was to reach Pluto around 2012, and the Kuiper Belt around 2017.

The Pluto debate

Planet X?

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 disk cast doubt on the idea that it could be 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.

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.

Minor planet?

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.

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. Due to its small size and unusual orbit, however, recently there has been a well publicized debate regarding Pluto's classification as a major or as a minor planet, with increasing momentum for recognizing dual status. [1] Pluto is one of the largest known members of the main Kuiper Belt, the area of the solar system beyond the orbit of Neptune; one scattered Kuiper Belt object, Eris, has been found that is larger than Pluto, and they have several companions in the Kuiper Belt, including 2003 EL61 and Makemake (2005 FY₉).

The Kuiper belt is believed to be the source for all short-period comets, and Pluto, like other KBOs, shares features in common with comets. The solar wind is gradually sublimating Pluto's surface into space, in the manner of a comet. If Pluto were placed near the Sun, it would develop a tail, like comets do.

Hundreds of other Kuiper Belt objects have been discovered since September 1992, the first being (15760) 1992 QB1. The continued discovery of these objects, especially that 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 (TNOs)?

TNOs 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.

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 [2]. Eventually, the number 10,000 was assigned to an "ordinary" asteroid, 10000 Myriostos.

The debate centers on whether 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 are as well. Others argue that the term "minor planet" should refer to an astronomical object more than about 360 kilometres in diameter, at which point the object has a tendency to become round under its own gravity; 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 4,879.4 kilometres and Ceres, the largest known asteroid with a mean diameter of 950 kilometres. This definition 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 kilometres 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 kilometres on its diameter, close to Pluto's 2,320 kilometres.

On July 29, 2005, a Trans-Neptunian object called 2003 UB₃₁₃ (nicknamed "Xena", now Eris) was announced, which on the basis of its magnitude and simple albedo considerations was 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. Eris is the largest object yet discovered in the solar system since Neptune in 1846, although it is only slightly larger than Pluto. 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 Trans-Neptunian objects are known to have satellites; and Eris's spectrum suggests that it has a similar surface composition to Pluto, as well as a moon (nicknamed "Gabrielle"), discovered in September 2005. Trans-Neptunian object 2003 EL61 (nicknamed "Santa") has two moons (one of which is nicknamed "Rudolph") and is the fourth largest TNO behind Eris, Pluto, and Makemake (nicknamed "Easterbunny").

IAU meeting

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, 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.

In order to put these matters to rest, in August 2006, 3,000 astronomers and scientists of the IAU gathered in Prague, Czech Republic to deliberate and the organization plans to publish an official definition of "planet" in early September 2006, ruling on whether to call Pluto a planet or a Kuiper Belt Object (KBO). [3] [4] In August 2006, National Public Radio (NPR) reported that the IAU intends to recommend that Pluto remain classified as a planet, though perhaps belong to a subcategory called "Dwarf Planets". Such categorization means that the status of some asteroids and KBOs may also be defined as planets in the future. [5]

See also: Definition of planet

See also

• Pluto in popular culture
• 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
• Lowell Observatory History, Mission, Important Discoveries
• 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.
• Windows to the universe – Pluto
• Space.com article on second and third moons
• NASA's Hubble Reveals Possible New Moons Around Pluto – 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.
• A simulation of Pluto's orbit, showing its resonance with Neptune.
• NPR article on IAU's anticipated recommendation of Pluto's status as a planet.
• Experts meet to decide Pluto fate - BBC News 14th August 2006

Special characters

Category:Trans-Neptunian objects Pluto-Charon system