science and technology

Pluto, also designated 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun. Originally considered a planet, Pluto has since been recognised as the largest member of a distinct region called the Kuiper belt. Like other members of the belt, it is primarily composed of rock and ice and is relatively small; approximately a fifth the mass of the Earth's Moon and a third its volume. It has an eccentric orbit that takes it from 29 to 49 AU (4.3 7.3 billion km / 2.7 4.5 billion mi) from the Sun, and is highly inclined with respect to the planets. As a result, Pluto occasionally comes closer to the Sun than the planet Neptune.

Pluto and its largest satellite, Charon, are often considered a binary system because the barycenter of their orbits does not lie within either body. However, the International Astronomical Union (IAU) has yet to formalize a definition for binary dwarf planets, and until it passes such a ruling, Charon remains a moon of Pluto. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.

From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st centuries however, many objects similar to Pluto were discovered in the outer solar system, most notably the scattered disc object Eris, which is 27% more massive than Pluto. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto from planethood, and reclassified it under the new category of dwarf planet along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.

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. This idea was mentioned by her grandfather to a former Astronomer Royal who cabled his astronomer colleagues in America. After favourable consideration which was almost unanimous, the name Pluto was officially adopted and an announcement made by Slipher on May 1, 1930

Appearance and Composition

Mass and Size

Pluto is not only much smaller and less massive than every other planet, it is also smaller and less massive than seven moons of other planets: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. However, Pluto is larger than any minor planet in the main asteroid belt, and was larger than any other object discovered in the trans-Neptunian Kuiper belt until 2003 UB313 in 2005. See List of solar system objects by mass and List of solar system objects by radius.

Pluto's mass and diameter were unknown for many decades after its discovery and could only be estimated. The discovery of its satellite Charon permitted determining the mass for the Pluto-Charon system by simple application of Newton's formulation of Kepler's third law. Meanwhile, its diameter is now known since telescopes using adaptive optics can resolve its disk.

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 determined 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 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 and Composition

Pluto's mean apparent magnitude is 15.1 with a maximum of 13.56. To see it, a telescope is required; around 30 cm aperture desirable.[25] It looks indistinct and star-like even in very large telescopes because its angular diameter is only 0.15". The colour of Pluto is light brown with a very slight tint of yellow.

Spectroscopic analysis of Pluto's surface reveals it is composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide. Distance and limits on telescope technology make it currently impossible to directly photograph surface details on Pluto. Images from the Hubble Space Telescope barely show any distinguishable surface definitions or markings.

The best images of Pluto derive from brightness maps created from close observations of eclipses by its largest moon, Charon. Using computer processing, observations are made in brightness factors as Pluto is eclipsed by Charon. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a gray spot. Using this technique, one can measure the total average brightness of the Pluto-Charon system and track changes in brightness over time.

Maps composed by the Hubble Space Telescope reveal that Pluto's surface is remarkably heterogeneous, a fact also evidenced by its lightcurve, and by periodic variations in its infrared spectra. The face of Pluto oriented toward Charon contains more methane ice, while the opposite face contains more nitrogen and carbon monoxide ice. This makes Pluto the second most contrasted body in the Solar System after Iapetus.

The Hubble Space Telescope places Pluto's density at between 1.8 and 2.1 g/cm3, suggesting its internal composition consists of roughly 50 70 percent rock and 30 50 percent ice. Because decay of radioactive minerals would eventually heat the ices enough for them to separate from rock, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. It is also possible that such heating may continue into the present time, creating a subsurface ocean of liquid water.

Mass and Size Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass on the basis of its presumed effect on Neptune and Uranus. In 1955, Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars. However, in 1976, David Cuikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time, and found it matched that for methane ice, which meant Pluto had to be exceptionally bright, and therefore could not be more than 1 percent the mass of the Earth.

The discovery of its satellite Charon in 1978 enabled a determination of the mass of the Pluto Charon system by application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect on Pluto was measured, estimates of Pluto's mass fell to 13.1 Yg; less than 0.24 percent that of the Earth.

Observations of Pluto in occultation with Charon were able to fix Pluto's diameter at roughly 2,390 km. With the invention of adaptive optics astronomers were able to accurately determine its shape.

Among the objects of the Solar System, Pluto is not only smaller and much less massive than any planet, but at less than 0.2 lunar masses it is also smaller than seven of the moons: Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton. Pluto is more than twice the diameter and a dozen times the mass of Ceres, a dwarf planet in the asteroid belt. However, it is smaller than the dwarf planet Eris, a trans-Neptunian object discovered in 2005.

Atmosphere

Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide, derived from the ices on its surface. As Pluto moves away from the Sun, its atmosphere gradually freezes and falls to the ground. As it edges closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto. Scientists have recently discovered, by use of the Submillimeter Array, that Pluto's temperature is 10 kelvins colder than expected.

Pluto was found to have an atmosphere from an occultation observation in 1985; the finding was confirmed and significantly strengthened by extensive observations of another occultation 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 atmospheric pressure was determined as 0.15 Pascals, roughly 1/700,000 that of Earth.

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory and by James L. Elliot of MIT and Jay Pasachoff of Williams College. Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pascals, even though Pluto was farther from the Sun than in 1988, and hence should be colder and have a less dense atmosphere. The most widely accepted hypothesis to explain this discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years; as a result extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere. Another stellar occultation was observed by the MIT-Williams College team of James Elliot and Jay Pasachoff and a Southwest Research Institute team led by Leslie Young on 12 June 2006 from sites in Australia.

In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.

Orbit

Pluto's orbit is markedly different to those of the planets. The planets all orbit the Sun close to a flat reference plane called the ecliptic, and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity leads to a small region of Pluto's orbit lying closer to the Sun than Neptune's. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999. Detailed calculations indicate that the previous such occurrence lasted only fourteen years from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it had again lasted for 20 years.

Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forwards and backwards in time), after intervals longer than 10 20 million years, it is impossible to determine exactly where Pluto will be because its position becomes too sensitive to unmeasurable details of the present state of the solar system. For example, at some specific time many millions of years from now, Pluto may be at aphelion or perihelion (or anywhere in between), with no way for us to predict which. This does not mean that the orbit of Pluto itself is unstable, however; only that its position along that orbit is impossible to determine far into the future. In fact, several resonances and other dynamical effects conspire to keep Pluto's orbit stable, safe from planetary collision or scattering.

Neptune-avoiding orbit

Despite Pluto's orbit apparently crossing that of Neptune when viewed from directly above the ecliptic, the two objects cannot collide. This is because their orbits are aligned so that Pluto and Neptune can never approach closely. Several factors contribute to this.

At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed in a top-down projection (right), it is also the farthest above the ecliptic. This means Pluto's orbit actually passes above that of Neptune, preventing a collision. Indeed, the part of Pluto's orbit that lies as close or closer to the Sun than that of Neptune lies about 8 AU above the ecliptic, and so a similar distance above Neptune's orbit. Pluto's ascending node, the point at which the orbit crosses the ecliptic, is currently separated from Neptune's by over 21°; their descending nodes are separated by a similar angular distance (see diagram). Since Neptune's orbit is almost flat with respect to the ecliptic, Pluto is far above it by the time the two orbits cross.

However, this alone is not enough to protect Pluto; perturbations from the planets, particularly Neptune, would adjust Pluto's orbit (e.g. orbital precession), so that over millions of years a collision could be possible. Some other effect(s) must therefore be in place. The most significant of these is a mean motion resonance with Neptune.

Pluto lies in the 3:2 mean motion resonance of Neptune: for every three orbits of Neptune around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is configured so that, in each 500-year cycle, the first time that Pluto is near perihelion Neptune is over 50° "behind" Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance "ahead" of Pluto. In fact, the minimum separation of Pluto and Neptune is over 17 AU; Pluto actually comes closer (11 AU) to Uranus than it does to Neptune.

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

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 completely 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.

At one point some researchers suggested that Pluto and its moon Charon were moons of Neptune that were knocked out of Neptune's orbit, but it is now thought that Pluto was never Neptune's moon. Triton's retrograde orbit suggests that it was originally an independent body much like Pluto which was captured by Neptune. Triton also shares many atmospherical and geological composition similarities with Pluto.

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 called "New Horizons" to Pluto, to be conducted by the Southwest Research Institute. Its launch window is between 11th January and 14th February 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 Kuiper Belt Objects by 2022.

Originally the Voyager 1 probe was planned to visit Pluto, but due to budget cuts and lack of interest the flyby was cancelled. It was redirected for a close flyby of 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, smaller, icy bodies in the area of the solar system beyond the orbit of Neptune. These objects are now deemed members of the Kuiper belt and are accordingly known as Kuiper Belt Objects (KBOs). The continued discovery of these objects, especially of Plutinos, began a debate that goes on to this day: is Pluto a planet or simply the largest (known) example of a Kuiper belt object?

Kuiper belt objects are 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 early 1999. 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.

Some scientists argue that "planet", from the Greek for "wanderer", is a designation that does not depend upon an object's particular size, formation or orbit. Yet others argue that not only is Pluto a planet but also some moons like Titan, Europa or Triton, or even the larger asteroids. Some agree 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 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 Kuiper belt and beyond 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 beyond the Kuiper belt, 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, along with the claim that it is at least as large as Pluto, and estimated to be half again larger. This caused some to refer to it as the "10th planet" of the solar system. 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 moon, Charon, and its atmosphere; these characteristics may not, however, be unique to Pluto: several other Kuiper belt objects (not including Sedna are known to have satellites; and 2003 UB313's spectrum suggests that it has a similar surface composition to Pluto.

It is interesting to note that, historically, the first four asteroids (1 Ceres, 2 Pallas, 3 Juno and 4 Vesta) were considered planets for several decades (their size was not accurately known at the time). Some astronomy texts in the early 19th century referred to the existence of eleven planets (including Uranus and the first four asteroids). In 1845, the first new asteroid in 38 years was discovered (5 Astraea), just one year before Neptune, and soon every year brought a few more asteroid discoveries.

Although they are still called "minor planets", they are no longer considered "planets". Thus there is precedent for the sort of "demotion" that some propose for Pluto (although Pluto has more than twice the diameter of Ceres and more than 10 times its mass).

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. The discoverers of 2003 UB313 have used the phrase "10th planet", indicating that they would prefer to add to the list rather than demote their own discovery.

Wikipedia