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For other uses, see Pluto (disambiguation).
Pluto (pronounced /ˈpluːtoʊ/, from Latin: Plūto, Greek: Πλούτων), also designated 134340 Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest memberPluto is the largest Kuiper belt object (KBO); According to Wikipedia convention, which treats the Scattered disc as distinct, Eris, although larger than Pluto, is not a KBO. of a distinct region called the Kuiper belt. Like other members of the belt, it is composed primarily 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 30 to 49 AU (4.4–7.4 billion km) 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 does.
Pluto and its largest moon, Charon, are often treated together as a binary system because the barycentre of their orbits does not lie within either body. C.B. Olkin, L.H. Wasserman, O.G. Franz (2003). The mass ratio of Charon to Pluto from Hubble Space Telescope astrometry with the fine guidance sensors-. Lowell Observatory 254–259. DOI:10.1016/S0019-1035(03)00136-2. Retrieved on 2007-03-13. The International Astronomical Union (IAU) has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, Charon is classified as a moon of Pluto. O. Gingerich (2006). The Path to Defining Planets. Harvard-Smithsonian Center for Astrophysics and IAU EC Planet Definition Committee chair. Retrieved on 2007-03-13. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005. B. Sicardy, W. Beisker et al. (2006). Observing Two Pluto Stellar Approaches In 2006: Results On Pluto\'s Atmosphere And Detection Of Hydra. Retrieved on 2007-03-13.
From its discovery in 1930 until 2006, Pluto was counted as 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, notably the scattered disc object Eris, which is 27% more massive than Pluto.Astronomers Measure Mass of Largest Dwarf Planet. hubblesite (2007). Retrieved on 2007-11-03. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which the IAU reclassified as a member of the new category of dwarf planets along with Eris and Ceres. A. Akwagyiram (2005-08-02). Farewell Pluto?. BBC News. Retrieved on 2006-03-05. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.T. B. Spahr (2006-09-07). MPEC 2006-R19 : EDITORIAL NOTICE. Minor Planet Center. Retrieved on 2006-09-07.D. Shiga (2006-09-07). Pluto added to official "minor planet" list. NewScientist. Retrieved on 2006-09-08.
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The discovery of Pluto was rooted in a misconception. When analyzing the orbits of the planets, scientists found that the orbit of Uranus didn\'t match their calculations. This discrepancy was thought to be the result of gravitational interactions with a more distant planet. Numerous stargazers published possible locations for this purported planet; however, Pluto is now known not to be massive enough to affect the orbits of the planets, and the discrepancy eventually turned out to have been caused by an overestimation of the mass of Neptune.
In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Hypothesising that the perturbations were caused by the gravitational pull of another planet, Le Verrier sent his calculations to German astronomer Johann Gottfried Galle. On September 23, 1846, the night following his receipt of the letter, Galle and his student Heinrich d\'Arrest found Neptune exactly where Le Verrier had predicted.K. Croswell (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. The Free Press, 43. ISBN 978-0684832524.
Observations of Neptune in the late 19th century caused astronomers to speculate that Uranus\' orbit was being disturbed by another planet in addition to Neptune. In 1905, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X".J. Rao (11 March 2005). Finding Pluto: Tough Task, Even 75 Years Later. SPACE.com. Retrieved on 2006-09-08. Lowell\'s hope in tracking down Planet X was to establish his scientific credibility, which had been dented by his widely derided belief that channel-like features visible on the surface of Mars were in fact canals constructed by an intelligent civilization.Croswell p. 48 By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search from 1905 until his death in 1916, but to no avail. Lowell\'s disappointment at not locating Planet X, according to one friend, "virtually killed him".Croswell p. 49
Constance Lowell, Percival Lowell\'s widow, subsequently embroiled the observatory in a decade-long legal battle to secure the observatory\'s million-dollar portion of Lowell\'s legacy for herself, which meant that its search for Planet X could not resume until 1929.Croswell, p. 50 In that year, the observatory\'s director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.
Tombaugh\'s task was systematically to image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates, to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 20 helped confirm the movement. Upon confirmation, Tombaugh walked into Slipher\'s office and declared, "Doctor Slipher, I have found your Planet X."Croswell p. 52 After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The new object would later be found on photographs dating back to March 19, 1915.W. G. Hoyt (1976). "W. H. Pickering\'s Planetary Predictions and the Discovery of Pluto". Isis 67 (4): 551–564.. Retrieved on 2007-06-27.
The right to name the new object belonged to the Lowell Observatory. Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did. Name suggestions poured in from all over the world. Constance Lowell proposed Zeus, then Lowell, and finally her own first name. These suggestions were disregarded.B. Mager. The Search Continues. Pluto: The Discovery of Planet X. Retrieved on 2007-03-27.
The name Pluto was first suggested by Venetia Burney (later Venetia Phair), an eleven-year-old schoolgirl in Oxford, England.P. Rincon (2006-01-13). The girl who named a planet. Pluto: The Discovery of Planet X. BBC News. Retrieved on 2007-04-12. Venetia was interested in classical mythology as well as astronomy, and considered the name, one of the alternate names of Hades, the Greek god of the Underworld, appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian of Oxford University\'s Bodleian Library. Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in America.K. M. Claxton. The Planet \'Pluto\'. Parents\' Union School Diamond Jubilee Magazine, 1891–1951 (Ambleside: PUS, 1951), p. 30–32. Retrieved on 2007-10-15.
The object was officially named on March 24, 1930. (Tuesday, May 27, 1930) "The Trans-Neptunian Body: Decision to call it Pluto". The Times\': 15. Each member of the Lowell Observatory was allowed to vote on a short-list of three: "Minerva" (which was already the name for an asteroid), "Cronus" (which had garnered a bad reputation after being suggested by an unpopular astronomer named Thomas Jefferson Jackson See), and Pluto. Pluto received every vote.Croswell pp. 54–55 The name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia five pounds as a reward.
The name Pluto was intended to evoke the initials of the astronomer Percival Lowell, a desire echoed in the P-L monogram that is Pluto\'s astronomical symbol (
).NASA\'s Solar System Exploration: Multimedia: Gallery: Pluto\'s Symbol. NASA. Retrieved on 2007-03-25.
Pluto\'s astrological symbol resembles that of Neptune (
), but has a circle in place of the middle prong of the trident (
).
In Chinese, Japanese, and Korean, the name was translated as underworld king star (冥王星),Planetary Linguistics. Retrieved on 2007-06-12. suggested by Houei Nojiri in 1930.Steve Renshaw and Saori Ihara (2000). A Tribute to Houei Nojiri. Retrieved on 2007-06-12. In Vietnamese it is named after Yama (Sao Diêm Vương), the Guardian of Hell in Buddhist mythology. Yama (Devanāgarī यम) is also used in India, as it is the deity of Hell in Hindu mythologies.
Once found, Pluto\'s faintness and lack of a resolvable disc cast doubt on the idea that it could be Lowell\'s Planet X. Throughout the mid-20th century, estimates of Pluto\'s mass were often revised downward. In 1978, the discovery of Pluto\'s moon Charon allowed the measurement of Pluto\'s mass for the first time. Its mass, roughly 0.2 percent that of the Earth, was far too small to account for the discrepancies in Uranus. Subsequent searches for an alternate Planet X, notably by Robert Harrington,P. K. Seidelmann and R. S. Harrington (1987). Planet X — The current status. U. S. Naval Observatory. Retrieved on 2007-11-04. failed. In 1993, Myles Standish used data from Voyager 2\'s 1989 flyby of Neptune, which had revised the planet\'s total mass downward by 0.5 percent, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.Ken Croswell (1993). Hopes Fade in hunt for Planet X. Retrieved on 2007-11-04. Today the overwhelming consensus among astronomers is that Planet X, as Lowell defined it, does not exist. Lowell had made a prediction of Planet X\'s position in 1915 that was fairly close to Pluto\'s actual position at that time; however, Ernest W. Brown concluded almost immediately that this was a coincidence, a view still held today.History I: The Lowell Observatory in 20th century Astronomy. The Astronomical Society of the Pacific (1994-06-28). Retrieved on 2006-03-05.
The largest plutinos compared in size, albedo and colour.
Pluto\'s distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizons spacecraft is expected to arrive there.Space Probe Heads To Pluto - Finally. CBS News (2006-01-19). Retrieved on 2007-04-14.
Pluto\'s apparent magnitude averages 15.1, brightening to 13.65 at perihelion.D. R. Williams (7 September 2006). Pluto Fact Sheet. NASA. Retrieved on 2007-03-24. To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.This month Pluto\'s apparent magnitude is m=14.1. Could we see it with an 11" reflector of focal length 3400 mm?. Singapore Science Centre. Retrieved on 2007-03-25. It looks indistinct and star-like even in very large telescopes because its angular diameter is only 0.11". It is light brown with a very slight tint of yellow.M. Cuk (September 2002). What color is each planet?. Curious about Astronomy?. Cornell University. Retrieved on 2007-03-25.
Spectroscopic analysis of Pluto\'s surface reveals it to be composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.Tobias C. Owen, Ted L. Roush et al. (6 August 1993). "Surface Ices and the Atmospheric Composition of Pluto". Science 261 (5122): 745–748. doi:10.1126/science.261.5122.745. Retrieved on 2007-03-29. Pluto. SolStation (2006). Retrieved on 2007-03-28. Distance and current limits on telescope technology make it impossible directly to photograph surface details on Pluto. Images from the Hubble Space Telescope barely show any distinguishable surface definitions or markings.Hubble Reveals Surface of Pluto for First Time. Hubblesite (1996). Retrieved on 2007-03-26.
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 dark spot. Using this technique, one can measure the total average brightness of the Pluto-Charon system and track changes in brightness over time.E. F. Young; R. P. Binzel; K. Crane (2000). "A Two-Color Map of Pluto Based on Mutual Event Lightcurves". Bulletin of the American Astronomical Society 32: 1083. Retrieved on 2007-03-26. 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.Alan Boyle. "Pluto regains its place on the fringe", MSNBC, 1999-02-11. Retrieved on 2007-03-20.
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 today, creating a subsurface ocean of liquid water.The Inside Story. New Horizons (2007). Retrieved on 2007-03-29.
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.Croswell p. 57 However, in 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto\'s albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth.Pluto\'s albedo is 1.3–2.0 times greater than that of Earth. Pluto Fact Sheet. NASA (7 September 2006). Retrieved on 2007-03-24.
The discovery of Pluto\'s 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 1.31×1022 kg—less than 0.24 percent that of the Earth.J. Davies (2001). Beyond Pluto (extract). Royal Observatory, Edinburgh. Retrieved on 2007-03-26. Observations of Pluto in occultation with Charon were able to fix Pluto\'s diameter at roughly 2,390 km.D. J. Tholen, M. W. Buie, R. P. Binzel, M. L. Frueh (1987). "Improved Orbital and Physical Parameters for the Pluto-Charon System". Science 237 (4814): 512–514. doi:10.1126/science.237.4814.512. Retrieved on 2007-03-26. With the invention of adaptive optics astronomers were able to determine its shape accurately.L. M. Close, W. J. Merline, D. J. Tholen, T. C. Owen, F. J. Roddier, C. Dumas, (2000). "Adaptive optics imaging of Pluto-Charon and the discovery of a moon around the Asteroid 45 Eugenia: the potential of adaptive optics in planetary astronomy". Proceedings of The International Society for Optical Engineering 4007: 787–795,. Retrieved on 2007-03-26.
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, Earth\'s 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.
Artist\'s conception of the New Horizons spacecraft passing over Pluto, showing its tenuous atmosphere
Pluto\'s atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide, derived from the ices on its surface.Ken Croswell (1992). Nitrogen in Pluto\'s Atmosphere. Retrieved on 2007-04-27. 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,T. Ker (2006). Astronomers: Pluto colder than expected. Space.com (via CNN.com). Retrieved on 2006-03-05. by use of the Submillimeter Array, that Pluto\'s temperature is 43 kelvins, 10 K 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.IAUC 4097 (1985). Retrieved on 2007-03-26. From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal, roughly 1/700,000 that of Earth.R. Johnston (2006). The atmospheres of Pluto and other trans-Neptunian objects. Retrieved on 2007-03-26.
In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory,B. Sicardy; T. Widemann, et al. (2003-07-10). "Large changes in Pluto\'s atmosphere as revealed by recent stellar occultations". Nature 424. Nature. doi:10.1038/nature01766. Retrieved on 2006-03-05. James L. Elliot of MIT,Pluto is undergoing global warming, researchers find. Massachusetts Institute of Technology (2002-10-09). Retrieved on 2007-03-20. and Jay Pasachoff of Williams College.Williams Scientists Contribute to New Finding About Pluto. Williams College (2003-07-09). Retrieved on 2007-03-20. The atmospheric pressure was estimated to be 0.3 pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.R. R. Britt (2003). Puzzling Seasons and Signs of Wind Found on Pluto. Space.com. Retrieved on 2007-03-26. Another stellar occultation was observed by the MIT-Williams College team of James Elliot, Jay Pasachoff, and a Southwest Research Institute team led by Leslie Young on 12 June, 2006 from sites in Australia. J. L. Elliot, M. J. Person, A. A. S. Gulbis, E. R. Adams, E. A. Kramer, C. A. Zuluaga, R. E. Pike, J. M. Pasachoff, S. P. Souza, B. A. Babcock, J. W. Gangestad, A. E. Jaskot, P. J. Francis, R. Lucas, A. S. Bosh (2006). The Size of Pluto\'s Atmosphere As Revealed by the 2006 June 12 Occultation. E Pasadena Division of Planetary Sciences. Retrieved on 2007-04-12.
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.A. Stern (November 1 2006). Making Old Horizons New. The PI\'s Perspective. Johns Hopkins University Applied Physics Laboratory. Retrieved on 2007-02-12.
Orbit of Pluto – ecliptic view. This \'side view\' of Pluto\'s orbit (in red) shows its large inclination to Neptune\'s orbit (in blue). The ecliptic is horizontal
Pluto\'s orbit is markedly different from 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 also lasted 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 forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, it is impossible to determine exactly where Pluto will be because its position becomes too sensitive to unmeasurably small details of the present state of the solar system.Gerald Jay Sussman; Jack Wisdom (1988). "Numerical evidence that the motion of Pluto is chaotic". Science 241: 433–437. Jack Wisdom; Matthew Holman (1991). "Symplectic maps for the n-body problem". Astronomical Journal 102: 1528–1538. For example, at any 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.
Orbit of Pluto — polar view. This \'view from above\' shows how Pluto\'s orbit (in red) is less circular than Neptune\'s (in blue), and how Pluto is sometimes closer to the Sun than Neptune. The darker halves of both orbits show where they pass below the plane of the ecliptic. The positions of both bodies are as of April 16, 2006; by April 2007 they had changed by about three pixels (~1 AU).
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.X.-S. Wan, T.-Y. Huang, and K. A. Innanen (2001). "The 1 : 1 Superresonance in Pluto\'s Motion". The Astronomical Journal 121: 1155–1162. doi:10.1086/318733. Retrieved on 2007-03-26. 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,Maxwell W. Hunter II (2004). Unmanned scientific exploration throughout the solar system. NASA Programs, Lockheed Missiles & Space Company. Retrieved on 2007-03-28. and so a similar distance above Neptune\'s orbit.Renu Malhotra (1997). Pluto\'s Orbit. Retrieved on 2007-03-26. Pluto\'s ascending node, the point at which the orbit crosses the ecliptic, is currently separated from Neptune\'s by over 21°;David R. Williams. Planetary Fact Sheet. NASA. Retrieved on 2007-03-31. 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.
This alone is not enough to protect Pluto; perturbations (e.g., orbital precession) from the planets, particularly Neptune, would adjust Pluto\'s orbit, so that over millions of years a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of these is a mean motion resonance with Neptune.
This diagram shows the relative positions of Pluto (red) and Neptune (blue) on selected dates. The size of Neptune and Pluto is depicted as inversely proportional to the distance between them to emphasise the closest approach in 1896.
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 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.Hannes Alfvén and Gustaf Arrhenius (1976). SP-345 Evolution of the Solar System. Retrieved on 2007-03-28. 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.
Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto\'s and Neptune\'s orbits does not change.J. G. Williams; G. S. Benson (1971). "Resonances in the Neptune-Pluto System". Astronomical Journal 76: 167. Retrieved on 2007-04-17. However, there are several other resonances and interactions that govern the details of their relative motion, and enhance Pluto\'s stability. These arise principally from two additional mechanisms (in addition to the 3:2 mean motion resonance).
First, Pluto\'s argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°. This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the solar system, preventing encounters with Neptune. This is a direct consequence of the Kozai mechanism, which relates the eccentricity of an orbit to its inclination, relative to a larger perturbing body — in this case Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto\'s perihelion to the orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000 years.
Second, the longitudes of ascending node of the two bodies — the points where they cross the ecliptic - are in near-resonance with the above libration. When the two longitudes are the same — that is, when one could draw a straight line through both nodes and the Sun — Pluto\'s perihelion lies exactly at 90°, and it comes closest to the Sun at its peak above Neptune\'s orbit. In other words, when Pluto most closely intersects the plane of Neptune\'s orbit, it must be at its farthest beyond it. This is known as the 1:1 superresonance.
To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise. After passing the ascending node, Pluto is interior to Neptune\'s orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune\'s expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, in accordance with Kepler\'s third law. As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune once again at the original node. The whole process takes about 20,000 years to complete.
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.
Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005. Guy Gugliotta. "Possible New Moons for Pluto." Washington Post. November 1, 2005. Retrieved on October 10, 2006.
The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius, the stable gravitational zone of Pluto\'s influence. 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."S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, J.R. Spencer (2006). "Characteristics and Origin of the Quadruple System at Pluto". Nature 439: 946–948.
The Pluto-Charon system is noteworthy for being the largest of the solar system\'s few binary systems, defined as those whose barycentre lies above the primary\'s surface (617 Patroclus is a smaller example).Derek C. Richardson and Kevin J. Walsh (2005). Binary Minor Planets. Department of Astronomy, University of Maryland. Retrieved on 2007-03-26. This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.B. Sicardy et al. (2006). Charon\'s size and an upper limit on its atmosphere from a stellar occultation. Retrieved on 2007-03-26. The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon. If one were standing on Pluto\'s near side, Charon would hover in the sky without moving; if one were to travel to the far side, one would never see Charon at all.Leslie Young (1997). The Once and Future Pluto. Southwest Research Institute, Boulder, Colorado. Retrieved on 2007-03-26. In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.Charon: An ice machine in the ultimate deep freeze. Gemini Observatory (2007). Retrieved on 2007-07-18.
| Name | Diameter (km) | Mass (kg) | Orbital radius (km) (barycentric) | Orbital period (d) | |
|---|---|---|---|---|---|
| Pluto | /ˈpluːtəʊ/ | 2306 (65% Moon) | 1.305 (7)×1022 (18% Moon) | 2,040 (100) (0.6% Moon) | 6.3872 (25% Moon) |
| Charon | /ˈʃɛərən, ˈkɛərən/ | 1205 (35% Moon) | 1.52 (7)×1021 (2% Moon) | 17,530 (90) (5% Moon) | |
Artist\'s concept of the surface of Hydra. Pluto with Charon (right) and Nix (bright dot on left).
Two additional moons of Pluto 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 officially named Pluto\'s newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.International Astronomical Union (2006-06-21). "IAU Circular No. 8723 - Satellites of Pluto". Press release. Retrieved on 2007-02-12.
These 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.F. R. Ward; RM Canup (25 August 2006). "Forced Resonant Migration of Pluto\'s Outer Satellites by Charon". Science 313 (5790): 1107–1109. doi:10.1126/science.1127293. Retrieved on 2007-02-12.
Observations of Nix and Hydra to determine individual characteristics are ongoing. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons\' spectral similarity to Charon suggests a 35% albedo similar to Charon\'s; this value results in diameter estimates of 46 kilometres for Nix and 61 kilometres for the brighter Hydra. 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% that of Charon, or 0.03% of Pluto\'s. H. A. Weaver; S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young (23 February 2006). "Discovery of two new satellites of Pluto". Nature 439 (7079): 943–945. doi:10.1038/nature04547. Retrieved on 2007-04-03.
The discovery of the two small moons suggests that Pluto may possess a variable ring system. Small body impacts can create debris that can form into planetary rings. Data from a deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggest that no ring system is present. If such a system exists, it is either tenuous like the rings of Jupiter or is tightly confined to less than 1000 km in width.Andrew J. Steffl; S. Alan Stern. "First Constraints on Rings in the Pluto System". The Astronomical Journal 133: 1485–1489. doi:10.1086/511770. astro-ph/0608036.
In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometres.A.J. Steffl; M.J. Mutchler, H.A. Weaver, S.A.Stern, D.D. Durda, D. Terrell, W.J. Merline, L.A. Young, E.F. Young, M.W. Buie, J.R. Spencer (2006). "New Constraints on Additional Satellites of the Pluto System". The Astronomical Journal 132: 614–619.
Plot of all known Kuiper belt objects, set against the four outer planets
Pluto\'s origin and identity have long puzzled astronomers. In the 1950s it was suggested that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. This notion has been heavily criticised because, as explained above, Pluto never actually comes near the planet.Pluto\'s Orbit. NASA New Horizons (2007). Retrieved on 2007-03-26.
Beginning in 1992, astronomers began to discover a large population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This belt, known as the Kuiper belt after one of the astronomers who first speculated on the nature of a trans-Neptunian population, is believed to be the source of many short-period comets. Astronomers now believe Pluto to be the largest of the known Kuiper belt objects (KBOs). Like other KBOs, Pluto shares features with comets; for example, the solar wind is gradually blowing Pluto\'s surface into space, in the manner of a comet. Colossal Cousin to a Comet?. New Horizons. Retrieved on 2006-06-23. If Pluto were placed near the Sun, it would develop a tail, as comets do. Neil deGrasse Tyson (1999). Space Topics: Pluto Top Ten: Pluto Is Not a Planet. The Planetary Society. Retrieved on 2006-06-23.
Though Pluto is the largest of the Kuiper belt objects discovered so far, Triton, which is slightly larger than Pluto, shares many atmospherical and geological composition similarities with Pluto and is believed to be a captured Kuiper belt object.
