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Pluto

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy); The
Planets

   CAPTION: Pluto Astronomical symbol of Pluto

                  Pluto in True Color
                       Discovery
   Discovered by         Clyde W. Tombaugh
   Discovered on         February 18, 1930
         Orbital characteristics ( Epoch J2000)
   Semi-major axis       5,906,376,272 km
                         39.481 686 77 AU
   Orbital circumference 36.530 T m
                         244.186 AU
   Eccentricity          0.248 807 66
   Perihelion            4,436,824,613 km
                         29.658 340 67 AU
   Aphelion              7,375,927,931 km
                         49.305 032 87 AU
   Orbital period        90,613.3055 d
                         (248.09 a)
   Synodic period        366.73 d
   Avg. Orbital Speed    4.666 km/s
   Max. Orbital Speed    6.112 km/s
   Min. Orbital Speed    3.676 km/s
   Inclination           17.141 75°
                         (11.88° to Sun's equator)
   Longitude of the
   ascending node        110.303 47°
   Argument of the
   perihelion            113.763 29°
   Number of satellites  3
                Physical characteristics
   Diameter              2390 km ^
                         (19% of Earth, or
                         1485 mi)
   Surface area          1.795×10^7 km²
                         (0.033 Earths)
   Volume                7.15×10^9 km³
                         (0.0066 Earths)
   Mass                  (1.305±0.007)×10^22 kg ^
                         (0.0021 Earths)
   Mean density          2.03±0.06 g/cm³ ^
   Equatorial gravity    0.58 m/s²
                         (0.059 gee)
   Escape velocity       1.2 km/s
   Rotation period       −6.387230 d
                         (6 d 9 h 17 m 36 s)
   Rotation velocity     47.18 km/h (at the equator)
   Axial tilt            119.59° (to orbit)
                         112.78° (to the ecliptic)
   Right ascension
   of North pole         133.045±0.02°
                         (8 h 52 min 11 s) ^
   Declination           -6.145±0.02°
   Albedo                0.49–0.66 (varies by 35%) ^
   Surface temp.
                         min  mean max
                         33 K 44 K 55 K
   Adjective             Plutonian
              Atmospheric characteristics
   Atmospheric pressure  0.30 pascals (summer maximum)
   Composition           nitrogen, methane

          Note: This article contains special characters.

   Pluto ( IPA: /ˈpluːtəʊ/), designated (134340) Pluto in the Minor Planet
   Centre catalogue, is the second-largest known dwarf planet in the solar
   system. It orbits between 29 and 49 AU from the Sun, and was the first
   Kuiper Belt Object to be discovered. Approximately one-fifth the mass
   of the Earth's Moon, Pluto is primarily composed of rock and ice. It
   has an eccentric orbit that is highly inclined with respect to the
   planets and takes it closer to the Sun than Neptune during a portion of
   its orbit. Pluto and its largest satellite, Charon, could be considered
   a binary system because they are closer in size than any of the other
   known planetoid/moon combinations in the solar system, and because the
   barycentre 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, so Charon is regarded as a moon of
   Pluto. Two smaller moons, Nix and Hydra, were discovered in 2005. Pluto
   is smaller than several of the natural satellites or moons in our solar
   system (see the list of solar system objects by radius).

   From its discovery by Clyde Tombaugh in 1930, Pluto was considered the
   ninth planet from the Sun. In the late 20th and early 21st century,
   many similar objects were discovered in the outer solar system, most
   notably the Trans-Neptunian object Eris which is slightly larger than
   Pluto. In August 2006 the IAU redefined the term "planet", and
   classified Pluto, Ceres, and Eris as dwarf planets. Pluto is also
   classified as the prototype of a family of trans-Neptunian objects.
   After the reclassification, Pluto was added to the list of minor
   planets and given the number 134340.

Discovery

   In 1930 Clyde Tombaugh was working on a project searching for a ninth
   planet at Lowell Observatory. Tombaugh's work was to systematically
   take pictures of the celestial sky in pairs, one to two weeks apart,
   then look for objects that had moved between images. On February 18,
   1930, Tombaugh discovered a possible moving object on photographic
   plates taken on January 23 and January 29 of that year. A
   lesser-quality photo taken on January 20 helped confirm the movement.
   After the observatory worked to obtain further confirmatory
   photographs, news of the discovery was telegraphed to the Harvard
   College Observatory on March 13, 1930. Pluto would later be found on
   photographs dating back to March 19, 1915.

Relations to Neptune and Uranus

   The history of how Pluto was discovered is intertwined with the
   discoveries of Neptune and Uranus. 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 perturbations in the orbit of Uranus. Theorizing the
   perturbations were caused by the gravitational pull of another planet,
   Johann Gottfried Galle discovered Neptune on September 23, 1846.

   Observations of Neptune in the late 19th century had astronomers
   starting to speculate that Neptune's orbit too was also being disturbed
   by another planet in a similar manner that Neptune was disturbing
   Uranus. 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
   calculations by Indian astronomer V.B. Ketakar which predicted a
   location for an undiscovered planet.

Percival Lowell's influence

   Internal structure of Pluto
   Enlarge
   Internal structure of Pluto

   Percival Lowell would have significant influence on Pluto's discovery.
   In 1905, Lowell Observatory (founded by Lowell in 1894) started an
   extensive project in search of a possible ninth planet. The work
   continued after Lowell's death in 1916. Lowell was searching for a
   theoretical Planet X to match observations seen in Uranus and Neptune.

   Pluto is too small to have the effect on Neptune's orbit that initiated
   the search. After the flyby of Neptune by Voyager 2 in 1989, it was
   conclusively demonstrated that the discrepancies in Neptune's orbit
   observed by 19th century astronomers were due instead to inaccurate
   estimates of Neptune's mass. Once found, Pluto's faintness and lack of
   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 was 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 still held today. Tombaugh's discovery is
   therefore even more surprising, given that Pluto's proximity to the
   region predicted by Pickering, Lowell, and Ketakar was likely a mere
   coincidence.

Naming

   The right to name the new object belonged to the Lowell Observatory and
   its director, Vesto Melvin Slipher. Tombaugh urged Slipher to suggest a
   name quickly for the new object before someone else did. Name
   suggestions poured in from all over the world. Constance Lowell,
   Percival Lowell's widow, proposed Zeus, then Lowell, and finally her
   own first name, none of which met with any enthusiasm. Mythological
   names, such as Cronus and Minerva, were high on a list of considered
   names.

   The name Pluto was first suggested by Venetia Burney (later Venetia
   Phair), at the time an eleven-year-old girl from Oxford, England.
   Venetia, who was interested in Classical mythology as well as
   astronomy, suggested the name, the Roman equivalent of Hades, in a
   conversation to her grandfather Falconer Madan, a former librarian of
   Oxford University's Bodleian Library. Madan passed the suggestion to
   Professor Herbert Hall Turner, Turner then cabled the suggestion to
   colleagues in America. After favourable consideration which was almost
   unanimous, the name Pluto was officially adopted and an announcement
   made on May 1, 1930. Upon the announcement, Madan gave Venetia five
   pounds as a reward.

   The name retained for the object is that of the Roman god Pluto, and it
   is also intended to evoke the initials of the astronomer Percival
   Lowell. In the Chinese, Japanese, and Korean languages, the name was
   translated as death king star (冥王星), suggested by Houei Nojiri in 1930.
   China started official use of the name in 1933. Japan used the
   pronunciation プルートー (purūtō); later Tokyo Observatory decided to adopt
   it, and it became the official name in Japan in 1943. In Vietnamese it
   is named after Yama (Sao Diêm Vương), the Guardian of Hell in Buddhist
   mythology.

Symbol

   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
   founded Lowell Observatory, the observatory from which Tombaugh
   discovered Pluto. Besides its astronomical symbol Pluto also has an
   astrological symbol. Pluto's astrological symbol resembles that of
   Neptune ( ), but has a circle in place of the middle prong of the
   trident ( ).

Physical characteristics

   Diagram of Pluto (top left) and its moons (top right) compared in size,
   albedo and color index with the largest plutinos: Orcus (bottom left)
   and Ixion (bottom right).
   Enlarge
   Diagram of Pluto (top left) and its moons (top right) compared in size,
   albedo and colour index with the largest plutinos: Orcus (bottom left)
   and Ixion (bottom right).

   Many details about Pluto remain unknown, mainly due to the fact that it
   has not been visited up close by spacecraft. Pluto's distance from
   Earth makes in-depth investigation difficult.

Appearance

   Pluto's apparent magnitude is fainter than 14 m and therefore a
   telescope is required for observation. To see it, a telescope of around
   30 cm aperture is desirable. It looks 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.

   Charon's discovery resulted in the calculation of Pluto's albedo's
   being revised upward; since Pluto was now seen as being far smaller
   than originally estimated, its capacity to reflect light must be
   greater than formerly believed. Current estimates place Pluto's albedo
   as marginally less than that of Venus, which is fairly high.

   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.

Mass and size

   Pluto's volume is about 0.66% that of Earth's
   Enlarge
   Pluto's volume is about 0.66% that of Earth's

   Pluto's diameter and mass were incorrectly overestimated for many
   decades after its discovery. Initially it was thought to be relatively
   large, with a mass comparable to Earth, but over time the estimates
   were revised sharply downward as observations were refined.

   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. 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 two objects instead of
   one, the estimated size of Pluto was revised downward. Observations
   were able to determine Pluto's diameter when it is at occultation with
   Charon, and its shape can be resolved by telescopes using adaptive
   optics.
   Pluto (bottom right) compared in size to the largest moons in the solar
   system (from left to right and top to bottom): Ganymede, Titan,
   Callisto, Io, the Moon, Europa, and Triton.
   Enlarge
   Pluto (bottom right) compared in size to the largest moons in the solar
   system (from left to right and top to bottom): Ganymede, Titan,
   Callisto, Io, the Moon, Europa, and Triton.

   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 and less massive 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 trans-Neptunian
   Kuiper belt object Eris, discovered in 2005. See List of solar system
   objects by mass and List of solar system objects by radius.

Atmosphere

   Pluto does not have a significant atmosphere. It has a thin envelope of
   gas that is most likely made up of nitrogen, methane, and carbon
   monoxide, that develops in equilibrium with solid nitrogen and carbon
   monoxide ices on the surface as it approaches the Sun. 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 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 1985 (IAU Circ. 4097; MNRAS 276, 571); 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 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
   James 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.

   In October, 2006, the spectroscopic discovery of ethane (C2H6) on
   Pluto's surface, presented by Dale Cruikshank of NASA/Ames Research
   Centre (a New Horizons co-investigator) and colleagues was announced.
   This ethane is produced from the photolysis or radiolysis (i.e., the
   chemical conversion driven by sunlight and charged particles) of frozen
   methane (CH4) on Pluto's surface and suspended in its atmosphere.

   The MIT-Williams College team of James Elliot and Jay Pasachoff and a
   Southwest Research Institute team led by Leslie Young observed a
   further occultation of a star by Pluto on 12 June 2006 from sites in
   Australia. (Elliot, J. L., Person, M. J., Gulbis, A. A. S., Adams, E.
   R., Kramer, E. A., Zuluaga, C. A., Pike, R. E., Pasachoff, J. M.,
   Souza, S. P., Babcock, B. A., Gangestad, J. W., Jaskot, A. E., Francis,
   P. J., Lucas, R., Bosh, A. S. 2006, "The Size of Pluto's Atmosphere As
   Revealed by the 2006 June 12 Occultation," Pasadena Division of
   Planetary Sciences meeting, October 2006.)

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. This makes
   Pluto the second most contrasted body in the Solar System after
   Iapetus.

Orbit

   Orbit of Pluto – ecliptic view. This 'side view' of Pluto's orbit (in
   red) shows how steeply inclined the orbit is in comparison to Neptune's
   more normal orbit (in blue)
   Enlarge
   Orbit of Pluto – ecliptic view. This 'side view' of Pluto's orbit (in
   red) shows how steeply inclined the orbit is in comparison to Neptune's
   more normal orbit (in blue)

   Pluto's orbit is very unusual in comparison to the planets of the solar
   system. The planets orbit the Sun close to an imaginary flat plane
   called the plane of the ecliptic, and have nearly circular orbits. In
   contrast, Pluto's orbit is highly inclined above the ecliptic (up to
   17° above it) and very eccentric (non-circular). Owing to the orbit’s
   inclination, Pluto's perihelion is well above (~8.0 AU) the ecliptic.
   The high eccentricity means that part of Pluto's orbit is closer to the
   Sun than Neptune's.

Heliocentric distance

   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 also
   shows 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 are marked as of April 16, 2006; in
   April 2007 they will have changed by about 1 pixel.
   Enlarge
   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 also
   shows 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 are marked as of April 16, 2006; in
   April 2007 they will have changed by about 1 pixel.

   Near perihelion, Pluto gets closer to the Sun than Neptune; the most
   recent occurrence of this phenomenon lasted from February 7, 1979
   through February 11, 1999. Mathematical calculations indicate that the
   previous occurrence lasted only fourteen years from July 11, 1735 to
   September 15, 1749. However, the same calculations indicate that Pluto
   was closer to the Sun than Neptune 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 with 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.

Trans-Neptunian object

   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 to facilitate comparison. The
   closest approach is in 1896.
   Enlarge
   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 to facilitate comparison. The
   closest approach is in 1896.

   Pluto's orbit is often described as 'crossing' that of Neptune. In
   fact, Pluto's nodes (the points at which the orbit crosses the
   ecliptic) are both situated outside Neptune’s orbit and are separated
   by a distance of 6.4 AU (that is, over six times the distance of the
   Earth from the Sun). Furthermore, due to the orbital resonance between
   them, Pluto executes 2 full cycles while Neptune makes 3; this means
   that when Neptune reaches the 'closest' point on the orbit, Pluto
   remains far behind and when Pluto in turn reaches that point, Neptune
   is far (over 50°) ahead. During the following orbit of Pluto, Neptune
   is half an orbit away. Consequently, Pluto never gets closer than 30 AU
   to Neptune at this point in its orbit.

   The actual closest approach between Neptune and Pluto occurs at the
   opposite part of the orbit, some 30 years after Pluto's aphelion (its
   last aphelion was in 1866) when Neptune catches up with Pluto (i.e.
   Neptune and Pluto have similar longitudes). The minimum distance was
   18.9 AU in June 1896. In other words, Pluto never approaches Neptune
   much closer than it approaches Saturn.

Comet comparison

   The Kuiper belt is believed to be the source for all short-period
   comets, and Pluto, like other Kuiper Belt objects, shares features in
   common with comets. The solar wind is gradually blowing 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.

Moons

   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.
   Enlarge
   Pluto and its three known moons. Pluto and Charon are the bright
   objects in the centre, 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.

Charon

   The Pluto-Charon system is noteworthy for being the largest of the few
   binary systems in the solar system, i.e. the barycenter lies above the
   primary's surface ( 617 Patroclus is a smaller example). This and the
   large size of Charon relative to Pluto lead some astronomers to call it
   a dwarf double planet. The system is also unusual among planetary
   systems in that they are both tidally locked to each other: Charon
   always presents the same face to Pluto, and Pluto also always presents
   the same face to Charon.

   Some researchers have theorized that Pluto and 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. Today it is
   widely accepted that Pluto never orbited Neptune.

   Pluto and Charon, compared to Earth's Moon
   Name

   ( Pronunciation key)
   Diameter
   (km) Mass
   (kg) Orbital radius (km) Orbital period (days)
   Pluto ploo'-toe
   /ˈpluːtəʊ/ 2306
   (65% Moon) 1.3×10^22
   (18% Moon) 2390
   (0.6% Moon) 6.3872
   (25% Moon)
   Charon shair'-ən
   /ˈʃɛərən/ 1205
   (35% Moon) 1.5×10^21
   (2% Moon) 19,570
   (5% Moon)

Nix and Hydra

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

   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 christened 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.

   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
   kilometers 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.

   Observations of Nix and Hydra are ongoing to determine individual
   characteristics. Hydra is sometimes brighter than Nix, speculating that
   it either is larger in dimension or different parts of its surface may
   vary in brightness. Sizes are estimated from albedos. The moons'
   spectral similarity with Charon suggests a 35% albedo similar to
   Charon's; this results in diameter estimates of 46 kilometers for Nix
   and 61 kilometers for 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% of
   Charon's mass, or 0.03% of Pluto's.

   With the discovery of the two small moons, Pluto may possess a variable
   ring system. Small body impacts can create debris that can form into a
   ring system. Data from a deep optical survey by the Advanced Camera for
   Surveys on the Hubble Space Telescope suggests that no ring system is
   present. If such a system exists, it is either tenuous like the Rings
   of Jupiter, or it is tightly confined to less than 1000km in width.

Distribution

   Artist's concept of the surface of Hydra. Pluto with Charon (right) and
   Nix (bright dot on left).
   Enlarge
   Artist's concept of the surface of Hydra. Pluto with Charon (right) and
   Nix (bright dot on left).

   The distribution of Plutonian moons is highly unusual compared to other
   observed systems. Moons could potentially orbit Pluto up to 53% (or
   69%, if retrograde) of the Hill sphere radius (stable gravitational
   zone of influence) of 6.0 million kilometers. In simple terms, an
   imaginary sphere is drawn around an object to represent the potential
   of an object to have other objects orbit it stably. 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."

Additional moons?

   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 kilometers.

Exploration of Pluto

   Photo of New Horizons, the first probe to Pluto, launched on January
   19, 2006 (it is planned to reach Pluto in July 2015)
   Enlarge
   Photo of New Horizons, the first probe to Pluto, launched on January
   19, 2006 (it is planned to reach Pluto in July 2015)

   Pluto presents significant challenges for space craft because of its
   small mass and great distance from Earth. Voyager 1 could have visited
   Pluto, but controllers opted instead for a close flyby of Saturn's moon
   Titan, which resulted in a trajectory incompatible with a Pluto flyby.
   Voyager 2 never had a plausible trajectory for reaching Pluto. In 2000,
   NASA cancelled the Pluto Kuiper Express mission, citing increasing
   costs and launch vehicle delays.

   The first spacecraft to visit Pluto will be NASA's New Horizons,
   launched on January 19, 2006. The craft will benefit from a gravity
   assist from Jupiter, and the closest approach to Pluto will be on July
   14, 2015. Observations of Pluto will begin 5 months prior to closest
   approach and will continue for at least a month after the encounter.

   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 will also photograph the surfaces of Pluto and
   Charon. The ashes of Pluto's discoverer, Clyde W. Tombaugh, are aboard
   the spacecraft.

   Discovery of moons Nix and Hydra may present unforeseen challenges for
   the probe. With the relatively low escape velocity of Nix and Hydra,
   collisions with Kuiper belt debris may produce a tenuous dusty ring.
   Were New Horizons to fly through such a ring system, there would be an
   increased potential for micrometeorite damage that could damage or
   disable the probe.

Planetary status controversy

   Pluto's official status as a planet has been a constant subject of
   controversy, fueled by the past lack of a clear definition of planet,
   since at least as early as 1992, when the first Kuiper Belt Object,
   (15760) 1992 QB[1], was discovered. Since then, further discoveries
   intensified the debate in the 21st century.

Omission from museum models

   Museum and planetarium directors occasionally created controversy by
   omitting Pluto from planetary models of the solar system. Some
   omissions were intentional; the Hayden Planetarium reopened after
   renovation in 2000 with a model of 8 planets without Pluto. The
   controversy made headlines in the media at the time.

Commemoration as a planet

   Pluto is shown as a planet on the Pioneer plaque, an inscription on the
   space probes Pioneer 10 and Pioneer 11, launched in the early 1970s.
   The plaque, intended to give information about the origin of the probes
   to any alien civilization that might in the future encounter the
   vehicles, includes a diagram of our solar system, showing nine planets.
   Similarly, an analog image contained within the Voyager Golden Record
   included on the probes Voyager 1 and Voyager 2 (also launched in the
   1970s) includes data regarding Pluto and again shows it as the ninth
   planet.

   Elements 92, 93, and 94 are named uranium, neptunium, and plutonium
   respectively after Uranus, Neptune, and Pluto.

New discoveries ignite debate

   Pluto compared to Eris, 2005 FY9, 2003 EL61, Sedna, Orcus, Quaoar, and
   Varuna compared to Earth (artist's impressions; no detailed photographs
   exist).
   Enlarge
   Pluto compared to Eris, 2005 FY[9], 2003 EL[61], Sedna, Orcus, Quaoar,
   and Varuna compared to Earth (artist's impressions; no detailed
   photographs exist).

   Continuing advances in telescope technology allowed for further
   discoveries of Trans-Neptunian objects in the 21st century, some of
   comparable size to that of Pluto. In 2002, 50000 Quaoar was discovered,
   with a 1,280 kilometers diameter, making it a bit more than half the
   size of Pluto. In 2004, the discoverers of 90377 Sedna placed an upper
   limit of 1,800 kilometers on its diameter, near Pluto's diameter of
   2,320 kilometers.

   On July 29, 2005, a Trans-Neptunian object called Eris was announced,
   which on the basis of its magnitude and simple albedo considerations is
   assumed to be slightly larger than Pluto. This was the largest object
   discovered in the solar system since Neptune in 1846. Discoverers and
   media initially called it the "tenth planet", although there was no
   official consensus at the time on whether to call it a planet. Others
   in the astronomical community considered the discovery to be the
   strongest argument for reclassifying Pluto as a minor planet.

   The last remaining distinguishing feature of Pluto was now its large
   moon, Charon, and its atmosphere; these characteristics are probably
   not unique to Pluto: several other Trans-Neptunian objects have
   satellites; and Eris' spectrum suggests that it has a similar surface
   composition to Pluto, as well as a moon, Dysnomia, discovered in
   September 2005. Trans-Neptunian object 2003 EL[61] (nicknamed "Santa")
   has two moons (one of which is nicknamed "Rudolph") and is the fourth
   largest TNO behind Eris, Pluto, and 2005 FY[9] (nicknamed
   "Easterbunny").

IAU Decision

   There are three main conditions for an object to be called a 'planet',
   according to the IAU resolution passed August 24, 2006.
    1. The object must be in orbit around the Sun.
    2. The object must be massive enough to be a sphere by its own
       gravitational force. More specifically, its own gravity should pull
       it into a shape of hydrostatic equilibrium.
    3. It must have cleared the neighbourhood around its orbit.

   Pluto fails to meet the third condition.

   The IAU further resolved that Pluto be classified in the simultaneously
   created dwarf planet category, and that it act as prototype for a
   yet-to-be-named category of trans-Neptunian objects, in which it would
   be separately, but concurrently, classified.

   Prior to this decision several other definitions had been proposed,
   some of which might have ruled out planetary status for Earth or
   Mercury or may have classified several of the asteroids as planets.
   This version was democratically chosen in a successful attempt at
   avoiding these non-traditional results.

Impact of the IAU decision

   There has been resistance amongst the astronomical community towards
   the reclassification. Alan Stern, principal investigator with NASA's "
   New Horizons" mission to Pluto, has publicly derided the IAU
   resolution, stating that "the definition stinks" albeit "for technical
   reasons." Stern's current contention is that by the terms of the new
   definition Earth, Mars, Jupiter and Neptune, all of which share their
   orbits with asteroids would be excluded. However, his own published
   writing has supported the new list of planets, as "our solar system
   clearly contains" eight planets that have cleared their neighborhoods.
   Others have supported the IAU. Mike Brown, the astronomer who
   discovered Eris, said "through this whole crazy circus-like procedure,
   somehow the right answer was stumbled on. It’s been a long time coming.
   Science is self-correcting eventually, even when strong emotions are
   involved."

   Among the general public, reception is mixed amidst widespread media
   coverage. Some have accepted the reclassification, while some are
   seeking to overturn the decision, with online petitions urging the IAU
   to consider reinstatement. A resolution introduced by some members of
   the California state assembly light-heartedly denounces the IAU for
   "scientific heresy," among other crimes. Others reject the change for
   sentimental reasons, citing that they have always known Pluto as a
   planet and will continue to do so regardless of the IAU decision.

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