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For other uses, see Pluto (disambiguation).
Pluto
Pluto in True Color
Pluto in True Color
Discovery
Discovered byClyde W. Tombaugh
Discovery dateFebruary 18, 1930
Designations
Designation
134340 Pluto
dwarf planet
AdjectivesPlutonian
SymbolAstronomical symbol of Pluto
Orbital characteristics
Epoch J2000
Aphelion7,375,927,931 km
(49.305 032 87 AU)
(4,583,200,123 miles)
Perihelion4,436,824,613 km
(29.658 340 67 AU)
(2,756,921,611 miles)
5,906,376,272 km
(39.481 686 77 AU)
(3,670,060,865 miles)
Eccentricity0.248 807 66
366.73 day
4.666 km/s
Inclination17.141 75°
(11.88° to Sun's equator)
110.303 47°
113.763 29°
Known satellites3
Physical characteristics
1195 km [1]
(19% of Earth, or
742.5 mi)
1.795×107 km²
(0.033 Earths)
Volume7.15×109 km³
(0.0066 Earths)
Mass(1.305±0.007)×1022 kg [2]
(0.0021 Earths)
Mean density
2.03±0.06 g/cm³ [3]
0.58 m/s²
(0.059 gee)
1.2 km/s
−6.387230 day
(6 day 9 h 17 m 36 s)
Equatorial rotation velocity
47.18 km/h (at the equator)
119.591 ± 0.014° (to orbit)[1][2]
North pole right ascension
133.046 ± 0.014°[1]
North pole declination
-6.145 ± 0.014°[1]
Albedo0.49–0.66 (varies by 35%) [4] [5]
Surface temp. min mean max
Kelvin 33 K 44 K 55 K
Atmosphere
Surface pressure
0.30 pascals (summer maximum)
Composition by volumenitrogen, methane

Pluto (IPA: BrE /ˈpluːtəʊ/, AmE /ˈplutoʊ/), also designated 134340 Pluto (see minor planet names), is the second-largest known dwarf planet in the solar system and the tenth largest observed body directly orbiting the Sun. 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 celestial pair combinations in the solar system, and because the barycenter of their orbits does not lie within either body.[3] However, the International Astronomical Union (IAU) has yet to formalize a definition for binary dwarf planets, so Charon is currently regarded as a moon of Pluto.[4] Two smaller moons, Nix and Hydra, were discovered in 2005.[5] 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 Solar System's ninth planet. 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. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which was then reclassified under the new category of dwarf planet along with Eris and Ceres.[6] Pluto is also classified as the prototype of a family of trans-Neptunian objects.[7][8] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[9][10]

Discovery

Discovery photographs of Pluto

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.[11]

False prediction

The history of how Pluto was discovered is intertwined with the discoveries of Neptune and Uranus. In the 1840s, using Newtonian mechanics, both Urbain Le Verrier and John Couch Adams had correctly predicted the position of the then-undiscovered planet Neptune after analyzing 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.[12]

Observations of Neptune in the late 19th century had astronomers starting to speculate that Uranus's orbit was also being disturbed by another planet in addition to Neptune. In 1905, Lowell Observatory (founded by Percival Lowell in 1894) started an extensive project in search of a possible ninth planet, which Lowell termed "Planet X".[13] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[14] The work continued after Lowell's death in 1916.

Pluto is too small to have the effect on Uranus's orbit that initiated the search. After the flyby of Neptune by Voyager 2 in 1989, it was conclusively demonstrated that the discrepancies in Uranus'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, which makes Tombaugh's discovery even more surprising.[15]

Naming

File:Venetia phair.jpg
Venetia Burney, the girl who named Pluto

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.[13] 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.[16]

The name Pluto was first suggested by Venetia Burney (later Venetia Phair), an eleven-year-old girl from Oxford, England.[17] 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.[18] Madan passed the suggestion to Professor Herbert Hall Turner, Turner then cabled the suggestion to colleagues in America.

The object was officially named on March 24 1930.[19] Each member of the Lowell Observatory was allowed to vote on a shortlist 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 single vote.[20] The name was announced on May 1, 1930.[17] Upon the announcement, Madan gave Venetia five pounds as a reward.[17]

The name that was 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 underworld king star (冥王星), suggested by Houei Nojiri in 1930. 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.

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.[21] Pluto's astrological symbol resembles that of Neptune ( ), but has a circle in place of the middle prong of the trident ( ).

Physical characteristics

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 the first space probe to the system.

Appearance

Pluto's mean apparent magnitude is 15.1 m with a maximum of 13.56 m.[22] To see it, a telescope of around 30 cm aperture is desirable.[23] It looks star-like even in very large telescopes because its angular diameter is only 0.15". The color of Pluto is light brown with a very slight tint of yellow.[24]

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.[25] 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.[26]

Mass and size

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

Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass based on 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.[27] 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.[27][28]

Pluto (bottom right) compared in size to the largest satellites in the solar system (from left to right and top to bottom): Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton.

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, Pluto's mass fell to 0.24 percent that of the Earth.[29] Observations were able to determine Pluto's diameter when it is at occultation with Charon[30] and its shape can be resolved by telescopes using adaptive optics.[31] Pluto's diameter is 2,390 km.

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. 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,[32] 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; 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.[33] From the rate of dimming, the atmosphere was determined to have a pressure of 0.15 Pa, roughly 1/700,000 that of Earth.[34]

In 2002, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy of the Paris Observatory[35] and by James Elliot of MIT[36] and Jay Pasachoff of Williams College.[37] 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.[38]

In October, 2006, the spectroscopic discovery of ethane (C2H6) on Pluto's surface, presented by Dale Cruikshank of NASA/Ames Research Center (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.[39]

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.[40]

Composition

Possible structure of Pluto.
1. Frozen methane
2. Water ice
3. Silicate and water ice
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

Spectroscopic analysis of Pluto's surface reveals it is compsed of over 98 percent nitrogen ice, with traces of methane and carbon monoxide.[41][42] It 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 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.[43] 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 50-30 percent ices.[42] 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 diffrentiated, 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.[44]

Orbit

Pluto's orbit is very unusual in comparison to the major planets of the solar system. 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 to the ecliptic (over 17°) and strongly eccentric (non-circular).

The high eccentricity also leads to a small region of Pluto's orbit lying closer to the Sun than Neptune's. Pluto last passed through 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. The previous time around, between April 30, 1483 and July 23, 1503 this situation persisted again for around 20 years similarly to the 20th century. In fact, the length of periods when Pluto is closer to the Sun than Neptune lasts alternately for approximately thirteen and twenty years with minor variations [citation needed].

Neptune-avoiding 3:2 resonance

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.

Despite Pluto's orbit apparently crossing that of Neptune when viewed from directly above the ecliptic, the two objects never collide. Pluto orbits with a 3:2 mean motion resonance to Neptune; for every 3 orbits of Neptune around the Sun, Pluto makes two. The alignment of the orbits is such 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.

The closest approach between Neptune and Pluto occurs some 30 years after Pluto's aphelion. When Neptune catches up with Pluto (i.e. when Neptune and Pluto have similar longitudes), the minimum distance between them is 18.9 AU. They last made closest approach in June 1896. In other words, Pluto never approaches Neptune much closer than it approaches Saturn.[citation needed]

Other orbital factors

Furthermore, their orbits are so aligned that Pluto and Neptune never approach closely. Two factors contribute to this:

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 on April 16, 2006; by April 2007 they will have changed by about 1 pixel.

Pluto is affected by the Kozai mechanism, which causes its closest approach to the Sun to occur when it is farthest out of the plane of the Solar System.[45] This means that the part of Pluto's orbit that lies as close or closer to the Sun than Neptune lies about 8 AU above the ecliptic,[46] and hence also 8 AU above Neptune's orbit.[47] Pluto's nodes (the points at which the orbit crosses the ecliptic) are separated from Neptune's by a distance of 6.4 AU (that is, over six times the distance of the Earth from the Sun). (see diagram) [citation needed]

Additionally, even when looking down on the ecliptic from a polar view (see diagram), the locations of Pluto and Neptune on their orbits are so aligned that they never approach closely.[48] From Neptune's perspective, Pluto nears to within about 18 AU from the left in one orbit, then from the right in its next orbit 248 years later. The 3:2 resonance ensures that this set of alignments repeats approximately every two Pluto orbits, and is preserved qualitatively over millions of years[49] (see below).

As a result, the closest Pluto ever comes to Neptune is 17 AU. In fact, Pluto occasionally approaches Uranus significantly closer — only 11 AU.[48]

Long-term orbit evolution

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. There is, however, a libration of Pluto's perihelion around a point 90° away from Neptune. This libration has a period of around 20,000 years (about 80 Pluto orbits) and a double amplitude of of 76° around its average location with respect to Neptune[49].

To understand its nature, let us choose a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise. The majority of Neptune's gravitational effect on Pluto occurs around the time when Pluto makes its closest approach to Neptune during the 500 year cycle that includes 2 Pluto orbits and 3 of Neptune. Hence, when Pluto makes its closest approach approximately from behind, as in catching up to Neptune along its orbit, then the net effect of Neptune will be to transfer angular momentum to Pluto. The net result of this angular momentum transfer turns out to be that the alignment of Pluto's orbit changes so that, in the next 500 year cycle, the point of closest approach will be at a slightly different position relative to Neptune.[49] In fact, in the phase described above, it gradually moves towards a position which is approximately in opposition to Neptune, at Pluto's aphelion, over about 10 such cycles. Later, the slow change in the orbit continues in a similar fashion until the point of closest approach takes place approximately ahead of Neptune in its orbit. During this phase, the net effect of Neptune is again to pull Pluto towards itself, but this time, because of their different relative orientation, this has the effect of taking angular momentum away from Pluto. The point of closest approach heads toward a point ahead of Neptune, then starts to head back towards being located at aphelion again, and the whole libration repeats.[49][48]

Moons

Main article: Pluto's natural satellites
Pluto and its three known moons. Pluto and Charon are the bright objects in the center, the two smaller moons are at the right and bottom, farther out.
Artist's concept of the surface of Hydra. Pluto with Charon (right) and Nix (bright dot on left).

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.[50]

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."[51]

Charon

The Pluto-Charon system is noteworthy for being the largest of the solar system's few binary systems, i.e. whose barycenter lies above the primary's surface (617 Patroclus is a smaller example).[52] This and the large size of Charon relative to Pluto lead some astronomers to call it a dwarf double planet.[53] 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.[54]



Pluto and Charon, compared to Earth's Moon
Name

(Pronunciation key)

Diameter (km) Mass (kg) Orbital radius (km)
(barycentric) 		Orbital period (d)
Pluto ploo'-toe
/ˈpluːtəʊ/		 2306
(65% Moon)		 1.3×1022
(18% Moon)		 2390
(0.6% Moon)		 6.3872
(25% Moon)
Charon shair'-ən
/ˈʃɛərən/		 1205
(35% Moon)		 1.5×1021
(2% Moon)		 19,570
(5% Moon)

Nix and Hydra

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 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.[55]

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.[56]

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.[57]

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.[58]

Possibility of 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.[59]

Kuiper belt

Main article: Kuiper belt
File:Mainkuiperbelt.jpg
Artist's conception of the Kuiper belt, with Pluto's orbit (yellow) contrasted with Neptune's (white)

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 Triton, Neptune's largest moon. This notion has been heavily criticised, since, as explained above, Pluto never actually comes near the planet in its orbit.[60]

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 hypothesised its existence, is believed to be the source for all short-period comets. Astronomers now believe Pluto to be the largest of the Kuiper belt objects. 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.[61] If Pluto were placed near the Sun, it would develop a tail, like comets do.[62]

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.[63] Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population.

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.[64]

Exploration of Pluto

Main article: New Horizons
New Horizons, launched on January 19, 2006

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.[65] In 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.[66]

File:112806 pluto animation.gif
First Pluto sighting from New Horizons

The first spacecraft to visit Pluto will be NASA's New Horizons, launched on January 19, 2006. In early 2007 the craft made use of a gravity assist from Jupiter, and its closest approach to Pluto will be on July 14, 2015. Scientific observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter. New Horizons captured its first images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI).[67] The images, taken from a distance of approximately 4.2 billion kilometres (2.6 billion miles), confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.

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. Some of 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.[58]

Planetary status controversy

See also: Definition of planet

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 QB1, was discovered. Since then, further discoveries intensified the debate in the 21st century.

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.[68] 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.[69] Elements 92, 93, and 94 are named uranium, neptunium, and plutonium respectively after Uranus, Neptune, and Pluto. The Disney character Pluto, introduced in 1930, was also named in honour of the planet.[70]

New discoveries ignite debate

Error: Image is invalid or non-existent.

The discovery of the Kuiper belt and Pluto's relation to it led many to question whether Pluto could be considered separately from others in its population. In 2002, the KBO 50000 Quaoar was discovered, with a diameter of roughly 1,280 kilometres, about half that of Pluto.[71] In 2004, the discoverers of 90377 Sedna placed an upper limit of 1,800 kilometres on its diameter, near Pluto's diameter of 2,320 kilometres.[72]

On July 29, 2005, a Trans-Neptunian object later named Eris was announced, which on the basis of its magnitude and simple albedo considerations is assumed to be slightly larger than Pluto.[73] 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.[74] Others in the astronomical community considered the discovery to be the strongest argument for reclassifying Pluto as a minor planet.[75]

The last remaining distinguishing features of Pluto were 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.[76] It also possesses a moon, Dysnomia, discovered in September 2005. Trans-Neptunian object 2003 EL61 (nicknamed "Santa") has two moons (one of which is nicknamed "Rudolph") and is the fourth largest TNO behind Eris, Pluto, and 2005 FY9 (nicknamed "Easterbunny").

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.[77]

IAU Decision

Main article: 2006 redefinition of planet

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':

  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.[78]

Pluto fails to meet the third condition.[79] 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.

There has been resistance amongst the astronomical community towards the reclassification,[80] dubbed the "Great Pluto War" by some astronomers.[81][82]

File:New horizons Pluto.jpg
New Horizons spacecraft

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."[83] 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.[84] However, his own published writing has supported the new list of planets, as "our solar system clearly contains" eight planets that have cleared their neighbourhoods, however, he does not deny planetary classification to objects like Pluto & Ceres that have not "cleared their neighbourhood".[85] Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitoners against the definition.[86] 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."[87]

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.[88] The U.S. state of New Mexico's House of Representatives passed a resolution declaring that Pluto will always be considered a planet while overhead of the state, with March 13th being known as "Pluto Planet Day".[89] 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.[90]

The verb "pluto" (preterite and past participle: "plutoed") was coined in the aftermath of the decision. In January 2007, the American Dialect Society chose "plutoed" as its 2006 Word of the Year, defining "to pluto" as "to demote or devalue someone or something", an example being "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."[91]

See also

References

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  2. ^ Based on the orientation of Charon's orbit, which is assumed the same as Pluto's spin axis due to the mutual tidal locking.
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Further reading

  • Henderson, Mark (Oct. 30, 2005). "Pluto may lose status of planet". New Straits Times, p. F17.
  • Kaufmann III, William J., "Universe", 2nd Edition, pp.302–303
  • Pasachoff, Jay M., and Alex Filippenko, 2007, "The Cosmos: Astronomy in the New Millennium," 3rd Edition.

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