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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 body observed directly orbiting the Sun. Originally considered a planet, Pluto has since been recognised as the largest member of a distinct region called the Kuiper belt. Like other members of the belt, it is primarily composed of rock and ice and is relatively small; approximately a fifth the mass of the Earth's Moon and a third its volume. Pluto is smaller than seven moons in the Solar System. It has an eccentric orbit that takes it from 29 to 49 AU (4.3–7.3 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.

Pluto and its largest satellite, Charon, are often considered a binary system because the barycentre of their orbits does not lie within either body.[3] However, the International Astronomical Union (IAU) has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, Charon remains a moon of Pluto.[4] Pluto has two smaller moons, Nix and Hydra, discovered in 2005.[5].

From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st centuries however, many objects similar to Pluto were discovered in the outer solar system, most notably the scattered disc object Eris, which is slightly larger than Pluto. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto from planethood, and reclassified it under the new category of dwarf planet along with Eris and Ceres.[6] After the reclassification, Pluto was added to the list of minor planets and given the number 134340.[7][8]


Discovery photographs of Pluto
Discovery photographs of Pluto

In 1930 Clyde Tombaugh was searching for a ninth planet as part of a project at Lowell Observatory. His task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine if any objects had shifted position in that time. Using a machine called a blink comparator, he rapidly shifted each plate back and forth to create the illusion of movement. 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 photograph taken on January 20 helped confirm the movement. After the observatory obtained 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.[9]

Incorrect 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 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 precisely where Le Verrier had predicted.[10]

Observations of Neptune in the late 19th century caused astronomers to speculate that Uranus's orbit was 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".[11] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.[9] The work continued after Lowell's death in 1916.

Once found, Pluto's faintness and lack of a visible disc cast doubt on the idea that it could be Lowell's Planet X. Today, astronomers know that no one had actually predicted Pluto's existence, as it is far 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. Lowell had made a prediction of Planet X's position in 1915 which 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, which makes Tombaugh's discovery even more surprising.[12]


Venetia Burney, the girl who named Pluto
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 for the new object quickly before someone else did.[11] 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.[13] These suggestions met little enthusiasm, in large part because she had spent much of the preceding decade trying to wrest the observatory's million-dollar portion of Lowell's legacy for herself.[14]

The name Pluto was first suggested by Venetia Burney (later Venetia Phair) then an eleven-year-old schoolgirl in Oxford, England.[15] Venetia was interested in Classical mythology as well as astronomy, and considered the name, the Roman equivalent of Hades, 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.[16]

The object was officially named on March 24, 1930.[17] 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 single vote.[18] The name was announced on May 1, 1930.[15] Upon the announcement, Madan gave Venetia five pounds as a reward.[15]

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

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.

Physical characteristics

The largest plutinos compared in size, albedo and colour.
The largest plutinos compared in size, albedo and colour.
Possible structure of Pluto.  1. Frozen nitrogen  2. Water ice  3. Silicate and water ice
Possible structure of Pluto.
1. Frozen nitrogen
2. Water ice
3. Silicate and water ice

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.[20]

Appearance and composition

Pluto's mean apparent magnitude is 15.1 with a maximum of 13.56.[21] To see it, a telescope is required; around 30 cm aperture desirable.[22] It looks indistinct and star-like even in very large telescopes because its angular diameter is only 0.15". The colour of Pluto is light brown with a very slight tint of yellow.[23]

Spectroscopic analysis of Pluto's surface reveals it is composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.[24][25] 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.[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 gray spot. Using this technique, one can measure the total average brightness of the Pluto-Charon system and track changes in brightness over time.[27] Maps compsed 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.[28]

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.[25] Because decay of radioactive minerals would eventually heat the ices enough for them to separate from rock, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. It is also possible that such heating may continue into the present time, creating a subsurface ocean of liquid water.[29]

Mass and size

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

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.[30] 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.[30][31]

The discovery of its satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect on Pluto was measured, estimates of Pluto's mass fell to 13.1 Yg; less than 0.24 percent that of the Earth.[32] Observations of Pluto in occultation with Charon were able to fix Pluto's diameter at roughly 2,390 km.[33] With the invention of adaptive optics astronomers were able to accurately determine its shape.[34]

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.


Artist's conception of the New Horizons spacecraft passing over Pluto, showing its tenuous atmosphere
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.[35] 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,[36] by use of the Submillimeter Array, that Pluto's temperature is 10 kelvins colder than expected.

Pluto was found to have an atmosphere from an occultation observation in 1985; the finding was confirmed and significantly strengthened by extensive observations of another occultation in 1988. When an object with no atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.[37] From the rate of dimming, the atmospheric pressure was determined as 0.15 Pascals, roughly 1/700,000 that of Earth.[38]

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory[39] and by James Elliot of MIT[40] and Jay Pasachoff of Williams College.[41] Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pascals, even though Pluto was farther from the Sun than in 1988, and hence should be colder and have a less dense atmosphere. The most widely accepted hypothesis to explain this discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years; as a result extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.[42] Another stellar occultation was observed by the MIT-Williams College team of James Elliot and Jay Pasachoff and a Southwest Research Institute team led by Leslie Young on 12 June 2006 from sites in Australia.[43]

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.[44]


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

Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forwards and backwards in time), after intervals longer than 10–20 million years, it is impossible to determine exactly where Pluto will be because its position becomes too sensitive to unmeasurable details of the present state of the solar system.[45][46] For example, at some specific time many millions of years from now, Pluto may be at aphelion or perihelion (or anywhere inbetween), 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