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Pluto (IPA: BrE /ˈpluːtəʊ/, AmE /ˈpluto�
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Pluto (IPA: BrE /ˈpluːtəʊ/, AmE /ˈpluto��/), 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. 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. Pluto has two smaller moons, Nix and Hydra, discovered in 2005..
From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st centuries however, many objects similar to Pluto were discovered in the outer solar system, most notably the scattered disc object Eris, which is 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. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.
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.
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.
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". By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. 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.
The right to name the new object belonged to the Lowell Observatory and its director, Ves