Pluto Discovered

Pluto (134340 Pluto) is the tenth-most-massive known body directly orbiting the Sun, and the second-most-massive known dwarf planet, after Eris. Pluto is primarily made of rock and ice, and relatively small, about 1/6 the mass of the Moon and 1/3 its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. Hence Pluto periodically comes closer to the Sun than Neptune, but an orbital resonance with Neptune prevents the bodies from colliding.

Pluto was originally considered the ninth planet from the Sun. Its status as a major planet fell into question following further study of it and the outer Solar System over the next 75 years. Starting in 1977 with the discovery of the minor planet Chiron, numerous icy objects similar to Pluto with eccentric orbits were found. The scattered disc object Eris, discovered in 2005, is 27% more massive than Pluto.

In 1906, Percival Lowell - a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona, in 1894 - started an extensive project in search of a possible ninth planet, which he termed "Planet X". On February 18, 1930, Clyde W. Tombaugh, an assistant at the Lowell Observatory discovered Pluto - a possible moving object on photographic plates taken on January 23 and January 29 of that year. Tombaugh's task was to systematically image the night sky in pairs of photographs, then examine each pair and determine whether any objects had shifted position.

http://en.wikipedia.org/wiki/Pluto
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