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Pluto was discovered in 1930 by Clyde W. Tombaugh, who used a 13-inch (33-cm) photographic telescope at Lowell Obersvatory in Flagstaff, Ariz. The search for Pluto at Flagstaff had been initiated by the founder of the obervatory, Percival Lowell, who had estimated the ninth planet's posittion on the basis off its presumed gravitational pull on Uranus and Neptune. Scientist now know, however, that the mass of Pluto is insufficient to affect the orbits of Uranus and Neptune. The fact that Pluto was found relatively close to the position that had been predicted by Lowell was due to luck. After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish iff the mass of Neptune determined fron the Voyager2 encounter with Neptune is used. There is no tenth planet. Nevertheless, the discovery of Pluto was a direct result of a systematic search for such a planet, and cannot be attributed entirely to serendipity.
On the otherr hand, serendipity was responsible for the discovery of Pluto's satellite. Charon was discovered in 1978 by James W. Christy on photographic plates taken with the United States Naal Observatory's astrometric telescope near Flagstaff, Ariz. The photographs had been taken for the purpose of providing accurate position of Pluto to further refine its orbit, not to search for a satellite. The satellite appeared merely as a bump on the side of Pluto's image.
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