Pluto was discovered on February 18, 1930 by the American astronomer William Tombaugh Clyde( 1906-1997) from the Lowell Observatory in Flagstaff, Arizona, and was considered the ninth and smallest planet in the solar system by the International Astronomical Union and by the public opinion from then until 2006, although its membership in the group of planets of the solar system was always subject to controversy among astronomers. Even for many years there was a belief that Pluto was a satellite of Neptune that had ceased to be a satellite because it achieved a second cosmic speed. However, this theory was rejected in the 1970s. 1 In the 1940s, Urbain Le Verrier( 1811-1877) used Newtonian mechanics to predict the position of Neptune after analyzing perturbations in Uranus ' orbit. 2 Further observations of Neptune, at the end of the 19th century, led astronomers to surmise that another planet, besides Neptune, disturbed the orbit of Uranus. In 1906, Percival Lowell( 1855-1916)-a well-to-do Bostonian who had founded in 1894 The Lowell Observatory in Flagstaff, Arizona- started an intense search program for the ninth planet he called Planet X. 3 By 1909, he and William H. Pickering( 1855-1935) had suggested several celestial coordinates where that planet could be found. 4 Lowell and the members of his Observatory carried out the search, without obtaining results until the death of that in 1916. However, without knowing it, Lowell had photographed him in plates of March 19 and April 7, 1915 where it appeared as a weak object. 5 There are fourteen other known precovery observations, the most The old one was made at the Yerkes observatory on August 20, 1909.