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 .