Planets beyond Neptune
Percival Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune. Clyde Tombaugh’s discovery of Pluto in 1930 appeared to validate Lowell’s hypothesis, and Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planet,...