Pluto discovered, 1930

In 1894 the American businessman Percival Lowell founded an observatory in Flagstaff, Arizona, where he dedicated himself to astronomical research. His theories about canals on Mars resulted in the scientific community ostracising him and his observatory. He also theorised that the orbits of Uranus and Neptune showed the gravitational effects of an unknown Planet X, which he searched for without success from 1905 until his death in 1916. In 1919 William Henry Pickering, the Harvard astronomer and associate of Lowell, predicted the position of Planet X but a study of photographs taken at the Mount Wilson Observatory failed to locate the planet.

The search for Planet X continued at the Lowell Observatory. In 1929 a young astronomer called Clyde Tombaugh [pictured] started work there after having impressed them with drawings of his observations of Jupiter and Mars. He undertook a systematic review of photographs taken using a 13-inch astrograph. He used blink comparator to flick between photographs taken at different times in order to spot any moving objects. On 18th February 1930 Tombaugh noticed the sort of moving object that he was looking for when he compared two pictures taken on the 23rd and 29th of January. Subsequent observations confirmed that he had found a large stellar body orbiting the Sun.