After the discovery of Neptune in 1846, the search was on for other planets further out in the Solar System. One supposed unknown planet beyond Neptune was dubbed Planet X. By analysing small changes in the orbit of Uranus that were not produced by Neptune, astronomers could estimate where Planet X should be.
In 1929, a new powerful 13-inch photographic telescope was commissioned to study the outer edge of the Solar System. The discovery of Pluto was announced to the world on 13 March 1930.
In the 250 years it takes to orbit the Sun, Pluto follows a highly elliptical orbit - very different to that taken by the other planets. It is the smallest planet in the Solar System (smaller even than our Moon) and is the only one yet to be visited by a spacecraft.
