Pluto was photographed for the first time (but was not recognized as a planet) - March 19, 1915

By 1905, renowned astronomers Percival Lowell and William Henry Pickering suggested the possibility of an unknown ninth planet in the solar system. The same year, the Lowell Observatory began a massive project in search of the ninth planet, designated Planet X. The dwarf planet was photographed for the first time on March 19th, 1915 but wasn’t recognized as a planet until 1930 by the Lowell Observatory, fourteen years after Percival Lowell’s death.

The privilege on naming the new planet fell to Lowell Observatory’s director: Vesto Melvin Slipher. Ideas came in from across the globe. The name Pluto was suggested by an eleven year old girl – Venetia Burney – and was formally proposed by the girl’s grandfather, a librarian at Oxford University. An almost unanimous vote decided the debate and the planet was officially named Pluto on May 1st, 1930. Pluto is the Roman counterpart to the Greek Hades. In 2006, the International Astronomical Union agreed upon the definition of what a planet is. Pluto did not meet the guidelines and is now considered a dwarf planet.

http://en.wikipedia.org/wiki/Pluto