Pluto was discovered in February 18, 1930 by this guy:
Clyde Tombaugh
He discovered Pluto by comparing (blinking) photographic plates and was awarded the Jackson-Gwilt Medal and Gift by the Royal Astronomical Society for discovering Pluto.
Pluto as Displayed in Nasa
Now scientists think that the planet is an asteroid because of recent decoveries of 30 or so new objects in the outer part of the solar system, part of the Kuiper Disk.