Meteorologist Bill Steffen writes:

How many planets are there? Good question. Up until 1930 it was eight...then Clyde Tombough discovered Pluto (a name suggested by a little girl in England). When it was first observed, it was thought that Pluto was five times bigger than the Earth. Now we think Pluto might even be smaller than our moon. With the discovery of a 10th planet, Xena (actually bigger than Pluto), by Michael Brown last year...scientists have been rethinking what a planet really is.

Read Poor Pluto's Planetary Position Precariously Preserved
Read Kansas farm boy who discovered Pluto celebrated as American folk hero in the Duluth News Tribune
Also search NASA's Hubbel Space Telescope site for articles on and images of Pluto