This is a continuation of the excerpt from wikipedia:
Clyde Tombaugh‘s discovery of Pluto in 1930 appeared to validate Lowell’s hypothesis, and Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet.
Now remember, even before this latest discovery astronomers thought something might be there! THAT may in a sense be additional evidence in that something is there because they did not even have the tools we do now!
Check back for updates as they come in. Here are two different videos about it and one has a cool simulation of its orbit!
thanks to dispatchtribunal.com for the great info