The Discovery Of Pluto

Believe it or not, the discovery of Pluto was by accident. Astronomers had noted anomalies in the orbit of Neptune, and much like they did with Uranus' orbit, thought there was a planet lying outside Neptune's orbit which would give rise to the anomalies.  So astronomers began to look for it.

It wasn't until Kansas-born Clyde Tombaugh (it's only important that I mention he's from Kansas because so am I), using stereo imaging of photographic plates found what he thought was the new planet. However, the new object he found was Pluto.

Lowell Observatory Archives

Clyde Tombaugh

 

But after more measurements, the new planet was just not large enough to have the effect that they saw on Neptune. After some more calculations, Neptune's orbit was confirmed just using the masses of Neptune and Uranus.

So Pluto was there, but was not what was expected. If the orbit of Neptune had been calculated  correctly initially, it might have been a while longer before Pluto were discovered.