Clyde Tombaugh: Percival Lowell interpreted some of the — what they call “residuals” — slight irregularities in the orbit of Uranus, particularly, and later Neptune, as indicative of a mass out there as yet unseen — like the case of Neptune being discovered mathematically before it was seen, you see.  These residuals were very, very small. In fact, so small that they were questionable whether they were real or not, but it was the best that he had. So he predicted that there was a planet out there about seven times more massive than the earth, beyond the orbit of Neptune.  Of course, Pluto does not have that much mass.