Astronomers suspected another planet beyond Uranus when it was noted that Uranus was being slightly pulled out of orbit at points. This led to speculation that gravity from another planet was causing the pull.
As a planet is pulled by external gravitational forces, perturbations in its orbit path occur. Due to this fact, astronomers hypothesized that it was possible the orbit path of another planet existed. By observing the irregularities in Uranus's orbit, astronomers were able to calculate both the mass and location of the suspected planet.
Several astronomers, in particular John Couch Adams of Britain and Urbain Jean Joseph Leverrier of France, independently came to similar conclusions about this suspected planet. Another astronomer, Johann Gottfried Galle, used these calculations to search the skies. In 1846, he was able to observe Neptune, the eighth planet from the sun.