Previous planets had been discovered by just happening to see them in the sky. the last of those was the planet Uranus, first identified in 1781. By looking at the orbit of Uranus, astronomers noticed irregularities in it. Because of this they suspected that something was causing a gravitational pull on it. This led them to believe that there might be another planet beyond Uranus that was doing this. So they set about trying to calculate where it was and then to look for it. They had to identify how much of an irregularity there was in the orbit of Uranus, and what size of an object would cause it and how far it would be from Uranus at different times. To make the calculations as to where to find it took many years, before it was finally observed in 1846.