It was finally determined from the data obtained by the Infrared Astronomical Satellite in 1983 and analyzed over several years that Planet X does not exist. In 1993, it was discovered that the calculations of the period 1895-1905, on which the conclusions were based, were in error. This was verified by results determined from data received from Voyageur 2 in 1989 as it passed by the planet.
In the mid 20th century, astronomers noticed that Uranus and Neptune deviated slightly as they moved along their orbits. They thought that this might be caused by the gravitational pull of Pluto. However, this planet is too small and too distant to create such an effect. That meant that there was a larger planet beyond the orbit of Pluto.