Planets beyond Neptune
The planet Neptune was discovered in 1846. After this, many people thought that there might be more planets, which are further away from the sun than Neptune. In the early 20th century, Percival Lowell stated that there must be another planet-like body, farther out than Neptune. Lowell said that such a planet-like object could explain some irregularities in the orbits of the gas giants, in particular Uranus and Neptune. The gravity of a large unseen ninth planet could have perturbed Uranus enough to explain the irregularities. He called it "Planet X".