Following the discovery of the planet
Neptune in 1846, there was considerable speculation that another
planet might exist beyond its orbit. The search began in the mid-19th century and culminated at the start of the 20th with
Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the
orbits of the giant planets, particularly
Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have
perturbed Uranus enough to account for the irregularities.