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 gas giants , particularly Uranus and Neptune , speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities .
The gravity of a large unseen ninth planet could have perturbed Uranus enough to explain the irregularities .