Planets beyond Neptune

Hypothetical planets further than Neptune

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 continued 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 planet could have perturbed Uranus enough to account for the irregularities. Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, though Pluto was eventually determined to be too small for its gravity to affect the giant planets in 1978, resulting in a brief search for a tenth planet.

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