PLANET X - Hidden Planet Believed To Be The 9th Planet, Not Pluto

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.

Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet.

After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet, or whether it and its neighbors should, like the asteroids, be given their own separate classification. In 2006 the International Astronomical Union reclassified Pluto and its largest neighbors as dwarf planets, leaving Neptune the farthest known planet in the Solar System.

Planet X became a stand-in term for any undiscovered planet in the outer Solar System. As of March 2014, observations with the WISE telescope have ruled out the possibility of a Saturn-sized object out to 10,000 AU, and a Jupiter-sized or larger object out to 26,000 AU.

In 2014 and 2016, based on similarities of the orbits of a group of recently discovered trans-Neptunian objects, astronomers hypothesized the existence of a planet perhaps ten times the mass of the Earth, with a period of 10,000 to 20,000 years in a highly eccentric orbit averaging about 20 times as far out as Neptune. The hypothesized planet has been nicknamed Planet Nine, and efforts to image it have begun.