Planet X

In 1894, with the help of William Pickering, Percival Lowell, a wealthy Bostonian aristocrat, founded the Lowell Observatory in Flagstaff, Arizona. In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet, which he termed Planet X. The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (Planet X would, at the time, have been the ninth planet). Lowell's hope in tracking down Planet X was to establish his scientific credibility, which had eluded him thanks to his widely derided belief that channel-like features visible on the surface of Mars were canals constructed by an intelligent civilisation.

Lowell's first search focused on the ecliptic, the plane encompassed by the zodiac where the other planets in the Solar System lie. Using a 5-inch photographic camera, he manually examined over 200 three-hour exposures with a magnifying glass, but found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey. After revising his predicted possible locations, Lowell conducted a second search from 1913 to 1915. In 1915, he published his Memoir of a Trans-Neptunian Planet, in which he concluded that Planet X had a mass roughly seven times that of the Earth--about half that of Neptune--and a mean distance from the Sun of 43 AU. He assumed Planet X would be a large, low-density object with a high albedo, like the gas giants. As the result it would show a disc with diameter of about one arcsecond and an apparent magnitude of between 12 and 13--bright enough to be spotted.

Separately, in 1908, Pickering announced that, by analysing "kinks" in Uranus's orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed "Planet O", possessed a mean orbital radius of 51.9 AU and an orbital period of 373.5 years. However, plates taken at his observatory in Arequipa, Peru showed no evidence for the predicted planet, and British astronomer PH Cowell showed that the "kinks" observed in Uranus's orbit virtually disappeared once the planet's displacement of longitude was taken into account. Lowell himself, despite his close association with Pickering, dismissed Planet O out of hand, saying, "Ths planet is very properly designated "O", [for it] is nothing at all." Pickering would go on to suggest many other possible trans-Neptunian planets up to the year 1932, which he named P, Q, R, S, T and U, but none were ever detected.