In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Hypothesising that the perturbations were caused by the gravitational pull of another planet, Le Verrier sent his calculations to German astronomer Johann Gottfried Galle. On September 23, 1846, the night following his receipt of the letter, Galle and his student Heinrich d'Arrest found Neptune exactly where Le Verrier had predicted.

Observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet in addition to Neptune. In 1905, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X". Lowell's hope in tracking down Planet X was to establish his scientific credibility, which had been dented by his widely derided belief that channel-like features visible on the surface of Mars were in fact canals constructed by an intelligent civilisation. By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search from 1905 until his death in 1916, but to no avail. Lowell's disappointment at not locating Planet X, according to one friend, "virtually killed him".

Constance Lowell, Percival Lowell's widow, subsequently embroiled the observatory in a decade-long legal battle to secure the observatory's million-dollar portion of Lowell's legacy for herself, which meant that its search for Planet X could not resume until 1929. In that year, the observatory's director, Vesto Melvin Slipher, summarily handed the job of locating Planet X to Clyde Tombaugh, a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings. 

Tombaugh's task was systematically to image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates, to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 20 helped confirm the movement. Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet X." After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The new object would later be found on photographs dating back to March 19, 1915.

Planet X
ಗ್ರಹ ಸ್ಥಾನಮಾನದ ವಿವಾದ
Commemoration as a planet
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