Pluto, once believed to be the ninth planet, is discovered at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh.

The existence of an unknown ninth planet was first proposed by Percival Lowell, who theorized that wobbles in the orbits of Uranus and Neptune were caused by the gravitational pull of an unknown planetary body. Lowell calculated the approximate location of the hypothesized ninth planet and searched for more than a decade without success. However, in 1929, using the calculations of Powell and W.H. Pickering as a guide, the search for Pluto was resumed at the Lowell Observatory in Arizona. On February 18, 1930, Tombaugh discovered the tiny, distant planet by use of a new astronomic technique of photographic plates combined with a blink microscope. His finding was confirmed by several other astronomers, and on March 13, 1930--the anniversary of Lowell's birth and of William Hershel's discovery of Uranus--the discovery of Pluto was publicly announced.

With a surface temperature estimated at approximately -360 Fahrenheit, Pluto was appropriately given the Roman name for the god of the underworld in Greek mythology. Pluto's average distance from the sun is nearly four billion miles, and it takes approximately 248 years to complete one orbit. It also has the most elliptical and tilted orbit of any planet, and at its closest point to the sun it passes inside the orbit of Neptune, the eighth planet.

After its discovery, some astronomers questioned whether Pluto had sufficient mass to affect the orbits of Uranus and Neptune. In 1978, James Christy and Robert Harrington discovered Pluto's only known moon, Charon, which was determined to have a diameter of 737 miles to Pluto's 1,428 miles. Together, it was thought that Pluto and Charon formed a double-planet system, which was of ample enough mass to cause wobbles in Uranus' and Neptune's orbits. In August 2006, however, the International Astronomical Union announced that Pluto would no longer be considered a planet, due to new rules that said planets must "clear the neighborhood around its orbit." Since Pluto's oblong orbit overlaps that of Neptune, it was disqualified.