On this day in 1930, Pluto was discovered at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh. Pluto is usually the planet farthest from the sun.

What Do You Mean, Usually? Pluto's average distance from the sun is almost four million miles. One orbit around the sun takes 248 years. But its orbit is the most elliptical and tiled of any planet. When it is at its closet point to the sun, Pluto's orbit crosses Neptune's orbit. During that time, Neptune is farther from the sun than Pluto. For example, Neptune was farther from the sun between January 1979 and February 11 of 1999.

More Oddball Traits. Pluto rotates in the opposite direction from most of the other planets.

Uranus Wobbles But It Won't Fall Down? Percival Lowell first theorized the existence of an unknown ninth planet. Why? He thought the wobbles in Uranus' and Neptune's orbits were caused by gravitational pull from another planet. Lowell calculated where that planet might be and searched for it for decades. He never found it.

In Search of Pluto. The search resumed in 1929 at the Lowell Observatory, using a new technique that involved photographic plates and a blink microscope.

On This Day in 1930... Tombaugh discovered the tiny, distant planet.

Second the Motion. Tombaugh's finding was confirmed by other astronomers. That March, Pluto's discovery was announced.

1001 Names for Your Planet... Like many of the other planets, Pluto was given the Roman name of a god. In Roman mythology, Pluto was the god of the underworld. Like Hades, Pluto is dark.

Life on Pluto. Unlike Hades, it's cold. The surface temperature has been estimated at about -360 Fahrenheit.

About That Orbit Theory... After Pluto was finally located, astronomers questioned whether it was large enough to actually affect the orbits of Uranus and Neptune. It is far smaller than any of the other planets in the solar system. The solar system has moons that are bigger than Pluto, including our own. The discovery of Pluto's moon ~ Charon ~ in 1978 helped explain the apparent gravitational pull. Charon has a diameter of 737 miles. Added to Pluto's 1,428 mile diameter, Pluto and Charon form a double-planet system. The mass of that system is enough to cause wobbles in Uranus' and Neptune's orbits.

Pluto Vacations? Not so far. Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on Pluto's surface.