Artist conception of New Horizons spacecraft at the Solar System's ninth planet Pluto and its moon Charon (JHUAPL/SwRI)
New Horizons over Mysterious Pluto
Faraway History Planet Moons Spacecraft Go Now

Tombaugh Discovered Pluto in 1930

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Pluto was the most recent major planet to be discovered. It was found in 1930 during a sky survey by American amateur astronomer Clyde Tombaugh at Lowell Observatory in Arizona. Tombaugh died in 1997.

Hubble Space Telescope image of the Solar System's ninth planet Pluto and its moon Charon
Hubble sees Pluto and its moon Charon (NASA)
Pluto was the name for Hades in ancient Greek myth. Hades was god of the netherworld — the home of the dead. Similarly, to the ancient Romans, Pluto was god of the underworld.

The planet may have been given the name Pluto because it is so far from the Sun that it is in perpetual darkness. Another factor may have been Percival Lowell's initials PL. Lowell had founded the Lowell Observatory in Arizona in 1894.

Pluto has been a favorite with people on our inner Solar System planet ever since discovery of the faraway body was one of the biggest news stories of 1930. After the planet discovery became popular, Walt Disney named Mickey Mouse's dog in its honor.

Moon. Pluto has a natural satellite, Charon, which is larger in proportion to its planet than any moon of any planet in the Solar System.

The moon Charon, which was discovered in 1978 by Jim Christy, may be a chunk that broke off Pluto in a collision between the planet and some other large object.

Before Charon was found to be a separate body, blurred-together images of Pluto and Charon led astronomers to believe Pluto was a much larger single body. Today, some astronomers think of Pluto and Charon as a double planet rather than planet and moon.

The word Charon is pronounced "KAIR en." Alternatively, some pronounce it "SHAHR en."

The moon is named for the ferryman who conveyed the dead across the River Styx or the River Acheron to Hades. In Greek mythology, Styx was the river across which the souls of the dead were ferried, one of the five rivers of Hades. Another of the five was Acheron, the river of woe.

A long way off. Pluto's average distance from the Sun is 3.67 billion miles. That's 39.5 times the average distance between the Sun and Earth.

The small planet is estimated to be 1,440 miles in diameter. That's only 70 percent as big as Earth's Moon. In fact, Pluto is smaller than seven of the Solar System's moons — Io, Europa, Ganymede, Callisto, Titan, Triton and Earth's Moon.

Prior to the discovery of Charon, Earth's Moon held the distinction of being the largest moon relative to the size of its planet.

Other planets. How does Pluto differ from the eight major planets?
Pluto At a Glance
Discovered: 1930
Diameter: 1,430 mi.
Orbit: 3.7 billion mi.
Day: 6 Earth days 9 hours
Year: 248 Earth years
Satellite: Charon
Crossing Neptune. From 1979 to 1999, Pluto was the eighth planet from the Sun. In 1999, it crossed Neptune's path to again become the Solar System's most distant planet. It will remain in the ninth position for 228 years before crossing inside Neptune's orbit again.

Pluto and Netpune will never collide because their orbits are very different. Pluto can be visualized as above Neptune in the orbital plane. The two bodies always are separated by more than a billion miles.

Pluto's origin. The dwarf planet's origin is unknown. Some astronomers suggest Pluto may have been a moon of Neptune that escaped. Others say Pluto might be a comet or asteroid which wandered in amongst the planets from a region just outside the Solar System known as the Kuiper Belt, which is said to contain comets that orbit the Sun in a plane similar to the major planets.

Most of the little we know about Pluto has been learned since the late 1970s. Many of the key questions about Pluto and its natural satellite Charon await the close-up observation of an unmanned robot-probe spaceflight mission from Earth.   New Horizons >>

Pluto was the only one of the nine planets that had not been visited by a probe from Earth. A one-way trip from Earth to Pluto would take an interplanetary probe a dozen years. New Horizons will arrive at Pluto in 2015.


Pluto was reclassified into a new category of dwarf planets on August 24, 2006, by some 2,500 astronomers from 75 nations, meeting as the International Astronomical Union.

They demoted the small planet leaving only eight major planets in the Solar System – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The IAU definition covers planets and other Solar System bodies, except satellites: Pluto was defined as a dwarf planet and recognized as the prototype of a new category of Trans-Neptunian Objects. Pluto circles the Sun in an oblong orbit overlapping the orbit of Neptune's.


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