|New Horizons over Mysterious Pluto|
Tombaugh Discovered Pluto in 1930
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.
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.
Hubble sees Pluto and its moon Charon (NASA)
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?
- Body. Pluto is is a small, cold, dark, frozen, rugged, uneven, craggy place. By comparison, the nearest planets to Pluto in the outer Solar System are giant gas-bags. Pluto seems to be composed mostly of rock and ice with a thin atmosphere of nitrogen, carbon monoxide and methane. Pluto's mass is only 0.2 percent of planet Earth's mass.
- Orbit. Pluto has a strange orbit. It was the only one to cross another planet's orbit. Pluto's orbit tilts up from the Solar System plane. It takes Pluto 6 days 9 hours to rotate and 248 Earth years to complete an orbit around the Sun. It's orbit is off-center in relation to the Sun. That leads Pluto across planet Neptune's path.
- Size. Pluto is less than half the size of any major planet. It's only about two-thirds the size of Earth's Moon. Pluto's diameter is 1,430 miles. That's a mere18 percent of planet Earth's diameter.
- Moon. Pluto's natural satellite, Charon, is larger in proportion to its planet than any planet's moon.
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 At a Glance
Diameter: 1,430 mi.
Orbit: 3.7 billion mi.
Day: 6 Earth days 9 hours
Year: 248 Earth years
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:
- Planet is a celestial body that:
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
- has cleared the neighborhood around its orbit.
- Dwarf planet is a celestial body that
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
- has not cleared the neighbourhood around its orbit, and
- is not a satellite.
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.
- Small Solar System Bodies are all other objects, except satellites, orbiting the Sun. The category includes most of the Solar System asteroids, most Trans-Neptunian Objects, comets, and other small bodies.
Learn more about Pluto...
About Pluto, Charon, the new moons and the Kuiper Belt
- Facts about Pluto SEDS Nine Planets
- New Moons Around Pluto Images and information
- Facts about the Kuiper Belt and the Oort Cloud
- Exploring Pluto Smithsonian Institution National Air and Space Museum
- The Struggles to Find the Ninth Planet by Clyde W. Tombaugh
- Many Pluto Web Pages
About New Horizons spacecraft and former Pluto-Kuiper Express
About the IAU Pluto Status Change Proposal
- Much Ado About Pluto NASA story
- The status of Pluto IAU Press Release 1999
- What is a planet? IAU discussion of Pluto's status
About the mystery object Quaoar beyond Pluto
- Quaoar FAQ CalTech
- The Kuiper Belt and the Oort Cloud SEDS
- Artist illustrations of Quaoar STScI
- Comparison of Quaoar with the Moon NASA artist
- Comparison of Quaoar with North America NASA artist
About Astrophysics and Pluto
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