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Observing the Sky Solar System Satellites Navy Research Resources

Solar System: Outer Planets - Pluto

The Disputed Planet

Pluto, the ninth planet, was the last planet discovered (in 1930), the first and only planet discovered by an American (Clyde Tombaugh), and it is the only planet never visited by a spacecraft or probe.

Pluto is usually farther from the Sun than any of the planets; however, due to the eccentricity of its 248-year orbit, Pluto periodically crosses inside the orbit of Neptune. Neptune then spends 20 years as the most distant planet from the Sun. This last occurred from 1979 through 1999, and will occur again in 2226.


Courtesy of NASA JPL

Pluto is the smallest planet in our solar system, and as a matter of fact, is smaller than our Moon and six other moons (Io, Europa, Ganymede, Callisto, Titan, and Triton)! Pluto’s own moon, Charon, was discovered in 1978 by James Christy at the U.S. Naval Observatory, in Washington, D.C.

Rocky, icy little Pluto resembles a comet more than it does the other planets, and its orbit is suspiciously similar to those of the Kuiper Belt objects, which are comets-in-waiting. Scientists are still debating if Pluto should be considered a planet at all.

Pluto can be seen with a good amateur telescope, but it may take careful observations over several months to actually find it. NASA is developing a mission, called New Horizons, that could provide a closer view of the mysterious planet by the year 2015. Still in the planning stages, the mission could launch a spacecraft as soon as 2006 to study Pluto’s and Charon’s surface properties, geologies, interior makeups, and atmospheres. The craft would then fly on to study some Kuiper Belt Objects in the year 2026.