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Is it true that Pluto is not always the outermost planet in the solar system?

Yes. Pluto travels on a tilted orbit that crosses the plane shared by all other planetary orbits. (A plane is an imaginary surface that runs through all the points of an orbit.) Although Pluto's orbit is usually outside that of its closest neighbor, Neptune, at times it crosses over Neptune's orbit. For example, Pluto moved inside Neptune's orbit on January 23, 1979, making Neptune the farthest planet from the sun. Pluto will remain inside Neptune' s orbit until March 15, 1999.

Pluto was discovered in 1930 by American astronomer (a scientist specializing in the study of matter in outer space) Clyde Tombaugh (1906-). It is by far the smallest planet in the solar system, with a diameter of 1,457 miles (2,344 kilometers). Pluto is thought to be composed of rock and water ice. There are bright areas on the planet's...

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