Word: tombaugh
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Dates: during 1970-1979
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...been 42 years since Clyde Tombaugh, at Arizona's Lowell Observatory, discovered the last and outermost of the solar system's nine known planets. But many astronomers have never given up hope of finding a tenth planet even farther from the sun. They have been encouraged in their search by irregularities in the orbit of the eighth planet, Neptune, which some suspect could be caused by the gravitational tug of a mysterious "Planet X." Until now, however, all efforts to sight Planet X have failed...