Pluto , ninth planet from the Sun and outermost known member of the solar system. Pluto was discovered as the result of a telescopic search inaugurated in 1905 by the American astronomer Percival Lowell, who conjectured the existence of a distant planet beyond Neptune as the cause of slight perturbations (see Orbit) in the motions of Uranus. Continued by members of the Lowell Observatory staff, the search ended successfully in 1930, when the American astronomer Clyde William Tombaugh found Pluto near the position Lowell had predicted. The new planet’s mass, however, seemed insufficient to account for the perturbations of Neptune, and the search for a possible tenth planet continues. Pluto revolves about the Sun once in 247.7 years at an average distance of 5.9 billion km (3.67 billion mi). The orbit is so eccentric that at certain points along its path Pluto is closer to the Sun than is Neptune. No possibility of collision exists, however, because Pluto’s orbit is inclined by more than 17.2° to the plane of the ecliptic and never actually crosses Neptune’s path.
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