Journey Into Space: Gravity, Orbits, and Collisions
Space Astronomy and Space
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Is Pluto Really a Planet?

Ever since the discovery of Pluto, astronomers have wondered whether it is really a planet. Pluto's small mass, its unusual orbit, and its tilted orbital plane make it very different from any other planet. Even some of the satellites, or moons, of the other planets are more massive than Pluto.

PlutoSome astronomers think that Pluto resembles an asteroid more than a planet. The largest known asteroid, Ceres, has a diameter that is almost half that of Pluto, and Ceres has nearly one eighth of Pluto's mass. Like Pluto, many asteroids also have orbits that are elliptical and tilted. Most astronomers, however, still choose to classify Pluto as a planet, even though it is not a typical one. Whatever it is, Pluto is certainly a unique member of the solar system.

(Photo: NASA)
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