Pluto; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Is Pluto really a planet? Until about six years ago, the question would have seemed silly. But in the early 1990s, astronomers found a region of orbiting bodies just beyond Neptune. The region, which was dubbed the Kuiper belt, is populated mostly by bodies too small to be planets and also by comets with relatively short periods, meaning that they approach the sun at least once every couple of centuries.

Most astronomers still consider Pluto a planet. Although its mass is only 1/400 that of Earth, it is still easily the largest Kuiperlike object. Also, Pluto seems to be more reflective than the other bodies in the Kuiper belt. Tradition may also have something to do with it; Pluto has been regarded as a planet since Clyde Tombaugh discovered it in 1930.