Is Pluto a Planet? A Historical Journey through the Solar System.

IS PLUTO A PLANET? A Historical Journey through the Solar System DAVID A. WEINTRAUB

The discovery of various large objects in the outer solar system has called into question the definition of a planet. In August, an astronomers' group answered the question posed in the title of Weintraub's book with a controversial "no." Weintraub, a professor of astronomy, reviews the notion of planet. To ancient observers, planets were considered wandering stars because they appeared to rise and set in an otherwise fixed sky. Aristotle established an Earth-centered view of the universe that made the sun, the moon, Mercury, venus, Mars, Jupiter, and Saturn the original seven planets. Nicolas Copernicus in 1515 observed that Earth revolves around the sun, not vice versa. Johannes Kepler defined a planet as an object that revolves around the sun in accordance with his three laws of planetary motion. The number of planets would be in flux throughout the 18th century, culminating in the discovery of Uranus in 1781. Pluto was declared a planet in 1930. In the 1940s, Gerald Kuiper asserted the existence of a solar system reservoir of materials left over from the formation of the planets. During the late 1990s and early 2000s, astronomers discovered hundreds of objects there, some of which were large enough to rival Pluto. Weintraub describes astronomers' current attempts to reconcile these objects with the definition of a planet, including the creation of planet subsets. Finally, he provides an appendix with information about the now-demoted planet Pluto. Princeton, 2007, 254 p., b&w images, hardcover, $27.95.