Earlier this year, when astronomers officially "demoted" Pluto from its status as the ninth planet in our solar system, they little expected the public rancor that followed the decision. Vanderbilt astronomer Weintraub places the Pluto controversy in context in his judicious, lively account of the development of our solar system and the evolution of the meaning of the word planet
, from Aristotle's theories to recent decrees by the International Astronomical Union. Assuming a geocentric universe, Aristotle argued that Mercury, Venus, the Moon, the Sun, Mars, Jupiter and Saturn were the only seven planets in the celestial realms. Later scientists—notably Kepler, Copernicus and Galileo—revolutionized astronomy by demonstrating that Earth and the other planets revolved elliptically rather than in perfect circular movements around the sun. By the mid-18th century, astronomers discovered other celestial bodies—comets, asteroids and moons—that often acted like planets by orbiting the sun and threw the definition of a planet into even more confusion. Weintraub effectively shows that Pluto is a planet by most definitions, but so are several other objects in the Kuiper asteroid belt. Weintraub's provocative, engaging study points to the richness and complexity of our solar system and its many possible planets. (Jan.)
