When Is a Planet Not a Planet?: The Story of Pluto
Scott takes the 2006 downgrading of Pluto from planet to dwarf planet as a teachable moment for discussing questions such as how the number of planets has changed through the centuries, what can be called a planet, and how scientists come to conclusions—and occasionally change their minds. Following a section on early astronomy, a succinct, timely, and somewhat surprising account of planetary history begins with Herschel's 1781 discovery of the seventh planet, Uranus. Twenty years later, Piazzi found Ceres, which was considered the eighth planet. In 1846, Galle discovered Neptune, bringing the total to nine. When objects similar to Ceres were found in the same orbit, all (including Ceres) were placed in a new category: asteroids. In other words, even before the discovery of Pluto, there were once nine planets, but one of them was demoted. Beautifully designed, the book includes many well-captioned, color illustrations, from period portraits to NASA images to artist's conceptions. A glossary and lists of recommended books and Web sites are appended. A good choice for updating astronomy collections. Phelan, Carolyn