PLANET QUEST: The Epic Discovery of Alien Solar Systems.

By Ken Croswell. Free Press. 324 pp. $25

Until a few years ago, only three human beings in history could claim to have discovered a planet, and only one of them— the late Clyde Tombaugh, discoverer of Pluto—lived in the 20th century. Today, however, a growing number of astronomers can make that claim. We now know of more planets beyond our solar system than within it, all of them discovered in the 1990s.

Croswell, an astronomer and the author of The Alchemy of the Heavens (1995), tells the stories behind these and earlier breakthroughs. We learn of the discoveries of Uranus, Neptune, and Pluto (the three planets in our solar system not visible to the naked eye), the failed search for a "Planet X" beyond Pluto’s orbit, and the quest for planets outside our solar system. The first discovery of an extrasolar planet was made in 1992 by Alexander Wolszczan, and was soon followed by a raft of similar breakthroughs by Swiss and American astronomers.

Unfortunately, the history of searches for planets—whether inside or outside our own system—has not always been a happy one. Croswell explains why claims for the discovery of other planets get made in the first place, and how the continued refusal of the data to back up some claims eventually leads to their rejection. He also explains why these searches always involve indirect evidence—usually the distortion of a star’s motion by the gravitational pull of its partner—rather than direct observation.

Croswell re-creates one of the shining moments of 20th-century science. In 1991, English astronomer Andrew Lyne and his team announced the detection of planets around a pulsar (a dead star), which seemed to be the first extrasolar planets. In 1992, however, Lyne found a flaw in his data that invalidated his conclusion. Rather than send a terse letter of retraction to a professional journal, Lyne stood up and explained his error before a gathering of the American Astronomical Society in Atlanta. When he finished, the auditorium of astronomers gave him a standing ovation.

If Croswell’s book has a weakness, it is his excessive attention to side issues and even nonissues, including a chapter-long semantic quibble over whether a "brown dwarf" is or is not a star. As a result, the main story about modern planetary discoveries doesn’t begin until page 180.

That said, Croswell’s command of the nuts and bolts of the profession enables him to explain what would otherwise be rather esoteric debates. A nice touch is his inclusion of interviews with a number of astronomers involved in the story, together with thumbnail sketches of their careers and accounts of how they came to be astronomers. No parent reading this book can fail to be impressed by these scientists’ testimonies to their earliest shaping experiences: "My parents bought me a telescope" or "My father showed me the constellations."

—James Trefil