PLANET QUEST

A lively, timely history of the search for extrasolar planets- -today's hottest astronomical game. Croswell, an astronomer and journalist (The Alchemy of the Heavens, 1995), relates how, beginning with William Herschel's 1781 discovery of Uranus, the search for new planets became a holy grail for astronomers; Neptune, Pluto, and the asteroids followed in due course. (``Planet X,'' believed by some astronomers to account for perturbations in the orbit of Uranus, remains undiscovered.) But planets beyond our own system seemed too remote for even the best telescopes to spot—the nearest star system being 25 trillion miles away, and others millions of times farther than that. At that distance, only indirect methods can apply, in particular, measuring minute fluctuations in the motions of stars, which a sufficiently large planet would cause. Such fluctuations have been reported, and ascribed to distant planets, since the 1940s. But until very recently, better observations have usually deflated the discoverers' claims. (One prominent astronomer's claim of periodic motions of Barnard's Star was finally explained by a periodic wobble in his telescope.) The space age made newer techniques available. A large planet would be expected to emit large amounts of infrared light, and when the bright star Vega was found to be unexpectedly energetic in the infrared, it was taken by some as evidence of planets. (A ring of dust is the more likely answer.) These and other false alarms were the entire story until 1995, when two Swiss astronomers reported a large object in orbit around 51 Pegasi, now considered the first observation of an extrasolar planet. Croswell provides engaging portraits of the astronomers (from Giordano Bruno through Geoffrey Marcy, one of those who discovered 51 Pegasi's planet) as well as a clear, lively summary of the scientific material. A thoroughly readable addition to the astronomy bookshelf. (illustrations)