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With a book title like Extraterrestrials: Where Are They? (Cambridge University Press, 拢11.95, ISBN 0 52 1 44803 4), this seems destined to appeal only to devotees of television鈥檚 X-Files. They will be disappointed, however, because it is not a volume filled with stories of apocryphal and spurious alien abductions but a collection of well-researched and intelligent essays edited by Ben Zuckerman and Michael H. Hart. Chapters in this updated version of the 1982 book include the history of the search for extraterrestrial life, the possibilities for interstellar travel and what conditions would support life on other worlds, including the mathematical probability or otherwise of Earthlife being alone in the Universe.