Ask any group of people, regardless of the group: "do you believe that there is life beyond Earth?" The answer is always a resounding, "yes." Ask them what evidence they have for believing this and the response is less enthusiastic. Notwithstanding the wackos who claim visitations of aliens, there is not one scintilla of evidence thus far produced to suggest that life on this planet has company anywhere else in the universe. That fact may change soon, and "The Crowded Universe: The Search for Living Planets" chronicles the process whereby this may happen. It is a stunning story, recasting scientists as detectives developing and using new tools to expand knowledge of our exciting universe.



Scientist Alan Boss, on the staff of the Carnegie Institution of Washington's Department of Terrestrial Magnetism, has found a second career as an interpreter of the scientific enterprise for the general public. His earlier book, "Looking for Earths: The Race to Find New Solar Systems" (Wiley, 1998), successfully opened the search for the first discoveries of planets around other stars to a much broader audience than ever reads the scholarly literature. "The Crowded Universe: The Search for Living Planets" continues that theme, carrying the story to the present. In the process, Boss chronicles how the first detection of extrasolar planets rocked the scientific world in 1995 and has given impetus to the search. Using new instruments, technologies, and techniques a loose confederation of scientists around the world are engaged in detecting and cataloguing the number of extrasolar planets around other stars. More than 330 have thus far been discovered, but all of them are giants similar to Jupiter and Saturn rather than terrestrial, Earth-like plants.



That may change soon, however, and Boss is convinced that in the next few years we will find Earths in abundance, some of which will be enough like ours to conclude that they are indisputably alive. Boss insists that life is not only possible elsewhere in the universe but is the normal state. He may well be right, and this book is an explication of how we came to this point in time as well as an analysis of how and why expectations for the discovery of Earth-like planets are so positive.



He discusses how scientific theories about planetary formation have changed radically in the past decade, leading many to conclude that the conditions that spawned life on Earth also took place elsewhere. Boss also uses the excitement of seeking life beyond Earth as the fundamental rationale for continued support in the United States for a robust space exploration program. Failure to do so, Boss contends, would mean that the U.S. would be a spectator in what could arguably be the most profound discovery in human history--extraterrestrial life.



Alan Boss may well be right; indeed, I hope he is. Perhaps it is somewhat like the tagline from the "X-Files," the 1990s television series concerning the search for extraterrestrial visitation of Earth, "I Want to Believe." But hopes have been dashed so often in looking for life beyond Earth that I must, if only for sanity's sake, take a skeptical view and not get too excited by the possibility.



I am reminded of the classic cognitive dissonance model defined by Leon Festinger in his seminal 1956 book, "When Prophecy Fails." Festinger asked the question, what happens when a prediction to which a social group subscribes fails completely and without ambiguity? What happens to its faithful supporters? Reason would suggest that members of the group would abandon the ideas that proved faulty. But true believers do not automatically abandon their cause when reality intrudes in discomforting ways. They rarely admit that they were wrong or change their behavior. Instead they modify just enough of their beliefs to hang on to its essence. We have seen this many times in the search for life beyond Earth. We expected to find life on Mars in 1976 when Viking landed there. We found that Mars is dead. We modified belief only modestly to suggest that perhaps Mars once long ago harbored life and began looking for signs of its extinction, and then we began looking for evidence of past water on Mars, the fundamental building block of life, and continue doing so to the present.



What has happened repeatedly, we adjust our belief ever so slightly. But we never seem to consider the possibility that we might be alone in the universe. Is Alan Boss engaging in wishful thinking by believing that Earth-like planets beyond this solar system are common? Will his predictions prove out, or once again are we placing hope in efforts that will eventually fail to detect evidence of life? I hope the answer to both questions is "no." The only way to know is to continue efforts to learn the answer. Like Boss, I hope the U.S. continues to pursue this question aggressively. Meantime, I will remain a hopeful skeptic.