Astrobiology, the science that deals with life elsewhere in the universe, continues to attract the attention of students, the public, and those scientists who enjoy letting their minds roam freely through the speculation that remains an essential part of searching for life beyond Earth. Since the previous edition of this book appeared, a single rock from Mars temporarily stunned the world with the tantalizing possibility that we had found the first evidence for extraterrestrial life. Although this conclusion now seems dubious, no doubt exists that the last few years have also brought the first sure discoveries of planets in orbit around sunlike stars. In fact, astronomers have now found so many of these planets (more than 50) that the thrill has temporarily vanished from their announcements, to return quickly on the day when we find not Jupiter-sized gas giants, as all extrasolar planets have so far proven to be, but the first extrasolar planets similar to our Earth.
Two decades ago, when the first edition of this book appeared, any textbook dealing with astrobiology appeared destined for modest use, simply because it speculated so largely and so widely, through many disparate fields of science and even sociology. We were surprised by the large number of instructors who have happily joined in this speculation and have risked teaching astrobiology despite not themselves being experts in all the fields of science it embraces. We now see more clearly that astrobiology’s appeal to students can easily override the fact that this field of study requires scientific speculation; indeed we suspect that some instructors, like ourselves, delight in showing students that the joy of science lies even more in what we don’t know than in what we do. To teach critical thinking, few subjects can match the search for life in the universe, in which speculation ranges from a careful attempt to assess hypotheses about the origin and evolution of life to the most incredible reports of extraterrestrial visitors to Earth.
This book can serve as a textbook for any introductory astronomy course with a focus on the search for life in the universe, and for many biology and geology courses that choose to emphasize the cosmic aspects of their fields of interest. Students in courses taught by ourselves and our colleagues seem nearly unanimous in welcoming an interdisciplinary course that centers on the search for life, hardly surprising in view of the public’s interest in the subject. That interest, fanned by sensational movies and television programs, starts with the human prejudice that we must be the center of the universe, the natural subject of attention of any extraterrestrial civilization. Teaching a course on extraterrestrial life takes the students along a journey that science has already made, from the belief that we on Earth are immensely special, perhaps unique, to the realization that our planet orbits a representative star in the outer reaches of a typical giant spiral galaxy, with millions of possible sites for life, some of which might be far more advanced than ourselves and without the least interest in making contact. The intellectual journey culminates with an assessment of how hard we may have to work if we hope to find other civilizations with whom to exchange our views of life, and of how long we may have to wait before any such exchange occurs.
In preparing this new edition of our book, we have changed the order of the introductory astronomy chapters (Chapters 2-6) so that they follow the conventional order, reaching outward from the Earth to the universe at large. We have revised the biology chapters (7-10) to include new approaches to understanding evolution on Earth, and to achieve greater clarity in presenting the interaction between the development of life on our planet and elsewhere in the cosmos. The planetary-astronomy chapters (11-15) reflect the new results from spacecraft and other investigations of the solar system, and a new chapter (Chapters 17) presents the discovery of extrasolar planets. Chapters 16-22 take us through the universe on a quest to answer the question of how we might find intelligent civilizations, if they exist, and what conclusions we can draw from the fact that we have yet to find definitive proof that life exists anywhere beyond the Earth.
In preparing this new edition of The Search for Life in the Universe, we owe a particular debt of gratitude to Dana Backman, Barbara Bowman, Richard Gammon, William Irvine, David Koerner, Alan Rosan, Jill Tarter, and Juliette Winterer, who took the trouble to read through part or all of our manuscript and provide us with detailed comments, which we have done our best to incorporate into this new edition. We would also like to thank other scientists who helped us with the text and illustrations for this edition, Gibor Basri, Ken Brecher, Frank Drake, David Hollenbach, Judith Lengyel, Steve Maran, Geoff Marcy, Larry Marschall, Chris McKay, David Morrison, Guy Ottewell, J. William Schopf, Seth Shostak, Frank Shu, Michael Soule, Hyron Spinrad, Larry Squire, Woody Sullivan, and Jill Tarter. To work with University Science Books, and thus with Bruce and Kathy Armbruster, Jane Ellis, Robert Ishi, and Susanna Tadlock, has been a special treat. Many happy hours have also been spent in consultation with Jon Lomberg, our good friend and outstanding astronomy artist, who provided the illustrations for this edition.