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Key Words bioastronomy, cosmobiology, exobiology, SETI
Abstract
Astrobiology is the study of the living universe. Astronomy provides the context for the origin and evolution of life on Earth. Conversely, discoveries about the terrestrial biosphere-from extremophilic microbes to the evolution of intelligence-inform our thinking about prospects for life elsewhere. Astrobiology includes the search for extraterrestrial life via in situ exploration, spectroscopy of solar and extrasolar planetary atmospheres, and the search for extraterrestrial intelligence. This review situates astrobiology within philosophical issues of the definition of life and the biological compatibility of the universe. It reviews the habitability of the Galaxy in general and of planets and moons in particular, and summarizes current controversies in origins-of-life research and in evidence for the earliest life on Earth. It critiques certain "rare Earth" and "anthropic" arguments, and considers four approaches to deciding whether intelligent life exists elsewhere in the Galaxy. It concludes that astrobiology must also speak to the future of human civilization.
1. THE SCIENCE OF ASTROBIOLOGY
The NASA Astrobiology Institute (NAI) defines astrobiology as "the study of the living universe" (NAI 2004). Implicit in this definition is that astrobiology includes the study of terrestrial biology. This is appropriate, for our approach to the search for life elsewhere is influenced by our knowledge of life on Earth, and our understanding of the origin and evolution of life on Earth is informed by our study of conditions on other worlds and of Earth's solar system and galactic environment. In this respect astrobiology is now understood more broadly than when the term was first coined, apparently by Laurence Lafleur (1941), who defined it as "the consideration of life in the universe elsewhere than on earth."
The word astrobiology was used by the astronomer Otto Struve (1955), and was adopted by NASA in 1995 (NAI 2004). Prior to that, the word exobiology was most commonly used in U.S. space exploration. The term exobiology dates to an agenda-setting paper by the biologist Joshua Lederberg (1960), and the word is still used occasionally within NASA and, more commonly, by the European Space Agency (ESA). "Astrobiology" avoids the combination of Greek and Latin roots in "exobiology," but has the drawback of seeming to imply that one is somehow studying the biology of stars (astron...