title: Exobiology: a critical review.
abstract: The quest for life beyond the Earth can be implemented in three ways. First, we may study contemporary terrestrial biology and organic chemistry to approach the problem of the origin of life on Earth. If it appears relatively easy for life to have emerged in the primitive terrestrial environment, it may follow that the origin of life is a fairly general planetary phenomenon. Second, we can investigate the physical environments of the planets, and determine whether the conditions there are so severe that living processes are entirely excluded, even allowing for the adaptability of life. Third, the planets may be studied for direct evidence of indigenous life forms. All three approaches are beset with uncertainties, and it is important to state at the outset that no completely convincing evidence exists for extraterrestrial life. The problem often reduces to probability considerations, and to estimates of observational reliability. At convenient places in the marshalling of evidence, I shall try to pause and give brief expression to alternative interpretations. In almost all cases, an optimistic view can be found which holds that the evidence is strongly suggestive of, or at the worst, not inconsistent with, the existence of extraterrestrial life; and a pessimistic view can be found, which holds that the evidence adduced in favor of extraterrestrial life is unconvincing, irrelevant, or has an alternative, non-biological explanation. I leave it to the reader to pick his own way among the factions.
authors.affiliation: Harvard University, Cambridge, Mass., USA.
authors.forename: C
authors.initials: C
authors.lastname: Sagan
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D004247:DNA; D019419:Evolution, Planetary; D018559:Exobiology; D005118:Extraterrestrial Environment; D018535:Mars; D000069497:Origin of Life; D016083:Planets; D012313:RNA
chemical_list: D012313:RNA; D004247:DNA