Moving Beyond Definitions in the Search for Extraterrestrial Life

According to the 2015 Astrobiology Strategy, a central goal of astrobiology is to provide a definition of life. A similar claim is made in the 2018 CRC Handbook of Astrobiology. Yet despite efforts, there remains no consensus on a definition of life. This essay explores an alternative strategy for searching for extraterrestrial life: Search for potentially biological anomalies (as opposed to life per se) using tentative (vs. defining) criteria. The function of tentative criteria is not, like that of defining criteria, to provide an estimate (via a decision procedure) of the likelihood that an extraterrestrial phenomenon is the product of life. Instead, it is to identify phenomena that resist classification as living or nonliving as worthy of further investigation for novel life. For as the history of science reveals, anomalies are a driving force behind scientific discovery and yet (when encountered) are rarely recognized for what they represent because they violate core theoretical beliefs about the phenomena concerned. While the proposed strategy resembles that of current life-detection missions, insofar as it advocates the use of a variety of lines of evidence (biosignatures), it differs from these approaches in ways that increase the likelihood of noticing truly novel forms of life, as opposed to dismissing them as just another poorly understood abiological phenomenon. Moreover, the strategy under consideration would be just as effective at detecting forms of life closely resembling our own as a definition of life.