Agnostic Biosignatures: Expanding the Search for Life in the Solar System

The search for life beyond Earth has long been a scientific goal, but as this quest expands, astrobiologists must grapple not only with where life may be but also with what life can be. Researchers now realize that our descriptions of biosignatures—the physical and chemical clues used to discern the presence of life—must expand to reflect fundamental features of living systems, not just analog expressions that assume a common heritage with Earth. These new biosignatures are thus agnostic in their relationship to life on Earth. New interdisciplinary endeavors now design generalized biosignatures that first consider the criteria and functions of life in any physical setting. These frameworks challenge the way we both seek and define life. Here we summarize the history of these efforts from their disciplinary roots and provide the conceptual foundations of the pursuit to sufficiently describe life without relying on definitions or familiar features. ▪ Agnostic biosignatures are indications of past or present biology that are not based on molecular or metabolic signs of life on Earth. ▪ The generalized search for life is rooted in both paleobiology and physics, where life is discerned from context and fundamental first principles. ▪ This work summarizes universal features and theories of life that recognize how biological systems differ from abiotic chemical systems. ▪ Proposed frameworks for a generalized search for life are described, as well as systematic needs for effective extraplanetary deployment.