The Futility of Exoplanet Biosignatures

TL;DR

This paper argues that detecting life on exoplanets is fundamentally limited without a comprehensive theory of life, emphasizing the need for unambiguous biosignatures and a better understanding of life's principles.

Contribution

It highlights the importance of developing a theory of life to improve the detection of biosignatures and critiques current approaches based on indirect indicators.

Findings

Detection of biosignatures is ambiguous without a theory of life.

Focus should be on unambiguous features of life from multiple research areas.

Current methods may not conclusively identify extraterrestrial life.

Abstract

The ultimate goal of astrobiology is to determine the distribution and diversity of life in the universe. But as the word "biosignature" suggests, what will be detected is not life itself, but an observation implicating a particular process associated with living systems. Technical constraints and our limited access to other worlds suggest we are more likely to detect an out-of-equilibrium suite of gasses than a writhing octopus. Yet, anything short of a writhing octopus will raise skepticism among astrobiologists about what has been detected. Resolving that skepticism requires a theory to delineate processes due to life and those due solely to abiotic mechanisms. This poses an existential question for the endeavor of life detection: How do astrobiologists plan to detect life via features shared between non-living and living systems? We argue that you cannot without an underlying theory of life. We illustrate this by analyzing the hypothetical detection of an "Earth 2.0" exoplanet. In the absence of a theory of life, we argue the community should focus on identifying unambiguous features of life via four areas of active research: understanding the principles of life on Earth, building life in the lab, detecting life in the solar system and searching for technosignatures. Ultimately, we ask, what exactly do astrobiologists hope to learn by searching for life?