Identifying life beyond Earth is one of astrobiology’s greatest challenges, as our understanding is largely shaped by terrestrial biology. A biosignature is generally defined as a substance, object, or pattern that indicates a biological origin. However, many potential biosignatures can also be produced abiotically, leading to false positives. Agnostic biosignatures aim to overcome this limitation by identifying life without relying on specific assumptions about its nature. The difficulty in defining life itself complicates the interpretation of observational data, and some researchers express skepticism that any remote signature could robustly indicate non-technological life. The focus should be on detecting what life does rather than what life is. Traditional biosignature searches often assume Earth-like conditions, such as rocky, watery planets, which limits the scope of potential discoveries.
Energy expenditure and chemical ordering could serve as indicators of life, regardless of its specific metabolic processes. Energy-ordered resource stratification, where chemical resources are spatially arranged based on their energy content, emerges as a novel criterion for biosignatures. This stratification is a robust consequence of ecological interactions and resource competition within ecosystems. In contrast, abiotic processes lack an inherent reason to produce energy-ordered stratification, as reaction rates and energy yields are uncorrelated. Since all life requires energy, expenditures of energy could lead to physically or chemically ordered systems that serve as biosignatures. A minimal model demonstrates how self-replication and ecological interactions lead to energy-ordered stratification without assuming any specific molecular detail or metabolism, making it an agnostic approach. Such patterns are observed in many Earthly contexts. The emergence of competing ecologies can thus produce an agnostic energetic biosignature, relevant for both sample return missions and detecting ancient life signatures on Earth.
Challenges in Distinguishing Biotic from Abiotic Processes
Distinguishing biotic from abiotic processes is challenging, as intricately ordered structures can arise through non-biological means. Some argue that truly agnostic biosignatures may not exist, as all proposed signatures require assumptions regarding metabolism, morphology, or chirality. A criterion that differentiates biotic from abiotically generated order must be independent of specific energy-harvesting mechanisms. Abiotic chemical reaction rates and energy yields are set by independent parameters and are generally uncorrelated. Even biosignatures linked to specific metabolisms are typically considered in the context of ecosystem-level processes.
Several alternative agnostic biosignature proposals exist. Some focus on detecting life signs that are not specific to Earth or any hypothetical life form. These proposals often rely on restricted habitability concepts or anomaly detection. Analyzing anomalous features at a population scale, based on models that predict what life does rather than what it is, offers a promising approach. Other proposals include searching for polyelectrolytes, homochirality, and using machine learning to distinguish biological from abiotic materials.
The Role of Information in Defining Life
Information is a fundamental characteristic of living systems, as life actively acquires, processes, and utilizes information to respond to changing conditions. Theoretical frameworks such as “semantic information” and “fitness value of information” highlight the role of information in abiogenesis. The transition from information-neutral to information-rich systems is a key step in the emergence of life. Information-centric approaches, such as epsilon machine reconstruction and Jensen-Shannon divergence for spectral analysis, show promise in biosignature identification.
Remote data limitations necessitate in situ analyses to complement remote sensing. Theoretical and experimental approaches must be integrated to better understand life’s informational dynamics and universal principles. Expanding research on non-carbon-based life and alternative biochemical solvents can broaden the scope of habitability. Studying life in extreme environments informs both astrobiology and human life support research. Binding pattern analysis of nucleic acid molecules may serve as an agnostic biosignature. Developing methods to detect life beyond conventional habitable zones, such as through vegetation red edge signatures or biosignature gases, is crucial for advancing the field.
An Ongoing Quest
The search for agnostic biosignatures is vital in the broader quest for extraterrestrial life. A multidisciplinary approach, combining biology, chemistry, physics, and information theory, is considered crucial for success. Future discoveries in this field could be transformative, reshaping our understanding of life in the universe. However, confirming biosignatures as definitive indicators of life requires planetary context, as no single signature is likely sufficient to claim the presence of life. The search for life beyond the solar system remains a compelling and monumental endeavour.
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