We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection. Key Words: Exoplanets—Biosignatures—Life detection—Bayesian analysis. Astrobiology 18, 779–824.

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Attempts to identify universal properties of life are limited by the single example of biochemistry on Earth. Using a global database of 28,146 annotated genomes and metagenomes, we report universal scaling laws governing the topology of biochemical networks across levels of organization from individuals, to ecosystems, to the biosphere as a whole. We show the three domains of life are topologically distinguishable, while nonetheless conforming to the same universal scaling laws. Comparing real biochemical networks to networks composed of randomly sampled biochemical reactions reveals the observed scaling is not a product of shared biochemistry alone, but is instead attributable to universal constraints on how global biochemistry is partitioned into individuals. The reemergence of the same regularities across levels of organization hints at general principles governing biochemical network architecture.There is increasing interest in whether life has universal properties, not tied to its specific chemical instantiation (1). Universal biology, if it exists, would have important implications for constraining the origins of life, engineering synthetic life and guiding the search for life beyond Earth (2, 3). Systems biology provides promising tools for uncovering such general principles. One approach has been to study the topology of organismal metabolic networks, revealing common 'scale-free' structure for all three domains of life (4). Another approach is identification of scaling laws describing remarkable consistency in scaling of biomass, biodiversity, and metabolic rate (5-9). However, so far it has been difficult to unify these patterns across all levels of organization where living processes persist, from individuals to ecosystems to the biosphere. A more convincing case for universal biology would be made by demonstrating all life on Earth shares similar properties, independent of biochemical diversity or level of organization. Here we show biochemical networks conform to universal scaling laws governing their global topology, which tightly constrain not only the network structure of individuals, but also that of ecosystems and the biosphere as a whole.

Studying biochemical networks at a planetary scale reveals a deeper level of organization than what has been understood so far.

A central question in origins of life research is how non-entailed chemical processes, which simply dissipate chemical energy because they can do so due to immediate reaction kinetics and thermodynamics,...