Beyond Mediocrity: How Common is Life?

Abstract

The probability that life spontaneously emerges in a suitable environment (abiogenesis) is one of the major unknowns in astrobiology. Assessing its value is impeded by the lack of an accepted theory for the origin of life, and is further complicated by the existence of selection biases. Appealing uncritically to some version of the ``Principle of Mediocrity'' -- namely, the supposed typicality of what transpired on Earth -- is problematic on empirical or logical grounds. In this paper, we adopt a Bayesian statistical approach to put on rigorous footing the inference of lower bounds for the probability of abiogenesis, based on current and future evidence. We demonstrate that the single datum that life has appeared at least once on Earth merely sets weak constraints on the minimal probability of abiogenesis. In fact, the {\it a priori} probability assigned to this event (viz., optimistic, pessimistic or agnostic prior) exerts the strongest influence on the final result. We also show that the existence of a large number of habitable worlds does not necessarily imply, by itself, a high probability that life should be common in the universe. Instead, as delineated before, the choice of prior, which is subject to uncertainty (i.e., admits multiple scenarios), strongly influences the likelihood of life being common. If habitable worlds are uncommon, for an agnostic prior, a deterministic scenario for the origin of life might be favoured over one where abiogenesis is a fluke event.

arXiv comment7 pages, 2 figures. Published in MNRAS

References in corpus (27)

• One or more bound planets per Milky Way star from microlensing observations
• Exoplanetary Atmospheres: Key Insights, Challenges and Prospects
• Exoplanet Biosignatures: Observational Prospects
• Bayesian analysis of the astrobiological implications of life's early emergence on Earth
• Superhabitable Worlds
• Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs
• Physical constraints for the evolution of life on exoplanets
• A New Empirical Constraint on the Prevalence of Technological Species in the Universe
• Relative Likelihood for Life as a Function of Cosmic Time
• Quantifying the Origins of Life on a Planetary Scale
• An Objective Bayesian Analysis of Life's Early Start and Our Late Arrival
• Subsurface Exolife
• A comprehensive comparison of the Sun to other stars: searching for self-selection effects
• Dependence of Biological Activity on the Surface Water Fraction of Planets
• Is Life Most Likely Around Sun-like Stars?
• Why do we find ourselves around a yellow star instead of a red star?
• Dissolving the Fermi Paradox
• Emergence of life in an inflationary universe
• Role of stellar physics in regulating the critical steps for life
• Copernicanism and the Typicality in Time
• Land Fraction Diversity on Earth-like Planets and Implications for their Habitability
• Quantifying the information impact of future searches for exoplanetary biosignatures
• Habitability is a continuous property of nature
• A Bayesian Analysis of Technological Intelligence in Land and Oceans
• Optimal Target Stars in the Search for Life
• The Noonday Argument: Fine-Graining, Indexicals, and the Nature of Copernican Reasoning
• False positives and the challenge of testing the alien hypothesis

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• Bioverse: Potentially Observable Exoplanet Biosignature Patterns Under the UV Threshold Hypothesis for the Origin of Life