Search

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets-Biosignatures-Habitability markers-Photosynthesis-Planetary surfaces-Atmospheres-Spectroscopy-Cryptic biospheres-False positives. Astrobiology 18, 663-708.

In the coming years and decades, advanced space- and ground-basedobservatories will allow an unprecedented opportunity to probe the atmospheresand surfaces of potentially habitable exoplanets for signatures of life. Lifeon Earth, through its gaseous products and reflectance and scatteringproperties, has left its fingerprint on the spectrum of our planet. Aided bythe universality of the laws of physics and chemistry, we turn to Earth'sbiosphere, both in the present and through geologic time, for analog signaturesthat will aid in the search for life elsewhere. Considering the insights gainedfrom modern and ancient Earth, and the broader array of hypothetical exoplanetpossibilities, we have compiled a state-of-the-art overview of our currentunderstanding of potential exoplanet biosignatures including gaseous, surface,and temporal biosignatures. We additionally survey biogenic spectral featuresthat are well-known in the specialist literature but have not yet been robustlyvetted in the context of exoplanet biosignatures. We briefly review advances inassessing biosignature plausibility, including novel methods for determiningchemical disequilibrium from remotely obtainable data and assessment tools fordetermining the minimum biomass required for a given atmospheric signature. Wefocus particularly on advances made since the seminal review by Des Marais etal. (2002). The purpose of this work is not to propose new biosignaturesstrategies, a goal left to companion papers in this series, but to review thecurrent literature, draw meaningful connections between seemingly disparateareas, and clear the way for a path forward.