Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

In the coming years and decades, advanced space- and ground-basednobservatories will allow an unprecedented opportunity to probe the atmospheresnand surfaces of potentially habitable exoplanets for signatures of life. Lifenon Earth, through its gaseous products and reflectance and scatteringnproperties, has left its fingerprint on the spectrum of our planet. Aided bynthe universality of the laws of physics and chemistry, we turn to Earth’snbiosphere, both in the present and through geologic time, for analog signaturesnthat will aid in the search for life elsewhere. Considering the insights gainednfrom modern and ancient Earth, and the broader array of hypothetical exoplanetnpossibilities, we have compiled a state-of-the-art overview of our currentnunderstanding of potential exoplanet biosignatures including gaseous, surface,nand temporal biosignatures. We additionally survey biogenic spectral featuresnthat are well-known in the specialist literature but have not yet been robustlynvetted in the context of exoplanet biosignatures. We briefly review advances innassessing biosignature plausibility, including novel methods for determiningnchemical disequilibrium from remotely obtainable data and assessment tools forndetermining the minimum biomass required for a given atmospheric signature. Wenfocus particularly on advances made since the seminal review by Des Marais etnal. (2002). The purpose of this work is not to propose new biosignaturesnstrategies, a goal left to companion papers in this series, but to review thencurrent literature, draw meaningful connections between seemingly disparatenareas, and clear the way for a path forward.