The Detectability of Earth's Biosignatures Across Time

Pallé, Enric
Bibliographical reference

Handbook of Exoplanets, ISBN 978-3-319-55332-0. Springer International Publishing AG, part of Springer Nature, 2018, id.70

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Over the past two decades, enormous advances in the detection of exoplanets have taken place. Currently, we have discovered hundreds of Earth-sized planets, several of them within the habitable zone of their star. In the coming years, the efforts will concentrate in the characterization of these planets and their atmospheres to try to detect the presence of biosignatures. However, even if we discovered a second Earth, it is very unlikely that it would present a stage of evolution similar to the present-day Earth. Our planet has been far from static since its formation about 4.5 Ga ago; on the contrary, during this time, it has undergone multiple changes in its atmospheric composition, its temperature structure, its continental distribution, and even changes in the forms of life that inhabit it. All these changes have affected the global properties of Earth as seen from an astronomical distance. Thus, it is of interest not only to characterize the observables of the Earth as it is today but also at different epochs. Here we review the detectability of the Earth's globally averaged properties over time. This includes atmospheric composition and biosignatures and surface properties that can be interpreted as signs of habitability (bioclues). The resulting picture is that truly unambiguous biosignatures are only detectable for about 1/4 of the Earth's history. For the rest of the time, we rely on detectable bioclues that can only establish an statistical likelihood for the presence of life on a given planet.
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