Abstract

The search for life has had a new enthusiastic restart in the last two decades thanks to the large number of new worlds discovered. The about 4100 exoplanets found so far, show a large diversity of planets, from hot giants to rocky planets orbiting small and cold stars. Most of them are very different from those of the Solar System and one of the striking case is that of the super-Earths, rocky planets with masses ranging between 1 and 10 M ⊕ with dimensions up to twice those of Earth. In the right environment, these planets could be the cradle of alien life that could modify the chemical composition of their atmospheres. So, the search for life signatures requires as the first step the knowledge of planet atmospheres, the main objective of future exoplanetary space explorations. Indeed, the quest for the determination of the chemical composition of those planetary atmospheres rises also more general interest than that given by the mere directory of the atmospheric compounds. It opens out to the more general speculation on what such detection might tell us about the presence of life on those planets. As, for now, we have only one example of life in the universe, we are bound to study terrestrial organisms to assess possibilities of life on other planets and guide our search for possible extinct or extant life on other planetary bodies. In this review, we try to answer the three questions that also in this special search, mark the beginning of every research: what? where? how?

Highlights

  • Since the ancient times, philosophers try to answer to the question “are we alone?” Up to now no certain answer has been given due mainly to the huge technological challenge in unveiling extant life on alien worlds

  • The search for life in the Galaxy received a big boost by the bonanza of new worlds discovered so far, if we are compelled to tackle with the distances at which these worlds are

  • In a meeting held in the 1975, Lovelock et al [28] supported the idea of Lippincott et al [27] that the O2 –CH4 disequilibrium was strong evidence for life, and from on, CH4 was established as a biosignature gas [28,29]

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Summary

Introduction

Philosophers try to answer to the question “are we alone?” Up to now no certain answer has been given due mainly to the huge technological challenge in unveiling extant life (if any) on alien worlds. Instead the term bioindicators defines all the other gases that are or could be indicative of biological processes but can be produced abiotically (e.g., on Earth, O3 is the photochemical by-product of O2 ) Their quantity and detection, along with other atmospheric species, all within a certain context (for instance, the properties of the star and the planet) points toward a biological origin. These gases should be ubiquitous by-products of carbon-based biochemistry, even if the details of alien biochemistry are significantly different from the biochemistry on Earth.

Atmospheric Biosignatures
Metabolic Biosignatures
Organic Matter Building By–Products
Secondary Metabolic Biosignatures
Bioindicators
Surface and Industrial Biosignatures
False Posivitves
Where to Find a Habitable World?
Method
The Long Way to Exoplanet Atmospheres Characterization
Perspectives
Findings
Summary and Conclusions
Full Text
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