Signatures interpretation & biomarkers

A wide diverstity of spectral features should be found among terrestrial exoplanets. In addition to the expected variety of planet bulk compositions, orbits and stars, the atmospheres, and thus the planetary spectra, should exhibit an even broader range of properties. The atmosphere represents only one millionth of the mass of our planet but this tiny fraction insures the Earth its habitable climate and produces most of its spectral signatures. As already observed in the Solar System, the composition of this small fraction of atmospheric volatiles can be extremely variable from one terrestrial planet to another.

Knowing the main characteristics of a star-planet system (age, mass and radius of the planet, luminosity and spectral stype of the star, orbital elements) does not allow us to predict the main atmospheric properties: surface pressure, main composition, nature and coverage of the clouds, circulation. Those are determined by outgassing/ingassing during the formation of the planet and during its further evolution, atmospheric escape, geochemical volatile cycles, and possibly the interaction with a biosphere, as on Earth. These atmospheric properties will thus have to be inferred from spectral and photometric measurements, which will require the ability to self-consistently model an atmosphere and its resulting spectrum for a given set of stellar, orbital, planetary and atmospheric parameters, and to explore the vast corresponding parameter space.

Most of the current atmosphere models are extrapolated from the case of the Earth, Venus and Mars. Even in such case, the coupling between radiative transfer, photochemistry, circulation and clouds makes the modeling complex and the results difficult to validate. In order to be able to characterize in sufficient depth the atmosphere of terrestrial exoplanets, we will have to develop more robust and versatile models. However challenging, this goal is a definitive requirement to allow us to search for inhabited worlds by looking for biologically-induced spectral features. The ability to assess the biological origin of a signature indeed requires a deep characterization of the planet and its atmosphere.