Spectral appearance and detectability of biosignatures of terrestrial planets in the habitable zone of M-dwarfs

Small extrasolar planets around M-dwarfs are prime targets in the search for habitable surface conditions and biosignatures with current and near-future telescopes like JWST and E-ELT. For the characterization of their atmosphere, model calculations are needed to predict and interpret potential planetary conditions.

Here we investigate the atmospheric conditions and spectral appearance of virtual Earth-like exoplanets in the Habitable Zone of different observed M-dwarf stars. As input for our climate-chemistry-model we use spectra of 10 observed M1V to M5.5V stars, which differ in the UV SED by several orders of magnitude. With a line-by-line radiative transfer model we calculate synthetic emission and transmission spectra using the resulting atmospheric composition and temperature profiles.

Due to weak stellar UV emissions at wavelengths higher than 0.2 µm, planets orbiting M-stars show an increase in the abundance of certain biomarkers and bioindicators (CH4, H2O, N2O) compared to the Earth around the Sun. The ozone profiles show a high dependency on the UV radiation below 0.2 µm. High UVC radiation can increase the ozone mixing ratio in the stratosphere by several orders of magnitude in comparison to a low UVC environment. The profiles of many biosignatures however lack a strong dependency on the stellar spectral type.

To investigate the impact of these photochemical responses upon the spectroscopic detectability of absorption bands with JWST and E-ELT we calculate Signal-to-Noise-Ratios for all the modelled atmospheric scenarios. 

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