2011A&A...534A..26V

Were a potentially habitable planet to be discovered, the next step would be the search for an atmosphere and its characterization. Eventually, surface conditions, hence habitability, and biomarkers as indicators for life would be assessed. The super-Earth candidate Gliese (GL) 581d is the first potentially habitable extrasolar planet so far discovered. Therefore, GL581d is used to illustrate a hypothetical detailed spectroscopic characterization of such planets. Atmospheric profiles for a wide range of possible one-dimensional (1D) radiative-convective model scenarios of GL581d were used to calculate high-resolution synthetic emission and transmission spectra. Atmospheres were assumed to be composed of N₂, CO₂, and H₂O. From the spectra, signal-to-noise ratios (SNRs) were calculated for a telescope such as the planned James Webb Space Telescope (JWST). Exposure times were set to be equal to the duration of one transit. The presence of the model atmospheres can be clearly inferred from the calculated synthetic spectra thanks to strong water and carbon-dioxide absorption bands. Surface temperatures can be inferred for model scenarios with optically thin spectral windows. Dense, CO₂-rich (potentially habitable) scenarios do not enable us to determine the surface temperatures nor assess habitability. Degeneracies between CO₂ concentration and surface pressure complicate the interpretation of the calculated spectra, hence the determination of atmospheric conditions. Still, inferring approximative CO₂ concentrations and surface pressures is possible. In practice, detecting atmospheric signals is challenging because the calculated SNR values are well below unity in most of the cases. The SNR for a single transit was only barely larger than unity in some near-IR bands for transmission spectroscopy. Most interestingly, the false-positive detection of biomarker candidates such as methane and ozone might be possible in low resolution spectra because CO₂ absorption bands overlap biomarker spectral bands. This can be avoided, however, by observing all main CO₂ IR bands instead of concentrating on, e.g., the 4.3 or 15µm bands only. Furthermore, a masking of ozone signatures by CO₂ absorption bands is shown to be possible. Simulations imply that such a false-negative detection of ozone would be possible even for rather high ozone concentrations of up to 10^–5.