Spectral features of Earth-like planets and their detectability at different orbital distances around F, G, and K-type stars

¹ Institut für Methodik der Fernerkundung, Deutsches Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen, 82234 Weßling, Germany
e-mail: pascal.hedelt@dlr.de
² Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
³ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁴ Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, Rutherfordstr. 2, 12489 Berlin, Germany
⁵ Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany

Received: 18 July 2011
Accepted: 22 February 2013

Abstract

Context. In recent years, more and more transiting terrestrial extrasolar planets have been found. Spectroscopy already yielded the detection of molecular absorption bands in the atmospheres of Jupiter and Neptune-sized exoplanets. Detecting spectral features in the atmosphere of terrestrial planets is the next great challenge for exoplanet characterization.

Aims. We investigate the spectral appearance of Earth-like exoplanets in the habitable zone (HZ) of different main sequence (F, G, and K-type) stars at different orbital distances. We furthermore discuss for which of these scenarios biomarker absorption bands and related compounds may be detected during primary or secondary transit with near-future telescopes and instruments.

Methods. Atmospheric profiles from a 1D cloud-free atmospheric climate-photochemistry model were used to compute primary and secondary eclipse infrared spectra. The spectra were analyzed taking into account different filter bandpasses of two photometric instruments planned to be mounted to the James Webb Space Telescope (JWST). We analyzed in which filters and for which scenarios molecular absorption bands are detectable when using the space-borne JWST or the ground-based European Extremely Large Telescope (E-ELT).

Results. Absorption bands of carbon dioxide (CO₂), water (H₂O), methane (CH₄) and ozone (O₃) are clearly visible in both high-resolution spectra as well as in the filters of photometric instruments. However, only during primary eclipse absorption bands of CO₂, H₂O and O₃ are detectable for all scenarios when using photometric instruments and an E-ELT-like telescope setup. CH₄ is only detectable at the outer HZ of the K-type star since here the atmospheric modeling results in very high abundances. Since the detectable CO₂ and H₂O absorption bands overlap, separate bands need to be observed to prove their existence in the planetary atmosphere. In order to detect H₂O in a separate band, a ratio S/N > 7 needs to be achieved for E-ELT observations, e.g. by co-adding at least 10 transit observations. Using a space-borne telescope like the JWST enables the detection of CO₂ at 4.3 μm, which is not possible for ground-based observations due to the Earth’s atmospheric absorption. Hence combining observations of space-borne and ground-based telescopes might allow to detect the presence of the biomarker molecule O₃ and the related compounds H₂O and CO₂ in a planetary atmosphere. Other absorption bands using the JWST can only be detected for much higher S/Ns, which is not achievable by just co-adding transit observations since this would be far beyond the planned mission time of JWST.