Atmospheric modeling and detectability of potential biosignatures on terrestrial planets orbiting low mass stars

Kurzfassung

The first atmospheres to be characterized on potentially habitable, rocky exoplanets will likely be orbiting cooler stars. Theory and observations of the solar system suggest that these important atmospheres could be very diverse — including e.g. massive hydrogen primordial, thick carbon dioxide or nitrogen atmospheres. Predicting the spectral signals from such a wide diversity of objects is one of the most fascinating and central topics in exoplanet science. This work presents the first consistent investigations of such diverse atmospheres over a wide parameter range. Previous studies in the literature lacked the necessary coupling for climate physics and photochemistry over the wide range of atmospheres possible. This physically consistent study calculates the observational times needed to detect spectral signals with future telescopes and therefore presents a major step forward for atmospheric spectral characterization of potentially habitable, terrestrial exoplanets. The thesis led to three successful first author papers which are already well-cited and the author was additionally invited by international groups to co-author numerous other scientific papers. The main results of the three first author papers are now briefly summarized. Wunderlich et al. (2019, Paper I) investigated the impact of the spectral energy distribution (SED) of M dwarfs upon climate and photochemical processes in the atmospheres of Earth-like planets. Compared to Earth, the abundances of methane (CH4) increase by factors of several hundreds in the atmospheres of planets around early M dwarfs and by factors of several thousands for planets around late M dwarfs. This leads to a significant increase of molecular features in transmission spectroscopy and enhanced detectability of CH4. For cloud-free conditions, the detection of CH4 with the James Webb Space Telescope (JWST) might be feasible within ten transits in the atmospheres of planets orbiting early M dwarfs at a distance up to ∼10 pc and planets orbiting late M dwarfs up to ∼30 pc. The nearby planetary system of the late M dwarf TRAPPIST-1 includes three potentially habitable, rocky planets. Wunderlich et al. (2020, Paper II) introduced a new photochemical scheme, which is part of a coupled convective– climate–photochemistry model (1D-TERRA). The model was used to simulate potential atmospheres of TRAPPIST-1 e and f, assuming different surface conditions and varying amounts of carbon dioxide (CO2). In dry CO2-rich atmospheres molecular oxygen (O2) and carbon monoxide (CO) might be produced abiotically and could reach up to a few percent in mixing ratios. Whereas results suggest that a detection of O2 will likely not be feasible with JWST or the Extremely Large Telescope (ELT) CO might be detectable by co-adding a few tens of transits in dry CO2-rich atmospheres. A detection of CH4 might be only possible on planets with wet atmospheres and a biosphere, suggesting that emissions of CH4 could be related to life. The potentially habitable planet LHS 1140 b orbiting a mid M dwarf is another excellent target for future atmospheric characterization. Recent observations suggest that the planet has a clear, H2-dominated atmosphere. Wunderlich et al. (2021, Paper III) investigated the impact of different CH4 concentrations upon climate, chemistry and detectability of spectral features for H2-dominated atmospheres on LHS 1140 b. The destruction of the potential biosignatures ammonia (NH3), phosphine (PH3), chloromethane (CH3Cl), and nitrous oxide (N2O) shows a weak dependence on the concentrations of CH4. For low abundances of CH4 only five to ten transits are required to detect these molecules with JWST or ELT. However, for CH4 surface mixing ratios of a few percent only a detection of PH3 and N2O might be feasible. In summary, the three publications of this thesis show that the characterization of potentially habitable, rocky exoplanets will be at the limit of the JWST and ELT capabilities. In Earth-like and CO2-dominated atmospheres the detection of CO2, CH4 and CO might be feasible for close targets such as the TRAPPIST-1 planets. Potential biosignatures such as NH3, PH3, CH3Cl and N2O might be detectable in H2-dominated atmospheres.