An Integrated Investigation of Biogenic Halomethane and Methylated Gases as Exoplanet Biosignature Candidates
Year: 2025
Type: dissertation
Abstract: One of the most exciting opportunities presented by the study of extrasolar planets is the ability to begin to answer the age-old, civilizational question “are we alone in the universe”. In this decade, robust attempts to answer this question are coming into reach with instruments such as the Extremely Large Telescope class, and NASA‘s next flagship telescope, the Habitable Worlds Observatory (HWO) designed to survey nearby exoplanets for signs of a global biosphere.However, once these highly capable instruments become available, the question of what signs of life to target remains without strong consensus. Since atmospheres are the planetary component most accessible to remote observation, prevalent and abundant gaseous biological products on the Earth such as oxygen (O2) and methane (CH4) were first identified as biosignature candidates. However, false positive scenarios have been uncovered for these gases (Luger & Barnes, 2015; Meadows et al., 2018a) which hamper their applicability and motivate the search for additional biosignature candidates, especially those with high biological specificity (i.e., clearly point to biological origins). These challenges necessitate the identification and analysis of additional biosignature candidates.Using a vertically integrated combination of photochemical, spectral, and instrumental models, I have explored the applicability of methylated and halomethane gases, expanding on the initial studies of a few methylated gases (i.e. Segura et al., 2005; Domagal-Goldman et al., 2011) to consider structurally similar gases. This includes other methylated gas candidates primarily formed by the biological enzymatic attachment of a CH3 group to an environmental substrate, and halomethanes formed by the halogenation processes leading to halogen substitutions into methane. This work builds on and describes a class of volatile compounds generated from common biological processes, deeply rooted in Earth’s history and connected to fundamental biological requirements.My results show that a next generation mid-infrared telescope could detect methylated gases around favorable target terrestrial exoplanets, such as those in the TRAPPIST-1 system, with modern Earth-like atmospheres. I demonstrated an additional photochemical and spectral build up elect when the impacts of multiple methylated gases (i.e. CH3Cl, CH3Br) are considered together. Mission concepts such as the Origins Space Telescope (Battersby et al., 2018) or the European Space Agency’s Large Interferometer for Exoplanets (LIFE, Quanz et al. (2022)) would be capable of detecting these gases. In Chapter 3, I build on this work and show that methylated biosignatures, specifically methyl chloride, are within current reach of JWST, if present, on sub Neptune-sized planets with hydrogen dominated atmospheres (i.e., “Hycean” planets). Chapter 4 reports the findings of two pilot studies combining laboratory and field studies with the photochemical and spectral modeling used in the first two chapters. These results show that generation of input data for specific exoplanet environments can impact the simulation results and lays the groundwork for future studies of this kind. In total, this work establishes an additional pathway for searching for life beyond the Earth and demonstrates preliminary feasibility for Earth-like and Hycean planets.
Author: Leung, Michaela
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7019953441