Project B2/212/P1/PORTAL -PhOtotrophy on Rocky habiTAble pLanets BRAIN-be 2.0 (Belgian Research Action through Interdisciplinary Networks) 4

Context Star light represents an unlimited and efficient source of energy that is abundantly used by life on Earth for billions of years and at the basis of trophic chains, so it could play also a key role in the development and sustainability of other biospheres elsewhere in the Universe. Tackling the questions of the universality and the diversity of phototrophic metabolisms is thus a critical step in our study of life on a cosmic scale-the purpose of the interdisciplinary field of astrobiology. Within the last three decades, thousands of exoplanets have been discovered, including a few dozen that are potentially habitable. The project "PhOtotrophy on Rocky habiTAble pLanets" (PORTAL) addresses the potential habitability of temperate rocky planets in orbit around very low-mass (<0.2 Msun; VLM) stars, and the possibility of detecting phototrophic life on such planets if it exists. Objectives The objectives are (1) to provide strong observational constraints on the physical and irradiative conditions at the surface of the planets orbiting in the habitable zone of the nearby dwarf star TRAPPIST-1, and (2) to use those constraints to investigate the possibilities of phototrophy in the infrared range and the detectability of their signatures, in samples from the early Earth and modern extreme habitats, to simulated exoplanet conditions in a new TRAPPIST biodome, to rocky exoplanets orbiting VLM stars. Our approach is multidisciplinary, combining expertise in astrophysics, internal geophysics, atmosphere-interior dynamics, geology, paleobiology and microbiology from an international consortium of scientists in Belgium, France and Germany. Conclusions PORTAL improves our understanding of the TRAPPIST-1 exoplanetary system. It provides new constraints on the high-energy stellar environment impacting atmospheric escape and photochemistry on the TRAPPIST-1 planets, methods for dealing with stellar contamination in atmosphere retrieval, models of possible atmospheres and their evolution around exoplanets of the TRAPPIST-1 system, or absence of atmosphere around certain exoplanets, discussion on exoplanet habitability around VLM stars. PORTAL also made important advances in our understanding of phototrophy on modern and early Earth. Finally, PORTAL provides a new experimental platform (a biodome) to investigate the possibility of phototrophy in the infra-red range, a system where oxygenic photosynthesis by model Earth microorganisms is performed and measured under controlled Trappist-1-like atmospheric and light conditions, and the possible biosignatures to detect remotely. This platform will also be useful for investigating other extraterrestrial and Early Earth conditions and for outreach and education.