Star light represents an unlimited and efficient source of energy that is abundantly used by life on Earth and at the basis of trophic chains, so it is natural to assume 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. For a handful of them, a detailed atmospheric characterization should be within reach of upcoming giant telescopes. A thorough assessment of the habitability of such late-type M-dwarfs is critical for a deep understanding of the universality and limits of Life. The emission of these cold stars peaks in the near-infrared (around 1 micron). Given their faintness, rocky planets have to orbit very close to them to be habitable. Depending on the planets’ atmospheric and magnetospheric properties, their proximity to the active host star could translate into fluxes of XUV photons and stellar winds on their surfaces dozens of times larger than for Earth. The capacity of these worlds to harbor Life should thus heavily rely on the efficiency of the mechanisms used by phototrophic organisms to harvest infrared photons and to protect from X-ray and UV photons and high-velocity charged particles.
On Earth, phototrophic organisms have evolved metabolisms to scavenge photons in the spectrum of visible light, but also to use the infra-red range. Several clades of anoxygenic and oxygenic phototrophs within the three domains of life have also developed additional strategies for protection against UV radiations, such as adaptation to shaded habitats and/or the synthesis of sunscreen pigments. Anoxygenic photosynthesis appeared more than 3.4 Ga, when Earth atmosphere was lacking free oxygen and an ozone layer, and its surface was exposed to strong UV radiations. Later oxygenic photosynthetic organisms, cyanobacteria, had a major impact on atmosphere composition and oceanic chemistry from at least 2.4 Ga, which contributed to the diversification of complex life (eukaryotes). Phototrophy thus can have a major impact on planets and life evolution.
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 to detect life on such planets. Our approach is multidisciplinary, combining expertise in astrophysics, internal geophysics, atmosphere-interior dynamics, geology, paleobiology and microbiology. The objectives are (1) to bring 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 infra-red 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 in orbit around VLM stars.
PoRTAL 