SIMBAD references

Super-Earth exoplanets are being discovered with increasing frequency and some will be able to retain stable H₂-dominated atmospheres. We study biosignature gases on exoplanets with thin H₂ atmospheres and habitable surface temperatures, using a model atmosphere with photochemistry and a biomass estimate framework for evaluating the plausibility of a range of biosignature gas candidates. We find that photochemically produced H atoms are the most abundant reactive species in H₂atmospheres. In atmospheres with high CO₂levels, atomic O is the major destructive species for some molecules. In Sun-Earth-like UV radiation environments, H (and in some cases O) will rapidly destroy nearly all biosignature gases of interest. The lower UV fluxes from UV-quiet M stars would produce a lower concentration of H (or O) for the same scenario, enabling some biosignature gases to accumulate. The favorability of low-UV radiation environments to accumulate detectable biosignature gases in an H₂ atmosphere is closely analogous to the case of oxidized atmospheres, where photochemically produced OH is the major destructive species. Most potential biosignature gases, such as dimethylsulfide and CH₃Cl, are therefore more favorable in low-UV, as compared with solar-like UV, environments. A few promising biosignature gas candidates, including NH₃ and N₂O, are favorable even in solar-like UV environments, as these gases are destroyed directly by photolysis and not by H (or O). A more subtle finding is that most gases produced by life that are fully hydrogenated forms of an element, such as CH₄ and H₂S, are not effective signs of life in an H₂-rich atmosphere because the dominant atmospheric chemistry will generate such gases abiologically, through photochemistry or geochemistry. Suitable biosignature gases in H₂-rich atmospheres for super-Earth exoplanets transiting M stars could potentially be detected in transmission spectra with the James Webb Space Telescope.