Searching for Biomarkers with Spektr-UF Observatory: Nitric Oxide Molecule in Atmospheres of Exoplanets near the Active Host Stars

Detection of an N₂–O₂ atmosphere on an Earth-like exoplanet may be one of the defining pieces of evidence for the presence of biological and geological activity on it. The nitric oxide molecule, in turn, is an indicator of such an atmosphere, and its spectral bands in the near UV range (γ-bands, 203–248 nm) can be potentially detected using a long-slit spectrograph (LSS) (resolution R = 1000) of the planned launch of the Spektr-UF space telescope (ST). Strong electron precipitations into the atmospheres of exoplanets that are in the potentially habitable zone near the active host stars can lead to an increase in the concentration of NO and, accordingly, increase the chances of detecting this biomarker. Based on the calculations using models of electron precipitation into the upper atmosphere of the planet, odd nitrogen chemistry, radiative transfer, and the Spektr-UF exposure time calculator, we determined the necessary observational conditions for detecting the NO biomarker. Thus, in a reasonable observational time (≤ 200 hours) with a signal-to-noise ratio of 3, using the Spektr-UF, it is possible to detect γ-bands of NO in the atmospheres of typical super-Earths and sub-Neptunes that are subjects of strong electron precipitation—at distances of up to 8 and 30 parsecs, respectively. The best observational targets for NO detection are exoplanets in the potential habitable zone near host stars with G–K spectral types.