On the Effects of M-Dwarf UV Variability on Detecting and Interpreting Biosignatures

In the coming decades, telescopes like JWST, LUVOIR, HabEx, and the ground-based extremely large telescopes will be capable of capturing spectra of transiting and non-transiting exoplanets. M-dwarf stars, arguably the best candidates for detecting resident terrestrial exoplanets, exhibit substantially different ultraviolet activity compared to stars like our Sun. These differences occur in multiple dimensions including spectral class, stellar age, flare activity, broadband UV flux, and the Lyman Alpha line. In this paper, we use a suite of photochemical, climate, and stellar models along with two atmospheric archetypes, the Pre-Industrial and Archean Earths, to analyze how these differences in UV radiation impact potential Earth-like planetary atmospheres, the spectra they produce, and gas surface fluxes we would infer based on those spectra. We find that some dimensions of this UV space, such as Lyman Alpha, have little impact on the chemistry of our atmospheres below 60km. However, two of the most promising biosignature gases, methane (CH4) and ozone (O3), can exhibit multiple order-of-magnitude changes in abundance and potentially significant changes in transit and reflectance spectra when orbiting stars of varying spectral classes, ages, and UV broadband flux levels. We also find that the inferred methane surface flux increases by over a factor of 5 when the noise floor of the stellar spectrum is consistent with the stars emission lines. Therefore, the context of the star’s age and spectral class, as well as stellar spectra with sufficiently high resolution and low noise floor, are required for any accurate interpretation of or inference stemming from the spectrum of a terrestrial exoplanet orbiting an M-dwarf star.