Observability of ultraviolet N I lines in the atmosphere of transiting Earth-like planets

Abstract

Nitrogen is a biosignature gas that cannot be maintained in its Earth-like ratio with CO$_2$ under abiotic conditions. It has also proven to be notoriously hard to detect at optical and infrared wavelengths. Fortunately, the ultraviolet region, which has only recently started being explored for terrestrial exoplanets, may provide new opportunities to characterise exoplanetary atmospheric nitrogen. In this work, the future prospects for detecting atomic nitrogen absorption lines in the transmission spectrum of an Earth-like planet orbiting in the habitable zone of a Sun-like star with LUVOIR are explored. Using the non-local thermodynamic equilibrium spectral synthesis code Cloudy, we produce a far-ultraviolet atomic transmission spectrum for an Earth-Sun-like system, and identify several nitrogen features, including both N I and N II lines. We calculate the number of transits required for 1$σ$ and 3$σ$ detections of the planetary N{\sc i} $\lambda1200$ triplet signal with the G120M grating of the LUMOS spectrograph designed for LUVOIR, as a function of distance to the system and stellar ultraviolet emission. The minimum number of transit observations necessary for 1$σ$ and 3$σ$ detections of atomic N are 188 and 1685, respectively, for a system located at a distance of one pc with 100 times the Solar ultraviolet flux. Given that the orbital period of an Earth-Sun system is one year, it is not feasible to detect atomic N in the transmission spectrum for these systems. Future studies in this direction should therefore focus on Earth-like planets orbiting in the habitable zone of M dwarfs.