Exoplanet Habitability: Potential O 2 /O 3 Biosignatures in the Ultraviolet

Abstract

Currently, the strongest remotely detectable biosignature in the Earth’s atmosphere is molecular oxygen (O2) produced during photosynthesis. However, recent studies of geochemical signatures on Earth-like exoplanets suggest that for most of them, atmospheric O2 would not be detectable by a remote observer, except during the last ~500 Myr of evolution. During a long period in the Earth’s history (2.0–0.7 Gyr ago), O2 was likely present in the atmosphere but in low concentrations, estimated at ~0.1–1% of the current level. Although spectral manifestations of O2 are weak at such low concentrations; however, ozone (O3) molecules, which are in a photochemical equilibrium with such low O2 concentrations, cause noticeable spectral features in the Hartley–Huggins UV band (~0.25 µm), with a weaker manifestation in the medium IR-region at about 9.7 µm. Thus, taking the Earth’s history as an informative example (proxy), it can be concluded that a category of exoplanets may exist for which the ordinary atmospheric biosignature can only be identified in the UV range. Accordingly, the article emphasizes the importance of planning for UV observation capabilities when designing future space telescopes for direct observations of exoplanets and their atmospheres, such as the World Space Observatory-UV (WSO-UV), Habitable Exoplanet Observatory (HabEx), or Large UV/Optical/Infrared Surveyor (LUVOIR), for the detection of ozone O3 in the atmospheres of planets with intermediate oxidation states. The article also discusses mitigation strategies for the so-called false positives, i.e., detection of O3 generated in abiotic processes. It also emphasizes the importance and broad implications of studying the Earth’s history as a window to understanding potential biosignatures for exoplanets and the importance of UV observations for identifying habitable exoplanets with next-generation space telescopes.