The Importance of UV Capabilities for Identifying Inhabited Exoplanets with Next Generation Space Telescopes

TL;DR

This paper highlights the critical role of UV observations in future space telescopes for detecting ozone as a biosignature on exoplanets, especially those with intermediate oxygen levels, informed by Earth's geochemical history.

Contribution

It emphasizes the importance of UV capabilities in upcoming telescopes for identifying biosignatures like O3 on exoplanets with low oxygen levels, considering Earth's historical context.

Findings

Ozone spectral features are detectable in UV even at low oxygen levels.

Earth's history provides insights into potential false negatives in biosignature detection.

UV observations are essential for identifying biosignatures on planets with intermediate oxygenation.

Abstract

The strongest remotely detectable signature of life on our planet today is the photosynthetically produced oxygen (O2) in our atmosphere. However, recent studies of Earth's geochemical proxy record suggest that for all but the last ~500 million years, atmospheric O2 would have been undetectable to a remote observer--and thus a potential false negative for life. During an extended period in Earth's middle history (2.0 - 0.7 billion years ago, Ga), O2 was likely present but in low concentrations, with pO2 estimates of ~0.1 - 1% of present day levels. Although O2 has a weak spectral impact at these low abundances, O3 in photochemical equilibrium with that O2 would produce notable spectral features in the UV Hartley-Huggins band (~0.25 um), with a weaker impact in the mid-IR band near 9.7 um. Thus, taking Earth history as an informative example, there likely exists a category of exoplanets for which conventional biosignatures can only be identified in the UV. In this paper, we emphasize the importance of UV capabilities in the design of future space-based direct imaging telescopes such as HabEx or LUVOIR to detect O3 on planets with intermediate oxygenation states. We also discuss strategies for mitigating against 'false positives'--that is, O3 produced by abiotic processes. More generally, this specific example highlights the broad implications of studying Earth history as a window into understanding potential exoplanet biosignatures.