Wavelength Requirements for Life Detection via Reflected Light Spectroscopy of Rocky Exoplanets.

Abstract

Searching for signs of life is a primary goal of the Habitable Worlds Observatory (HWO). However, merely detecting oxygen, methane, or other widely discussed biosignatures is insufficient evidence for a biosphere. In parallel with biosignature detection, exoplanet life detection additionally requires characterization of the broader physicochemical context to evaluate planetary habitability and the plausibility that life could produce a particular biosignature in a given environment. Life detection further requires that we can confidently rule out photochemical or geological phenomena that can mimic life (i.e., "false positives"). Evaluating false-positive scenarios may require different observatory specifications than biosignature detection surveys. Here, we explore the coronagraph requirements for assessing habitability and for cautiously excluding known false-positive (and false-negative) scenarios for oxygen and methane, the two most widely discussed biosignatures for Earth-like exoplanets. We find that broad wavelength coverage ranging from the near ultraviolet (UV; 0.26 µm) and extending into the near infrared (NIR; 1.7 µm) is necessary to contextualize these potential biosignatures with HWO. The short-wavelength cutoff is driven by the need to identify Proterozoic-like biospheres via O₃, whereas the long-wavelength cutoff is driven by the need to contextualize O₂ and CH₄ biosignatures via constraints on carbon-bearing atmospheric species. The ability to obtain spectra with signal-to-noise ratios of 20-40 across this 0.26-1.7 µm range (assuming R = 7 UV, R = 140 VIS, and R = 70 NIR) is also required. While not every Earth-analog biosignature and false positive can be unambiguously identified with these capabilities-and the plausibility and contextual clues of many biosignature false positives remain an area of active research-our minimal spectral recommendations would enable a broad search for Earth-like life assuming such observations are achievable for a statistically meaningful number of HWO targets.