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

TL;DR

This paper discusses the necessary wavelength coverage and spectral resolution for future space observatories to effectively detect and interpret biosignatures like oxygen and methane on rocky exoplanets, ensuring accurate assessment of habitability and false positives.

Contribution

It identifies specific wavelength ranges and spectral resolutions needed for life detection, emphasizing the importance of broad coverage from UV to near-infrared for exoplanet characterization.

Findings

Broad wavelength coverage from 0.26 to 1.7 μm is essential.

Spectral resolution of R=7 to R=140 is required across different bands.

Achieving SNR of 20-40 enables reliable biosignature interpretation.

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 ruling out 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 UV (0.26 $\mu$m) and extending into the near infrared (1.7 $\mu$m), is necessary for contextualizing biosignatures with HWO. The short wavelength cutoff is driven by the need to identify Proterozoic-like biospheres via O$_3$, whereas the long wavelength cutoff is driven by the need to contextualize O$_2$ and CH$_4$ biosignatures via constraints on C-bearing atmospheric species. The ability to obtain spectra with signal-to-noise ratios of 20-40 across this 0.26-1.7 $\mu$m range (assuming R=7 UV, R=140 VIS, and R=70 NIR) is also required. Without sufficiently broad wavelength coverage, we risk being unprepared to interpret biosignature detections and may ultimately be ill-equipped to confirm the detection of an Earth-like biosphere, which is a driving motivation of HWO..