Determining the Detectability of H ₂ O with Photometric Observations Using Bayesian Analysis for Remote Biosignature Identification on exo-Earths (BARBIE)

Abstract We examine the detectability of water (H 2 O) in the reflected-light spectrum of an Earth-like exoplanet assuming a photometric observational approach rather than spectroscopic. By quantifying the detectability as a function of normalized exposure time, resolving power ( R ), and amount of spectral points, we can constrain whether spectroscopy or photometry is the more efficient observing procedure to detect H 2 O at varying abundances by measuring the broad 0.94 μ m absorption feature using the Habitable Worlds Observatory (HWO). We simulate low-resolution spectroscopy ( R = 10, 20, and 30, presented as a photometric bandwidth fraction of 10%, 5%, and 3% herein) as a proxy for narrowband photometric observations, and constrain the wavelength range from 0.85 to 1.05 μ m to narrow focus on the 0.9 μ m feature. We then constrain the number of spectral points to two or three points at each bandwidth fraction to investigate the impact of spectral point placement on detectability. Additionally, we take the signal-to-noise ratios for strong H 2 O detection and calculate the resultant exoplanet yields assuming photometric observation and compare to the yields from higher-resolution spectroscopic observations under different noise instances, characterization wavelengths, noise floors, and aperture sizes. We find that H 2 O is strongly detectable at all bandwidth fractions depending on the spectral point placement, requiring a minimum of three spectral points, at a variety of normalized exposure times depending on the abundance of H 2 O. We also find that the detector noise is the main driver in determining whether photometry or spectroscopy results in higher yields. Photometry is the preferred observational method in high-noise cases, while spectroscopy is preferred in low-noise scenarios.