Biosignatures & Paleobiology

Determining the Detectability of H2O With Photometric Observations Using Bayesian Analysis For Remote Biosignature Identification On exoEarths (BARBIE)

By Keith Cowing

Press Release

December 9, 2025

Filed under astrochemistry, astronomy, atmosphere, Bayesian Analysis, biochemistry, Biosignatures, Habitable Worlds Observatory, organic chemistry, Spectroscopy, Water

Determining the Detectability of H2O With Photometric Observations Using Bayesian Analysis For Remote Biosignature Identification On exoEarths (BARBIE) — TOP: Heat map plots illustrating detection strength as a function of normalized exposure time and varying resolving abundance at varying Earth H2O abundances with 2 spectral points permitted at each bandwidth. lnB≥5 is a strong detection, 2.5≤lnB≤5 is a weak detection, and lnB≤2.5 is unconstrained. A modern abundance of H2O is considered to be 3×10−3 . With two spectral points regardless of bandwidth, H2O detectability is low, requiring higher abundances. BOTTOM: Heat map plots illustrating detection strength as a function of normalized exposure time and varying resolving abundance at varying Earth H2O abundances with 3 spectral points permitted at each bandwidth. lnB≥5 is a strong detection, 2.5≤lnB≤5 is a weak detection, and lnB≤2.5 is unconstrained. Note that the y-axis is cropped to 3.84, to more directly compare to Figure 2 which caps at 4.0. The key factor to photometric H2O detection is spectral point quantity. — astro-ph.EP

We examine the detectability of water (H₂O) 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 H₂O at varying abundances by measuring the broad 0.94 microns absorption feature using the Habitable Worlds Observatory (HWO).

We simulate low-resolution spectroscopy (R = 10, 20, 30, presented as photometric bandwidth fraction 10%, 5%, 3% herein) as a proxy for narrow-band photometric observations, and constrain the wavelength range from 0.85 – 1.05 microns, to narrow focus on the 0.9 microns feature. We then constrain the number of spectral points to 2 or 3 points at each bandwidth fraction to investigate the impact of spectral point placement on detectability.

Additionally, we take the signal-to-noise ratios (SNRs) for strong H₂O 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 wavelength, noise floors, and aperture sizes.

We find that H₂O is strongly detectable at all bandwidth fractions depending on the spectral point placement, requiring a minimum of 3 spectral points, at a variety of normalized exposure time depending on the abundance of H₂O.

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.

Natasha Latouf, Chris Stark, Avi Mandell, Vincent Kofman

Comments: Accepted for publication in the Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2512.07620 [astro-ph.EP] (or arXiv:2512.07620v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.07620
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Submission history
From: Natasha Latouf
[v1] Mon, 8 Dec 2025 15:07:05 UTC (8,736 KB)
https://arxiv.org/abs/2512.07620
Astrobiology, exoplanet,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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