![O2–O2 CIA impact on reflected-light spectra. Top: modern Earth composition (21 % O2). Bottom: otherwise identical O2-dominated composition (78 % O2), with N2 adjusted to keep total surface pressure fixed.[astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/11/Distinguishing.png?fit=1200%2C678&ssl=1)
[astro-ph.EP] Reflected-light spectra of Earth-sized exoplanets promise to probe habitability and biosignatures, but interpreting atmospheric oxygen can be challenging because O2 can arise from both biological and abiotic processes.
In addition, reflected-light retrievals exhibit degeneracies among atmospheric composition, clouds, surface reflectance, and gravity. We use the Bayesian retrieval framework ExoReLℜ, leveraging recent model developments that incorporate (i) O2-O2 collision-induced absorption (CIA), which produces the O4 bands, and (ii) a wavelength-dependent (step-function) surface albedo parameterization.
Focusing on a modern Earth analog, we compare retrievals with and without O2-O2 CIA using spectra within 0.25-1.8 μm, and perform signal-to-noise ratio (SNR) sweeps for two observing configurations motivated by the Habitable Worlds Observatory: a resolved-visible case (RVIS=140) and a broadband-visible case (three broad bins).
Omitting O2-O2 CIA allows the retrieval to fit the spectrum with an incorrect O2-dominated atmosphere. Including O2-O2 CIA leverages the O4 bands to break this degeneracy and recover the correct N2-dominated, O2-rich solution without requiring a tight mass prior. Adopting a robustness criterion of ≥95% posterior mass in the N2-dominated region, both configurations require SNR ≳20 at 0.75 μm (SNR ≈14 near the 1.27 μm O4 band for RNIR=70) to identify the correct dominant gas, with higher cloud fraction increasing this threshold.
Reducing near-IR coverage to 1.4 μm prevents recovery of the correct N2-dominated solution across the explored SNR range, and instead favors a spurious O2-dominated, CO2-rich atmosphere.
These results support near-IR coverage to ∼1.7 μm and SNR ≳20 at 0.75 μm for robust oxygen interpretation on Earth-like planets.
Nick Burri, Mario Damiano, Renyu Hu
Comments: 19 pages, 16 figures, 2 tables. Accepted for publication in The Astronomical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2610.42035 [astro-ph.EP] (or arXiv:2610.42035v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2610.42035
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Submission history
From: Nick Burri
[v1] Fri, 30 Oct 2026 17:01:07 UTC (2,856 KB)
https://arxiv.org/abs/2610.42035
Astrobiology,
