Coupled Photochemical-Climate Modeling of Plausible Tenuous Outgassed Atmospheres on the TRAPPIST-1 Planets

Available JWST observations TRAPPIST-1 system have suggested that several of the planets are likely airless, or possess a very tenuous atmosphere.

However, the high atmospheric escape rates expected for these planets suggest that any tenuous atmosphere must be replenished by constant outgassing, and past studies on modeling potential atmospheres for the planets have not widely considered surface pressures <1 bar.

Here, we show that tenuous atmospheres on the TRAPPIST-1 planets are likely possible, supported by constant plausible rates of water and/or CO₂ outgassing against assumed high escape rates (up to ~1030 s⁻¹).

We use a coupled photochemical-climate model and sample from a broad phase space of outgassing, surface deposition, and top-of-atmosphere escape rates to test hundreds of atmospheres per planet. Critically, our model also allows surface pressure to vary based on the balance of sources and sinks.

We find that 6 different compositional archetypes are generated via H₂O and/or CO₂ outgassing across our phase space, and atmospheres commonly fall between 10⁻⁴ — 1 bar. We find that potentially habitable surface environments are possible for TRAPPIST-1d and e at pressures between 0.05 — 2 bar and 0.5 — 1 bar, respectively.

Where possible, we compare our models to JWST observational data for TRAPPIST-1b, c, d, and e; all atmospheres found in this study for these planets match available transmission data to <3σ. However, emission data are consistent with atmospheric outcomes constrained to thin O₂-dominated compositions for TRAPPIST-1b (≲0.01 bars) and c (≲0.2 bars), which may or may not contain trace SO₂.

Transmission and emission spectra for all stable atmospheres found for T-1c (without trace SO₂). Top 3 rows (white background) and bottom 3 rows (grey background) show atmospheres sourced from H₂O and H₂O-CO₂ outgassing, respectively. Each row corresponds to a different atmospheric archetype (described in text), from top to bottom: H₂O-dominated, O₂– dominated (produced via H₂O outgassing), mixed H₂O₂, mixed CO₂-CO, O₂-dominated (produced via H₂O-CO₂ outgassing), and mixed O₂-H₂O-CO₂. Left column shows visible to near-infrared transmission spectra (0.001 – 6.5µm), middle column shows secondary eclipse emission spectra at thermal wavelengths (8 – 17µm), and right column shows nightside thermal emission at 15µm. Transmission spectra are colored by their fit (σ-deviation) to available T-1c transmission data (Radica et al. 2025), and emission data are colored by their combined fit to all available emission data (15µm day and night points; Zieba et al. 2023; Gillon et al. 2026). Spectral features are labeled with the absorbing gas. The overall best fitting atmospheres for T-1c include lower pressure (<0.2 bar) O₂-dominated atmospheres (regardless of outgassing source) and mixed O₂-H₂O atmospheres. — astro-ph.EP

[Editor’s NOTE] – The same information on all TRAPPIST-1 star system planets is provided in this paper.

Megan Gialluca, Victoria Meadows, Andrew Lincowski, Trent Thomas, Parker Hinton, David Brain, David Crisp

Comments: Accepted for publication in the Planetary Science Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2605.14101 [astro-ph.EP) (or arXiv:2605.14101v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2605.14101
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Submission history
From: Megan Gialluca
[v1] Wed, 13 May 2026 20:40:33 UTC (3,286 KB)
https://arxiv.org/abs/2605.14101

Astrobiology, exoplanet,