![Transmission spectra of TRAPPIST-1 c from the four JWST/NIRSpec PRISM epochs (4-pixel bins, 0.8β5.0 πm), offset vertically by 200 ppm per epoch for clarity and colour-coded by date, with the inverse-variance-weighted mean spectrum (black, plotted at its true depth). Grey bands mark the stellar H2O features near 1.4 and 1.85 πm. The dotted line marks 2.0 πm, the boundary of the region where the contamination is strongest (Section 3). The 2023 November 8 visit contains a flare during transit and the 2024 July 9 visit shows activity outside transit (Section 2.5). -- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/10/Detecting.png?fit=1200%2C671&ssl=1)
[astro-ph.EP] Transmission spectroscopy of rocky exoplanets orbiting late M dwarfs is limited by stellar contamination rather than photon noise.
We present a framework for this contamination-dominated regime, fitting all transit epochs jointly with a Gaussian process per epoch for the time-varying stellar contamination and a single atmospheric model shared across epochs.
We use pure CO2, H2O, and CH4 models scaled by their peak-to-trough amplitude, allowing us to measure how large a signal must be to be recovered. We calibrate this framework using injection-recovery tests and 17 archival JWST/PRISM transits of the airless sibling TRAPPIST-1 b to quantify the spurious signals that stellar contamination alone produces.
Applied to four JWST/NIRSpec PRISM transits of TRAPPIST-1 c, our framework recovers injected CO2 signals without bias and reaches detection thresholds of 146 (171) ppm for moderate (strong) evidence, compared with 96 ppm if there were no stellar contamination. CO2 and CH4 produce no spurious signals on the airless control planet, whereas H2O shows a persistent stellar floor of +55Β±34 ppm.
TRAPPIST-1 c is consistent with a featureless, airless baseline for all three molecules. Our 95 per cent upper limit of 90 ppm for CO2-shaped signals effectively rules out clear, isothermal pure-CO2 atmospheres (β₯0.1 bar) and N2-dominated atmospheres with 1 per cent CO2 (β₯1 bar) at the equilibrium temperature.
An empirical scaling study using 2 to 10 transits of TRAPPIST-1 b shows the CO2 detection threshold decreasing with the number of transits k as kβ0.6, reaching 80 ppm at ten transits, while water searches become limited by the stellar floor beyond about six transits. CO2atmospheres, by contrast, are detectable when we utilize our framework to account for stellar activity at the cost of roughly 1.5 to 2 times as many transits.
Alexander D. Rathcke, Lars A. Buchhave
Comments: 24 pages, 10 figures. To be submitted to MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2609.37722 [astro-ph.EP] (or arXiv:2609.37722v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.37722
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Submission history
From: Alexander Rathcke
[v1] Tue, 29 Sep 2026 14:49:21 UTC (292 KB)
https://arxiv.org/abs/2609.37722
Astrobiology, Astrochemistry,
