Detecting atmospheres in the presence of stellar contamination: an empirically calibrated framework applied to four JWST/NIRSpec PRISM transits of TRAPPIST-1c
Abstract
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 +55 ± 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 (≥0.1 bar) and N2-dominated atmospheres with 1 per cent CO2 (≥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 k−0.6, reaching 80 ppm at ten transits, while water searches become limited by the stellar floor beyond about six transits. CO2 atmospheres, 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.
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BibTeX
@misc{rathcke2026detecting,
title = {{Detecting atmospheres in the presence of stellar contamination: an empirically calibrated framework applied to four JWST/NIRSpec PRISM transits of TRAPPIST-1c}},
author = {Alexander D. Rathcke and Lars A. Buchhave},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.37722},
eprint = {2609.37722},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.37722},
note = {CC0},
}
RIS
TY - UNPB
TI - Detecting atmospheres in the presence of stellar contamination: an empirically calibrated framework applied to four JWST/NIRSpec PRISM transits of TRAPPIST-1c
AU - Alexander D. Rathcke
AU - Lars A. Buchhave
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2609.37722
UR - https://arxiv.org/abs/2609.37722
AB - 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 CO_2, H_2O, and CH_4 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 CO_2 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. CO_2 and CH_4 produce no spurious signals on the airless control planet, whereas H_2O shows a persistent stellar floor of +55 ± 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 CO_2-shaped signals effectively rules out clear, isothermal pure-CO_2 atmospheres (≥0.1 bar) and N_2-dominated atmospheres with 1 per cent CO_2 (≥1 bar) at the equilibrium temperature. An empirical scaling study using 2 to 10 transits of TRAPPIST-1 b shows the CO_2 detection threshold decreasing with the number of transits k as k^−0.6, reaching 80 ppm at ten transits, while water searches become limited by the stellar floor beyond about six transits. CO_2 atmospheres, 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.
ER -