Cloudy With A Chance Of Metals: Indications Of CO2 In The Atmosphere Of GJ 1214 b From High-resolution K-band Spectroscopy

Sub-Neptune exoplanets frequently exhibit muted transmission spectra, with GJ 1214 b being the most prominent example. Following years of intense observing campaigns yielding featureless planetary spectra, recent observations with JWST revealed the first possible atmospheric signatures. We present high-resolution transmission spectroscopy of GJ 1214 b based on eight transits obtained with the CRIRES+ spectrograph in the K band.

We used SYSREM to remove telluric and stellar signals and searched for signatures of H₂O, CO, CH₄, H₂S, NH₃, and CO₂ using the cross-correlation technique. We obtained non-detections for the first five molecules and used injection recovery tests to derive upper limits on the atmosphere. For CO₂ we measure a CCF signal at S/N ~ 3.6, with a detailed investigation showing no obvious indication that it is caused by correlated noise.

A Welch t-test confirmed the in-trail and out-of-trail distributions to be different at 3.4σ confidence. A Bayesian retrieval framework with free chemistry, resulted in volume mixing ratios corresponding to a metallicity of [M/H]=0.48+0.89−1.70, an opacity deck pressure of log10(Pc)=−3.04+2.52−1.53 and a planet temperature of T_iso=398+283−197 K, consistent with a value intermediate between the day- and night-side T-p’s derived from JWST data.

While these values correspond to relatively large signal amplitudes predicted for CO₂ features in the mid-infrared, they are compatible with JWST NIRSpec observations within the models’ 1.5σ uncertainties.

Further modelling and additional data are required to confirm the atmospheric signatures and obtain a comprehensive interpretation of low- and high-resolution data. Overall, our results support previous findings that CO₂ is likely to be a significant component of the atmosphere of GJ 1214 b.

L. Nortmann, D. Cont, F. Lesjak, A. D. Rains, A. Lavail, L. Boldt-Christmas, E. Nagel, A. Reiners, N. Piskunov, F. Yan, A. Hatzes, O. Kochukhov, D. Shulyak, U. Seemann, M. Rengel, A. Hahlin

Comments: 24 pages, 21 figures (16 main text, 8 appendix), Accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2604.15292 [astro-ph.EP] (or arXiv:2604.15292v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2604.15292
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Submission history
From: Lisa Nortmann
[v1] Thu, 16 Apr 2026 17:54:55 UTC (11,905 KB)
https://arxiv.org/abs/2604.15292

Astrobiology, Exoplanet,