Cloudy with a chance of metals: Indications of CO₂ in the atmosphere of GJ 1214 b from high-resolution K-band spectroscopy

¹ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
² Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany
³ Exzellenzcluster Origins, Boltzmannstraße 2, 85748 Garching, Germany
⁴ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁵ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁶ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁷ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, 31400 Toulouse, France
⁸ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
⁹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁰ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹² Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹³ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK

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Received: 10 December 2025
Accepted: 25 March 2026

Abstract

Context. Sub-Neptune extra-solar planets are abundant in the Milky Way, yet their atmospheric properties remain poorly understood. They frequently exhibit muted transmission spectra, with GJ1214b being the most prominent example. Following years of intense observing campaigns yielding featureless planetary spectra, more recent observations with JWST have revealed the first possible atmospheric signatures of H₂O, CH₄, and CO₂.

Aims. We present high-resolution transmission spectroscopy of GJ 1214 b based on eight transits obtained with the CRIRES⁺ spectrograph in the K band.

Methods. We used SYSREM to remove telluric and stellar signals from the data and searched for signatures of H₂O, CO, CH₄, H₂S, NH₃, and CO₂ using the cross-correlation technique.

Results. 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 cross-correlation signal at S/N ~3.6, with a detailed investigation of the signal showing no obvious indication that it is caused by correlated noise. A Welch t-test confirmed that the in-trail, in-transit distribution is significantly different from the out-of-trail distribution at a 3.4σ confidence. Interpreting the data using a Bayesian retrieval framework, with multiple molecular species and free chemistry, resulted in a retrieved planet temperature of T_iso = 398₋₁₉₇⁺²⁸³ K, consistent with a value intermediate between the day- and night-side temperatures from JWST-derived temperature-pressure profiles at high altitudes, as expected for the planetary terminator. In addition, a metallicity of [M/H]= 0.48_−1.70^+0.89 was derived from the abundances of the retrieved molecules, along with an opacity deck pressure of log₁₀(P_c) = −3.04_−1.53^+2.52. A simpler equilibrium chemistry retrieval assuming CO₂ as the sole opacity source returned a compatible temperature, with smaller formal uncertainties (T_iso = 509₋₅₉⁺¹⁰² K), slightly higher metallicity ([M/H]= 1.51_−0.75^+0.68), and higher opacity deck pressure (log₁₀(P_c) = −0.88_−2.48^+1.95). While these sets of 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.

Conclusions. 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.