Cloudy with a chance of metals: Indications of CO₂ in the atmosphere of GJ1214b from high-resolution K-band spectroscopy

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 GJ, 1214, b 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. 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 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. SYSREM 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. 4 σ confidence. Interpreting the data using a Bayesian retrieval framework, with multiple molecular species and free chemistry, resulted in a retrieved planet temperature of T_ =398^ iso +283 _ -197 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^ +0. 89 _ -1. 70 was derived from the abundances of the retrieved molecules, along with an opacity deck pressure of log_ 10 (P_) = -3. 04^ c +2. 52 _ -1. 53. A simpler equilibrium chemistry retrieval assuming CO₂ as the sole opacity source returned a compatible temperature, with smaller formal uncertainties (T_ =509^ iso +102 _ -59 K), slightly higher metallicity (M/H =1. 51^ +0. 68 _ -0. 75), and higher opacity deck pressure (log_ 10 (Pc) = -0. 88^ +1. 95 _ -2. 48). 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. 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.