Atmospheric constraints on GJ 1214 b from CRIRES⁺ and prospects for characterisation with ANDES

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, Genil, 18008 Granada, Spain
² Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁴ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
⁵ Department of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Department of Space Research and Space Technology, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 December 2025
Accepted: 2 February 2026

Abstract

Context. Sub-Neptune exoplanets such as GJ 1214 b provide a critical link between terrestrial and giant planets, yet atmospheric characterisation remains challenging due to high-altitude clouds and compressed atmospheres. JWST has recently hinted at molecular signals in GJ 1214 b, and ground-based high-resolution spectroscopy is potentially able to confirm them.

Aims. We aim to constrain the atmospheric composition of GJ 1214 b using all available transits observed with the upgraded CRIRES⁺ spectrograph on the Very Large Telescope (VLT) by searching for the signatures of water vapour, methane, and carbon dioxide.

Methods. We analysed eight CRIRES⁺ transit datasets covering the K band (1.90-2.45 μm) at a resolving power of R ≈ 100,000. We used the SysRem algorithm to correct for telluric and stellar contributions and employed the cross-correlation technique with templates from petitRADTRANS to search for H₂O, CH₄, and CO₂. Injection-recovery tests were performed across a grid of metallicities (Z) and cloud-deck pressures (p_c) to quantify detection limits. We also generated predictions for ANDES observations using end-to-end simulated datasets with EXoPLORE.

Results. We detect no significant H₂O, CH₄, or CO₂ signatures. Injection-recovery tests show that such non-detections exclude atmospheres with low-altitude clouds and moderate or low metallicities. CH₄ yields the tightest empirical limits, with CO₂ unexpectedly ruling out intermediate metallicities (∼100× solar) with clouds deeper due to its rapidly rising opacity in compressed high-Z atmospheres. Our constraints are in line with either a high-Z or a high-altitude aerosol layer, in agreement with recent JWST inferences.

Conclusions. The combined analysis of eight CRIRES⁺ datasets provides the most stringent high-resolution constraints on the atmospheric properties of GJ 1214 b to date. Planetary signals are likely buried below our current detection threshold, preventing confirmation of recent JWST-reported molecular hints. Simulations of a single transit observed with ANDES on the ELT predict modest improvements for H₂O, a substantially expanded detectable region for CH₄, and the strongest gains for CO₂, making the latter a particularly effective tracer for characterising high-metallicity atmospheres in sub-Neptunes.