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

TL;DR

This study uses high-resolution spectroscopy from CRIRES+ to constrain the atmospheric composition of GJ 1214 b, finding no significant molecular detections but setting limits on atmospheric metallicity and cloud properties, and explores future prospects with ANDES.

Contribution

It provides the most stringent high-resolution constraints on GJ 1214 b's atmosphere and evaluates the potential of ANDES for future atmospheric characterization.

Findings

No significant H2O, CH4, or CO2 signatures detected.

Excludes atmospheres with low-altitude clouds and low/medium metallicities.

CO2 observations can effectively probe high-metallicity atmospheres.

Abstract

In this study, we aim to constrain the atmospheric composition of GJ 1214 b using all available transits observed with the upgraded CRIRES+ spectrograph at the VLT by searching for the signatures of water vapour, methane, and carbon dioxide. We analysed eight CRIRES+ transit datasets covering the K band (1.90-2.45 microns) 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 across a grid of metallicities (Z) and cloud-deck pressures (pc) were performed to quantify detection limits. We also generated predictions for ANDES observations using end-to-end simulated datasets with EXoPLORE. 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 (~ 100xsolar) 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. The combined analysis of eight CRIRES+ datasets provides the most stringent high-resolution constraints on the atmospheric properties of GJ 1214 b to date. 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.