Ground-Based Spectrograph Finds Tentative CO₂ Signal in the Atmosphere of Sub-Neptune GJ 1214 b
A team led by L. Nortmann (University of Göttingen) reports a tentative detection of carbon dioxide in the atmosphere of the sub-Neptune exoplanet GJ 1214 b, using the CRIRES+ spectrograph on ESO's Very Large Telescope. The cross-correlation signal, at S/N ≈ 3.6 across eight transits, is the first molecular feature seen in GJ 1214 b from the ground and corroborates earlier JWST atmospheric hints.
Astronomers have identified a tentative signal of carbon dioxide in the atmosphere of GJ 1214 b — a sub-Neptune exoplanet roughly 40 light-years from Earth that has, for nearly two decades, refused to give up the composition of its atmosphere to observers.
The detection, reported in a paper submitted to arXiv on April 16 and accepted for publication in Astronomy & Astrophysics, was made with the CRIRES+ spectrograph on the European Southern Observatory's Very Large Telescope at Paranal, Chile. The team, led by L. Nortmann, combined eight K-band transit observations, applied the SYSREM algorithm to remove telluric and stellar contamination, and then cross-correlated the residuals against molecular templates for six candidate absorbers.
What they found
| Molecule | Result |
|---|---|
| CO₂ | CCF signal at S/N ≈ 3.6, confirmed by Welch t-test at 3.4σ |
| H₂O | Non-detection (upper limit) |
| CO | Non-detection (upper limit) |
| CH₄ | Non-detection (upper limit) |
| H₂S | Non-detection (upper limit) |
| NH₃ | Non-detection (upper limit) |
The 3.6 signal-to-noise cross-correlation is evidence-level, not a 5σ discovery-grade claim. The authors note that a detailed investigation found no obvious indication the peak is caused by correlated noise, and the Welch t-test on in-trail versus out-of-trail signal distributions rejected the null hypothesis at 3.4σ.
Why GJ 1214 b is a hard target
GJ 1214 b is a roughly 6-Earth-mass sub-Neptune orbiting a cool M-dwarf star every 1.58 days. Since its 2009 discovery by the MEarth ground-based transit survey, the planet has become the archetype of a class of worlds that produce unusually featureless transmission spectra — flat lines where atmospheric features ought to appear. The prevailing explanation is that a high-altitude haze or cloud layer obscures the deeper, molecule-bearing atmosphere from outside observers.
In late 2023, the James Webb Space Telescope produced the first hint that the planet had an atmosphere at all, tentatively identifying water vapor features in MIRI observations. The Nortmann CRIRES+ result is the first complementary ground-based signal and is consistent with a CO₂-bearing atmosphere sitting beneath the cloud deck.
Method
High-resolution cross-correlation spectroscopy is the dominant ground-based tool for exoplanet atmospheres. The planet's orbital motion Doppler-shifts its spectral lines by tens of kilometers per second between ingress and egress, producing a tell-tale tilted pattern in a time-series spectrogram. Stacking the 8-transit dataset and cross-correlating against a synthetic CO₂ template produces a peak at the planet's known radial-velocity semi-amplitude — that peak is the detection.
The CRIRES+ spectrograph (Cryogenic High-Resolution InfraRed Echelle Spectrograph), upgraded from the original CRIRES in 2021, operates at resolving power R ≈ 100,000 across the 0.95–5.3 µm infrared range — the wavelength window where CO₂ has strong ro-vibrational bands.
What it means
If confirmed at higher significance, the Nortmann result would make GJ 1214 b the first sub-Neptune with a ground-based molecular detection and would settle one long-running question — whether the muted JWST spectra reflect a cloud-topped but molecule-rich atmosphere rather than a clear, low-abundance one.
The paper is available on arXiv as 2604.15292. Co-authors include researchers from the University of Göttingen, Uppsala University, the Thüringer Landessternwarte Tautenburg, the Max Planck Institute for Solar System Research, and ESO.