TRAPPIST-1 appears to have near-solar elemental abundances, but its carbon isotope balance stands out. A new preprint estimates a C/O ratio of 0.60 +0.02/−0.02 and a 12C/13C ratio of 154 +17/−16, compared with a reported solar-system value of 89.
The analysis reports the first stellar C/O ratio and the first carbon and oxygen isotope ratios for TRAPPIST-1, inferred from joint high-resolution K-band and M-band spectra.
Reading the star in infrared
The dataset combined Keck/NIRSPEC M-band observations collected over three half-nights, with 3.36 hours of open-shutter time, and archival SPIRou K-band data made up of 17 exposures lasting 245.16 seconds each.
The M-band spectra were manually reduced and turned into one-dimensional spectra. The SPIRou data were processed through a pipeline, shifted to account for Earth’s motion, aligned, cleaned of outliers, resampled and combined. Synthetic spectra were then computed in a plane-parallel, hydrostatic atmospheric model using temperature structure, gas mixing ratios, gravity, and continuum and line opacities.
A Bayesian retrieval tested possible combinations of atmospheric and instrumental properties against the observations. Nineteen parameters were sampled, while spline continuum amplitudes were optimized during each likelihood calculation rather than sampled as separate parameters.
The two infrared bands played different roles: K-band data anchored the C/O ratio and metallicity, while M-band data drove the isotope-ratio constraints.
The isotope signals
The M-band retrieval detected 13CO and C18O, two less common forms of carbon monoxide. The reported log Bayes factors, scores used in the retrieval’s model comparison, were 136.7 for 13CO and 69.1 for C18O. A cross-correlation check supported the same detections, although it reused the same spectra and templates and was therefore not independent.
In the preferred flexible one-column model, the star’s metallicity was [M/H] = 0.00 +0.06/−0.06, described as near solar. Its C/O ratio was 0.60 +0.02/−0.02.
The inferred oxygen ratio, 16O/18O = 490 +78/−64, was reported as consistent within uncertainty with comparison values of 525 ± 21 for the Sun and 557 ± 30 for the present-day interstellar medium. The result therefore looks broadly solar-like on that measure, although its uncertainty range is much wider than the quoted comparison uncertainties.
The picture was less ordinary for carbon. The estimated 12C/13C ratio of 154 +17/−16 was above the reported solar-system value of 89. The authors caution that this ratio is not an independent age indicator, so it should not be treated as a standalone clock for the star.
The study did not detect C17O. Instead, it reported an approximately 99% lower limit: log10(12C16O/12C17O) ≥ 3.36, corresponding to 16O/17O > 2300 and 18O/17O > 4.3. These are limits rather than point estimates of the two ratios.
Why the model still matters
The detailed isotope results depend on the assumed atmospheric structure. C/O remained stable across the tested atmosphere parameterizations, but metallicity and both isotope ratios changed substantially when the pressure–temperature profile was altered. The carbon-to-oxygen result was therefore more stable to those tested choices than the isotope measurements.
An exploratory two-column model produced bolometric effective temperatures spanning about 2300 K to 2550 K while leaving C/O stable. For the tested wavelength range and signal-to-noise level, however, the high-resolution data did not distinguish that solution from the flexible one-column model. The test was prior-driven and exploratory.
The result is consequently best understood as a model-dependent estimate for TRAPPIST-1. The study’s retrieval sensitivity shows that changing the pressure–temperature profile can materially alter metallicity and isotope ratios even when the C/O ratio remains stable.
A result still under review
The study is an arXiv preprint, version arXiv:2608.26097v1, dated 26 August 2026. Its front matter labels it a draft version dated 27 August 2026.
The M-band raw data were reported as downloadable from the Keck Observatory Archive, and the final spectrum was provided in machine-readable Table 3.
Paper data and sources
Original title: The Isotopic and Elemental Abundances of Planet-Host Star TRAPPIST-1
Authors: Darío González Picos, Ian J. M. Crossfield, David Coria et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: 10.3847/1538-4357/ae9bbc
Original paper · Full text