Laboratoire cosmique : ce qu'un sub-Saturne autour d'une naine rouge révèle sur la naissance des planètes ✛ ✦

Résumé original

tel que rédigé par les auteurs, en anglais arXiv:2606.23774

We report the JWST NIRSpec/PRISM transit spectrum of TOI-6894b, an exceptional 420 K sub-Saturn that is the only known giant planet transiting a late M dwarf. Remarkably, both the light curve and the transit spectrum exhibit almost no stellar contamination. The spectrum is dominated by prominent absorption features from CH$$_4$$ and the photochemical product CS$$_2$$. For the first time in a transit spectrum, NH$$_3$$ is visually evident, while subtler features from H$$_2$$O, and CO$$_2$$ can also be seen. We significantly improve upon state-of-the-art photochemical reaction networks, and use our new network to run radiative-convective photochemical models at different metallicities. These models show that the spectrum--in particular the size of the NH$$_3$$ and CO$$_2$$ features relative to the CH$$_4$$ and H$$_2$$O features--is most consistent with a metallicity of 3--10$$\times$$ solar. Using a semi-free retrieval framework that perturbs the self-consistent model's abundance and temperature profiles to fit the data, we find that the planet's C/O, N/O, and S/O ratios are broadly consistent with solar values. A grid retrieval on 1D radiative-convective photochemical equilibrium (RCPE) models reveals a similar result: $$[M/H]=0.46 \pm 0.08$$ and C/O=$$0.69 \pm 0.06$$. The planet's atmospheric metallicity, abundance ratios, and bulk metal fraction are all strikingly similar to that of Jupiter, Saturn, and other gas giant exoplanets, despite orbiting a very low-mass star.