[astro-ph.EP] The JWST is revolutionizing our understanding of the temperate sub-Neptune population through atmospheric spectroscopy.
The nature of these planets remains debated, as their bulk properties are compatible with a range of interior scenarios, including mini-Neptunes, hycean worlds, and gas dwarfs, with different predicted atmospheric compositions. While theoretical studies have predicted compositional diagnostics for shallow- versus deep-atmosphere scenarios, there is a critical need for empirical constraints for a temperate planet that is a priori known to possess a deep H2-rich atmosphere.
The temperate exo-Neptune TOI-1231 b provides one such benchmark target. In this work, we present the JWST near-infrared (0.65–5.2 μm) transmission spectrum of TOI-1231 b, observed with NIRISS single-object slitless spectroscopy and NIRSpec G395H, representing the first for a temperate exo-Neptune. The density of TOI-1231 b requires a thick H2-rich atmosphere, making the planet a keystone reference case for testing mini-Neptune scenarios for sub-Neptunes.
We report a strong detection of CH4 (lnB=54.5-69.6) and moderate to strong evidence for CO2 (lnB=2.9-6.6). We do not find significant evidence for any other prominent molecule, although we find high 95% upper limits on the mixing ratios of NH3 and CO, both of which are expected in deep H2-rich atmospheres. We also do not find any significant evidence for sulfur-bearing species that have been inferred for some temperate sub-Neptunes.
This composition is consistent with expectations for a temperate Neptune possessing a deep H2-rich atmosphere with no distinct surface. We discuss the implications of our results for the characterization of temperate sub-Neptunes.
Subhajit Sarkar, Nikku Madhusudhan, Lorenzo Pica-Ciamarra, Måns Holmberg, Frances E. Rigby, Julianne I. Moses, Megan Mealing
Comments: Accepted for publication in ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2607.00973 [astro-ph.EP] (or arXiv:2607.00973v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2607.00973
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Submission history
From: Madhusudhan Nikku
[v1] Wed, 1 Jul 2026 14:09:52 UTC (8,791 KB)
https://arxiv.org/abs/2607.00973
Astrobiology, exoplanet,
