the generally unconstrained strength of vertical mixing is a
degeneracy between cases 3 and 4. At low mixing strength,
spite this caveat, the four considered cases occupy suffi-
ciently distinct regions of chemical space that their bulk
and the MMW, remain meaningful diagnostics of forma-
tion environment. We now test how well this classification

Article number, page 6 describes the population of sub-Neptunes recently characterized by JWST.

In the following, we compare the gas compositions predicted at the AMOI directly with observed sub-Neptune atmospheres, setting aside the additional complexity introduced by atmospheric structure and vertical mixing explored in the present Section.
3.2.

Comparison to characterized sub-Neptune atmospheres Atmospheric characterization of the sub-Neptune population with JWST has revealed a variety of transmission spectra that are either featureless, due to high altitude aerosols or high mean molecular weight atmospheres, or spectra where chemical absorption features can be robustly detected.

For sub-Neptunes with chemical features robustly detected from their transmission spectra (> 3 σ), the dominant atmospheric absorbers across JWST’s NIRISS and NIRSpec wavelength ranges have been either H2O or CH4, and the atmospheric mean molecular weights have been found to range between ∼2.3 g mol−1 (e.g., TOI-421 b Davenport et al. 2025) up to ∼18 g mol−1 (e.g., GJ9827 d Piaulet-Ghorayeb et al. 2024).

In the classification we present here, H2O and CH4 have also emerged as the dominant atmospheric gases across the four formation cases (in a H2 background for cases 1, 2, and 4), and the atmospheric mean molecular weights range between ∼2 – 20 g mol−1 (Figure 4).

It is therefore instructive to see how far our classification can accurately describe the observed population of sub-Neptune atmospheres based on retrieved abundances of H2O and CH4, and the atmospheric mean molecular weight, which we present in Figure 5.

For this population analysis we ignore the other carbon bearing species CO and CO2 as the detections of these molecules in sub-Neptune atmospheres, and their reported abundances, have been demonstrated to be model-dependent between different retrieval pipelines (e.g., Schmidt et al. 2025) preventing a trustworthy population-level inference.

In contrast, either CH4, or H2O, or both CH4 and H2O together, have been observed at > 3 σ in every sub-Neptune with any reported chemical detection to date, making these species suitable for population-level chemical analysis.

Across the entire population of sub-Neptunes with robust chemical detections from JWST so far, we find good agreement with our classification system within approximately one order of magnitude.

The sub-Neptunes with robust chemical detections, in order of decreasing equilibrium temperature, are: TOI-421 b (Davenport et al. 2025), GJ9827 d (Piaulet-Ghorayeb et al. 2024), GJ3470 b (Beatty et al. 2024), TOI-270 d (Benneke et al. 2024; Holmberg & Madhusudhan 2024; Felix et al. 2025a), TOI-732 c Rigby et al. (2025), LP791-18 c (Roy et al. 2025), and K2-18 b (Madhusudhan et al. 2023; Hu et al. 2025).

Of this population, TOI-421 b and GJ3470 d have low mean molecular weight atmospheres and robust detections of H2O.

TOI-421 b has a non-detection of CH4 and GJ3470 d has a detection of CH4 three orders of magnitude less than H2O (Davenport et al. 2025; Beatty et al. 2024).

H2O in a H2 background atmosphere with very low CH4 indicates either case 1 or case 2, therefore TOI-421 b and GJ3470 b can be explained by a formation scenario inside the water ice line.

K2-18 b and TOI-732 c also have relatively low mean molecular weight atmospheres, however there are robust detections of CH4 between ∼1 – 10% volume mixing ratio, and non-detections of H2O (Madhusudhan et al. 2023; Hu et al.
2025).

This is consistent with case 4 under the additional consideration that H2O can condense out of the observable region of the atmosphere, leading to its non-detection and lowering the mean molecular weight.