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.

TOI-270 d and LP791-18 c are also both consistent with case 4, with robust detections of CH4 ranging between ∼1 – 10% but with intermediate mean molecular weight atmospheres between ∼5 – 8 g mol−1 (Benneke et al. 2024; Roy et al. 2025), which would result from temperatures that are too high for H2O to have condensed out.

For TOI-270 d, there may be a similar or greater abundance of H2O detected as CH4 (Benneke et al. 2024), however the detection significance is below 3 σ and the abundance constraints vary between different retrieval analyses (Holmberg & Madhusudhan 2024; Felix et al. 2025a).

For LP791-18 c there is no clear detection of H2O although its reported upper limit is high and not strongly constraining (Roy et al. 2025), so an abundance of H2O similar to that of CH4, according to case 4, remains consistent with existing data.

Therefore K2-18 b, TOI-732 c, TOI-270 d and LP791-18 c are all consistent with formation beyond the water ice line.

In contrast to the rest of the population, GJ9827 d has a high mean molecular weight atmosphere and H2O detected at a volume mixing ratio > 30% (Piaulet-Ghorayeb et al. 2024).

This cannot be explained by a formation beyond the water ice line as that would result in a high CH4 abundance (case 3) or an intermediate mean molecular weight (case 4), however it can naturally be explained by a formation within the ice line as case 1 or 2, followed by fractionated mass loss enriching the H2O/H2 ratio (Valatsou et al. 2026).

This interpretation is supported by GJ9827 d’s proximity to the edge of the radius valley, and the observations of high energy stellar flaring activity during the transit observations (Piaulet-Ghorayeb et al. 2025).

The case of GJ9827 d exemplifies how our results challenge the conventional assumption that a water-rich atmosphere indicates formation beyond the water ice line: a methane-free water-rich atmosphere is, counterintuitively, a signature of formation inside it.

For the sub-Neptunes that have featureless transmission spectra, it is more difficult to determine a classification because a featureless spectrum is degenerate between a high mean molecular weight atmosphere (case 3) and highaltitude aerosols.

Despite their featureless spectra, some constraints have still been placed on the atmospheric mean molecular weights.

For TOI-836 c a mean molecular weight lower than ∼6 g mol−1 could be ruled out based on the amplitude of the spectral features, however if the spectrum is instead being truncated by high altitude aerosols, the mean molecular weight could be as low as 4.25 g mol−1 (Wallack et al. 2024).

TOI-836 c is therefore unlikely to belong to cases 1 or 2, but can’t yet be distinguished between cases 3 or 4.

For GJ436 b, and TOI-776 b and c, observations of the He escape rate have been used to infer upper limits on their atmospheric mean molecular weights of < 10.2, < 13.6, and < 12.8 g mol−1 respectively (Rogers et al. 2026), tentatively ruling out case 3 for each planet, unless the atmospheric vertical mixing is low (Figure 4).

TOI-776 c additionally has a lower limit on mean molecular weight of > 6 – 8 g mol−1 if high altitude aerosols are not responsible for the featureless spectrum (Teske et al. 2025), which could suggest case 3 with a low vertical mixing strength when combined with the upper limit from He escape.

GJ3090 b has also been a subject of interest for breaking the cloud-mean Article number, page 7 molecular weight degeneracy (Ahrer et al.