tios, and atmospheric metal mass fractions of TOI-270 d as well as K2-18 b within the correct order of magnitude.
CO2 further serves as a useful discriminant between soot-bearing (case 3) and soot-free water-rich (case 4) formation: case 4 predicts log(CO2) between roughly -5 and -2, whereas case 3 tends to yield lower values. The tightly constrained CO2 abundance and sub-unity C/O ratios of TOI-270 d appear more consistent with case 4, as does the low mean molecular weight of K2-18 b. However, the latter conclusion remains speculative given current observational uncertainties and its dependence on the assumed magma ocean interface temperature. Soot-bearing interiors can further enhance methane abundances and produce methane-dominated atmospheres, as possibly inferred for TOI-836 c if aerosol explanations can be ruled out. Additionally, the water-rich case 4 may help explain the longstanding non-detection of CO in sub-Neptune atmospheres: although CH4, H2O, and CO are dominant at depth, CO abundances can decrease by up to eight orders of magnitude under weak vertical mixing, compared to at most five orders of magnitude for CO2, suggesting that retrieved compositions carry information on both primordial building blocks and mixing efficiency.
Together, these case-by-case comparisons motivate a more general diagnostic. We demonstrate how the H2O/CH4 versus MMW parameter space provides a powerful and physically motivated diagnostic linking atmospheric observables to planetary building blocks, with the four formation cases occupying distinct regions separable at the order-of-magnitude level. Outliers from the predicted trends are naturally explained by fractionated atmospheric escape, as inferred for GJ9827 d (Piaulet-Ghorayeb et al. 2024; Krishnamurthy et al. 2023; Valatsou et al. 2026), and water condensation, as inferred for K2-18 b (Hu et al. 2025). Interestingly, representative retrieved atmospheres exist for almost all modeled cases, suggesting that sub-Neptunes may form across a broader range of disk environments than theoretical arguments favouring formation beyond the water ice line would imply (Venturini et al. 2020).
Looking forward, the most decisive observational test is a precise mean molecular weight measurement for a planet currently classified as case 3 (with soot) or case 4 (without soot) candidate. Case 3 and 4 predict MMW >17 and <7.5 respectively, which is a factor of two difference that is in principle measurable from the amplitude of spectral features across JWST’s full wavelength range, or from atmospheric escape observations sensitive to He and H. On the modeling side, the key uncertainty is the magma ocean interface temperature TAMOI which controls the predicted MMW spread for case 4. Interior structure models that jointly fit mass, radius, and atmospheric composition would help narrow this parameter and sharpen the predicted observational boundaries between cases. For planets where CO2 is already detected, such as K2-18 b, improved retrieval precision on the CO2 abundance and C/O ratio, which is feasible with additional JWST transits, would help resolve whether a soot component is required or whether water-rich formation alone suffices. Finally, the H2O/CH4 vs MMW diagnostic framework presented here should be applied systematically to the growing JWST sub-Neptune sample as new characterizations become available. The diversity of sub-Neptune atmospheres is ultimately a record of their formation history, but it can only be decoded by explicitly Article number, page 10
accounting for the chemical equilibration between planetary interiors and their atmospheres.