Mean molecular weight (μ) of mini- and super-Neptunes as a function of equilibrium temperature. The μ values are taken from the literature and shown in Table D1. We show TOI-1130 b in this figure. We calculate the mean molecular weight from the posterior distribution of the equilibrium chemistry retrieval (Section 4.3). The blue vertical line marks the critical temperature for the condensation of water (350 K) as proposed in N. Madhusudhan et al. (2025). The schematic on the right demonstrates this idea. Above the critical temperature, the water in the planet is vaporized and mixed with the H/He envelope to form either H₂–H₂O mixed envelopes (miscible envelope mini-Neptunes) or steam-dominated envelopes (steam worlds). The intermediate μ value of TOI-1130 b likely aligns it with the miscible envelope mini-Neptunes.