Potential interior structures of Uranus, Neptune, K2-18b and TOI-270 d if they have undergone hydrogen-water demixing. We note that alternatives models are also possible (see Sects. 3 and 4). For Uranus and Neptune, the sketches correspond to the cases with T_offset = 1100 K, in which both planets are fully demixed. However, we can’t exclude scenarios in which demixing is still ongoing or does not occur at all. The sketch for K2-18 b shows a mini-Neptune scenario with ongoing demixing, while the one for TOI-270 d corresponds to a case of full demixing.