Mass-radius diagram of sub-Neptunes with masses detected at the 3σ level (or better), including HD 5278 b (star). The objects are color-coded by their equilibrium temperature. The different curves depict internal structure models of a variable composition from Zeng & Sasselov (2013) and Zeng et al. (2019) (as reported in the legend): Earth-like (32.5% Fe + 67.5% MgSiO₃), pure rock (100% MgSiO₃), 50% Earth-like rocky core + 50% H₂O layer by mass, 100% H₂O mass, and Earth-like rocky core (99.7%) + 0.3% H₂ envelope by mass at a temperature of 1000 K. The location of Uranus and Neptune is also indicated. Data of transiting systems used for this and the following plots are taken from the TEPCat catalog as of September 2020. Along with TOI-130 b (T-130 for simplicity), Kepler-36 c and K2-292 b (K-36 and K2-292 for simplicity), which are discussed in the text, are also indicated in the plot.