M–R diagram of small planets around M-dwarfs, K-dwarfs, and FG-dwarfs from the PlanetS catalog. The planets are color-coded by their equilibrium temperature calculate within the catalog with a bond albedo of 0 and a total heat redistribution. The composition lines of pure-silicates (brown) from Zeng et al. (2016), Earth-like planets (yellow), and 50% water (blue) from Zeng et al. (2019) are displayed. The vertical dotted lines correspond to the minimum mass of the sub-Neptunes across spectral types (at 1.9, 3.4, and 4.3 M_⊕ for M-, K-, and FG-dwarfs) discussed in Sect. 4.1 and 4.4.