HCR sample on the mass-density diagram. The dots, connected by black dashed lines, are planet pairs that belong to this sample. The coloured dots represent those planets, for which we calculated interior distance I. The red colours indicate a pair of planets at a short interior distance, which means they are probably similar in the interior structure, and blue colours indicate planets that show larger separation in the interior from their neighbour. Light-grey curves represent the MRR interior curves calculated by Zeng et al. (2016). From the baseline of a silicate planet with 100% MgSiO₃, the red points in the direction of increasing Fe content, and each curve represents a 10% increase in Fe content up to 100% Fe. Similarly, the blue arrow points in the direction of increasing water content at 10% increments per curve. Solar System planets are added for comparison and plotted as purple stars.