Planetary normalised density of known rocky planets as a function of the estimated iron to silicate mass fraction of the protoplanetary disc. We also show the rocky Solar System planets represented by their symbols. For these, we also calculated the iron fraction from the abundances of the Sun. HD 23472 d, HD 23472 e, and HD 23472 f planets which are below the radius gap are shown with a pink circle. We note that HD 137496 b is behind HD 23472 e in the figure since their positions almost overlap. The black solid line shows the correlation for the known exoplanets, excluding the potential super-Mercuries (brown circles) identified in Adibekyan et al. (2021).