Iron-mass fractions, Fe_planet%, of the 57 rocky exoplanets as a function of their equilibrium temperature, T_eq. The data points are divided by the spectral type of their host stars into two groups: planets around M stars (red squares) and around FGK stars (blue triangles). The Fe_planet% of the two groups show different correlations with equilibrium temperature. Green stars denote the results for the four terrestrial planets in our Solar System.