Normalized densities ρ/ρ_Earth–like of the 57 exoplanets as a function of their iron-mass fractions Fe_planet%. The normalized density, ρ/ρ_Earth–like, of a planet is directly calculated from its mass and radius and ρ_Earth–like is the density of an Earth-like composition (core mass fractions of 32.3% and mantle molar ratios of Fe/Mg=0.12 and Si/Mg=0.81). The data of Mercury is marked as black star for reference. The exoplanets with both Fe_planet% > Fe_Mercury% and ρ/ρ_Earth–like> (ρ/ρ_Earth–like)Mercury are circled as super-Mercuries while the others are super-Earths. The distributions of Fe_planet% and ρ/ρ_Earth–like are showed in the top and right panels, respectively.