Left panel: mass-radius diagram of small (R_p ≤ 4 R_⊕) planets with mass and radius determinations better than 4σ and 10σ, respectively, color-coded by planet equilibrium temperatures. The different solid curves, from bottom to top, correspond to planet compositions of 100% iron, 33% iron core and 67% silicate mantle (Earth-like composition), 100% silicates, 50% rocky interior and 50% water, 100% water, rocky interiors and 1% or 2% hydrogen-dominated atmospheres (Zeng & Sasselov 2013). The gray dark circles indicate Venus (V), the Earth (E), Uranus (U), and Neptune (N). Right panel: same as left panel for planets orbiting FGK dwarfs, with color-coded equilibrium temperatures T_eq ≤ 600 K. Kepler-10 b and c are indicated with squares.