Mass-radius diagram of all small exoplanets (R_p ≤ 2 R_⊕ and M_p ≤ 30 M_⊕) with a measured mass and radius to a precision better than 20% as listed in the NASA Exoplanet Archive. The colours of the planets indicate the insolation in units of log 10 (F_⊕). Earth and Venus are plotted in red filled circles for comparison. The solid lines are theoretical mass-radius curves (Zeng et al. 2016); from top to bottom: 100% H₂O (blue solid line), a mixture of 50% H₂O and 50% MgSiO₃ (cyan dashed line), 100% MgSiO₃ (green solid line), a mixture of 75% MgSiO₃ and 25% Fe (magenta dashed line), a mixture of 50% MgSiO₃ and 50% Fe (brown solid line), a mixture of 25% MgSiO₃ and 75% Fe(red dashed line), and 100% Fe (orange solid line).