Mass-radius relationships of water planets in comparison with Uranus (U), Neptune (N), the three known Neptune-mass planets, and the two known super-Earth mass planets GJ1214b and CoRoT-7b. Solid line: T₁ = 1000 K, isothermal atmosphere, M_core = 0.25   M_⊕; dashed line: same as solid line but T₁ = 2000 K; dot-dashed line: same as solid line but adiabatic atmosphere; thin solid line: fit-formula for water planets from Fortney et al. (2007b). The thin dashed-dot line is the theoretical M − R relation for Mg-silicate planets from Valencia et al. (2010).