Mass-radius curves for exoplanets. The horizontal lines show planets of a fixed radius. An upper mass limit comes from the pure iron planet case. A lower mass limit comes from theoretical arguments, namely the inability of a small and low-mass planet to retain H and He. Further arguments about water loss may be used to argue that a planet cannot have a significant interior water fraction. The solid lines are homogeneous planets. From top to bottom the homogeneous planets are made of water ice (blue solid line); silicate (MgSiO3 perovskite; red solid line); and iron (Fe ϵ; green solid line). The non-solid lines are differentiated planets. The red dashed line is for silicate planets with 32.5% by mass iron cores and 67.5% silicate mantles (similar to Earth) and the red dotted line is for silicate planets with 70% by mass iron core and 30% silicate mantles (similar to Mercury). The blue dashed line is for water planets with 75% water ice, a 22% silicate shell and a 3% iron core; the blue dot-dashed line is for water planets with 45% water ice, a 48.5% silicate shell and a 6.5% iron core (similar to Ganymede); the blue dotted line is for water planets with 25% water ice, a 52.5% silicate shell and a 22.5% iron core. Curves taken from Seager et al. (2007).