Bruce Murray Space Image LibraryMass-radius diagram for a wide range of planetary sizes Mass-radius diagram for a wide range of planetary sizes The lines show the relationship between mass and radius for idealized planets. Some of them are for a homogeneous planet with the same composition from surface to core. From top to bottom the homogeneous planets are: hydrogen (cyan solid line); a hydrogen-helium mixture with 25% helium by mass (cyan dotted line); water/water ice (blue solid line); silicate (MgSiO3 or perovskite; red solid line); and iron (Fe; green solid line). The rest of the lines (all dashed or dotted) would be differentiated planets, with a layered structure. The red dashed line is for silicate planets with 32.5% by mass iron cores and 67.5% silicate mantles (similar to Earth). 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. The blue triangles are Solar System planets: from left to right Mars, Venus, Earth, Uranus, Neptune, Saturn, and Jupiter. The magenta squares denote the transiting exoplanets known as of 2007. Note at high masses, electron degeneracy pressure becomes important, so that with increasing mass the planet radius does not change. At highest masses, the radius actually shrinks with increasing mass. Seager et al. (s007). Caption modified from Koh Xuan Yang.