Mass-radius diagram of the known transiting planets with radius R_p ≤ 12 R_⊕, mass M_p< 500 M_⊕, and precision on the mass better than 30%. Green diamonds indicate the Solar System giant planets Jupiter, Saturn, Neptune, Uranus, and the terrestrial planets Earth and Venus (from right to left). The three dotted lines indicate isodensity curves of 0.5, 1.5, and 5 g cm^-3 (from top to bottom), and the blue solid lines show the mass and radius of planets consisting of pure water, 100% rocks, and 100% iron (Seager et al. 2007). The positions of Kepler-101b and c are plotted with red squares.