Mass-Radius relationship for 55 Cnc e. We show four different rocky compositions: no iron, Earth-like (33% iron core, 67% silicate mantle with 0.1 of iron by mol), super-Mercury (63% iron core, 37% silicate mantle no iron), and a pure iron planet. We consider two types of volatiles compositions: 0.1−0.01% of H-He (pink), and 10−50% of water (blue) above an Earth-like nucleus. We show our data for 55 Cnc e (red cross with label (1)), that reported by Winn et al. (2011) (blue and label (2)), and the data for the known transiting hot super-Earths, Kepler-10 b (K-10b – data from Batalha et al. 2011), and CoRoT-7 b (C-7b data from Bruntt et al. 2010; Hatzes et al. 2011). Uranus and Neptune are shown for reference.