Comparison of the synthetic and the observed M − R relationship. The synthetic population and all actual planets with known mass and radius (both in the solar system and around other stars) and a semimajor axis of at least 0.1 AU are shown. For extrasolar planets, error bars are given. The color code for the internal composition of the synthetic planets is the same as in Fig. 9. The synthetic population only contains planets with primordial H₂/He atmospheres, thus at low masses discoveries of planets lying below the synthetic population are expected. The two “forbidden” regions in the top left and bottom right are a consequence of the core accretion formation process (and the EOS). This leads to a characteristic, elongated “S” shape of the planetary M − R relationship.