Range of synthetic rocky planets that fit data (black dots) of a) M and R (case A), and b) M, R, and stellar abundance constraints (case B). Mass-radius curves for pure iron (red) and pure silicate spheres (blue, zero amount of FeO) were calculated independently. In case A, all synthetic planets that fit data (black dots) fall within the limits of a pure silicate (blue line) and pure iron sphere (red line). In case B, this region is reduced to a smaller area indicating that chemical bulk ratio (Fe/Si_◯) introduces strong additional constraints on the internal structure. Synthetic planets for which no internal structure could be derived within model assumptions (pure iron core surrounded by a rocky mantle) are indicated by gray open circles. For every circle in the plots an inverse problem has been solved. See main text for further details.