Radius, R, as a function of semimajor axis, a, of planets in the planetary systems where high-density exoplanets have been observed. The data points are color coded by their masses, M (in units of M_⊕). We plot only planets for which the radius and masses are known with sufficient precision to enable compositional estimates, i.e., σ_R/R <0.3 and σ_M/M <0.3. We distinguish between terrestrial planets (symbols with black edges) and volatile-rich planets (symbols with red edges) based on whether their R is greater or smaller than 2 R_⊕, respectively (see Section 2.1.2). Most of the known planets in the systems with high-density exoplanets are predominantly made of rock and iron, with no large volatile envelopes.