From Figure 1 of Szabó and Kiss. Panel A shows the density of each planet as a function of mass. The underlying dashed lines represent models from Fortney et al. 2007 for a range of planetary core masses. In the panel B, the authors have separated the planets into two mass-density subclasses. The lower two panels display the two subclasses of planets in radius-period space (panel C) and mass-period space (panel D).