M–R relation of exoplanets from the PlanetS Catalog (https://dace.unige.ch/exoplanets/). Planets that are highly irradiated by their host star (F_* > 2 × 10⁸ erg s⁻¹ cm²) are shown in gray as their radii are inflated. Top: planets shown with the M–R relation fit from Chen & Kipping (2017) and the associated uncertainly in the M–R relation slope. The change in the M–R relation is found to occur at a mass of ∼120 M_⊕. Bottom: planets shown with composition curves of H–He in a proto-solar ratio (black) and water (blue) assuming an effective temperature of 1500 K. It is clear that planets around or below Saturn’s mass are much more compact than planets with a composition that is dominated by H–He.