An exoplanet interior is the structure and composition beneath the layers directly sampled by observations. Mass and radius constrain the mean mass density, but a physical model also needs hydrostatic equilibrium, an equation of state, and assumptions about composition and thermal history. Dense material can be concentrated in a core or mixed into surrounding layers.
Solid-planet calculations show how iron-, silicate- and water-rich bodies occupy different broad mass-radius relations, with compression preventing one constant-density scaling from applying to every mass.[ref]
Mass-radius relationships for solid exoplanets by Sara Seager and others. Abstract: cold solid-planet models, composition families and non-power-law mass-radius relation
A light gas envelope can substantially alter the observed radius. Exoplanet interior-composition degeneracy therefore remains even when both mass and radius are measured precisely. New to topics? Read the docs here!