Exoplanet interior
= Exoplanet interior
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>.<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]{ref} 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.