How Internal Structure Shapes the Metallicity of Giant Exoplanets

Abstract

The composition and internal structure of gas giant exoplanets encode key information about their formation and evolution. We investigate how different assumed interior structures affect the inferred bulk metallicity and its correlation with planetary mass. For a sample of 44 giant exoplanets (0.12-5.98 MJ), we computed evolutionary models with CEPAM and retrieved their bulk metallicities under three structural hypotheses: core+envelope (CE), dilute core (DC), and fully mixed (FM). Across all structures, we recover a significant positive correlation between total heavy-element mass (MZ) and planetary mass (M), and a negative correlation between bulk metallicity (Z) and M (also for Z/Zstar vs M). Dilute core structures yield metallicities comparable to CE models, regardless of the assumed extent of the composition gradient. Increasing atmospheric metallicity augments the inferred bulk metallicity, as enhanced opacities slow planetary cooling. Non-adiabatic DC models can further increase the retrieved metallicity by up to 35 percent. We find that the mass-metallicity anti-correlation is primarily driven by low-mass, metal-rich planets (M < 0.2 MJ), and that massive planets (greater than about 1 MJ) can exhibit unexpectedly high metallicities (Z approximately 0.1-0.3). Improved constraints on convective mixing, combined with upcoming accurate measurements of planetary masses, radii, and atmospheric compositions from missions such as PLATO and Ariel, will provide further constraints on interior structure and formation models of gas giant planets.