Schematic figure showing the layers, composition, and parameters of our interior structure model, GASTLI. The parameters set by the user are the planet mass, CMF, envelope composition, equilibrium temperature (at null albedo), and internal temperature. The envelope composition is either determined by log(Fe/H) in solar units or as an envelope metal mass fraction, Z_env. The outputs computed by the interior structure model include the total planet radius, R = R_interior + Ɀ_atm, and the interior profiles (see text).