Envelope mass as a function of the mass of solids contained in the planet (M_Z). The mass of the core is fixed at 6 M_⊕. An accretion rate of solids of 3 × 10^-6M_⊕/yr was assumed, which corresponds to a luminosity of 10²⁷ erg/s (the same used in all the other results). The decrease of M_env for low metallicities (shown in the inset) is due to the increase in the gas opacity and to the fact that for these cases of low Z, a radiative zone develops in the outer layers. For Z> 0.1, the structures are almost fully convective. Therefore, the increase in κ_gas does not play a role, the dominant effect is the increase in M_env due to the inclusion of Z in the EOS.