Isothermal, isobaric projections of the Gibbs free-energy-of-mixing surface in the MgSiO₃–Fe–H₂ system at six different temperatures and pressures along a nominal T–P path through our fiducial sub-Neptune planet. Concentrations are in mole fractions. The dark gray region is the region of spontaneous exsolution into two phases and is defined by the spinodals (black curves). The blue curves are the binodals defining the coexisting compositions for two stable phases. Example pairs of coexisting compositions are shown by the tie lines between two binodes (blue points) on the binodals. The bulk composition of our reference sub-Neptune with 2 wt% H₂ is shown by the circled Neptune symbol. The bulk composition for Earth, including H in the metal core (E. D. Young et al. 2023), and the bulk composition of Neptune are also shown for reference.