PALEOS: Multiphase equations of state and mass–radius relations for exoplanet interiors

Modeling the interior of a rocky or water-rich exoplanet is fundamentally a thermodynamic closure problem. Every layer's density, temperature gradient, and phase state must follow from an equation of state (EoS) that stays self-consistent across the many orders of magnitude in pressure and temperature spanned between the surface and the core, but existing EoSs are scattered across disciplines, employ different formalisms, and rarely provide the full set of thermodynamic quantities needed for such a model. This presents fundamental hurdles for evolutionary models that rely on a self-consistent thermodynamic treatment of interior phase transitions. We present (Planetary Assemblage Layers: Equations of State), an open-source and extensible Python toolkit that consolidates published EoSs for iron and magnesium silicate (MgSiO_3) with the composite EoS for water (H_2O), which we further correct at high pressure and temperature, into a unified phase-aware and thermally responsive framework covering 17 thermodynamic phases. Vapor and supercritical silicate phases lie outside the current scope of the toolkit. For each material, and , showing that each admits two purely rocky interior solutions indistinguishable in mass and radius but occupying radically different geophysical states: one fully solid and the other hosting a deep magma ocean above a liquid iron core. Phase-aware thermally resolved EoSs are thus essential for translating astronomical observations into exoplanetary geophysics. aqua analytically derives the density, specific internal energy, entropy, heat capacities, thermal expansion, and adiabatic gradient from the underlying pressure--volume--temperature relations via Maxwell relations. The resulting EoSs are validated for thermodynamic consistency and compiled into lookup tables on regular pressure--temperature grids, which are publicly released. We validate the framework against the preliminary reference Earth model, recovering Earth's radius to 0.3% and lower-mantle densities to 3%. Using we computed a grid of 17,900 mass--radius relations spanning 0.1--100,Mearth for rocky (Fe + MgSiO_3) and water-rich (Earth-like core plus H_2O envelope) compositions at surface temperatures from 300 to 4000,K. Because these EoSs treat solid and molten states continuously, thermal expansion remains active from fully solid interiors into the magma-ocean regime. For low-mass silicate planets, thermal expansion inflates the radius by more than 1% above 1500,K and by up to 16% at 4000,K, a change as large as both the radius spread from bulk composition (the iron--rock--water degeneracy) and the typical transit radius measurement uncertainty. We demonstrate the resulting degeneracy on two ultrashort-period super-Earths, WASP-47,e TOI-1807,b