We adopt the definitions of planet bulk core composition cases as defined in Li et al. (2026), chemically equilibrate them globally, and investigate how the observable regions of their atmospheres differ between the different bulk compositions. The methodologies for chemical equilibration and atmospheric structure are outlined in Sections 2.1 and 2.2, respectively. The bulk composition cases that we consider are illustrated in Figure 1 and include planetary cores of the following compositions: (1) a purely rocky core, (2) a sootArticle number, page 2
rock core (24% soot, 76% rock), (3) a soot-water-rock core (20% soot, 25% water, 55% rock), and (4) a water-rock core (40% water, 60% rock). The four cases represent endmembers of planetary cores that were assembled within the soot line (case 1), between soot line and water ice line (case 2), and beyond the water ice line (case 3 and 4). Case 4 ignores any soot and can be a proxy for a scenario where interstellar refractory carbon may have been destroyed during heating processes in the protoplanetary disk (Anderson et al. 2017; Bergin et al. 2015). Case 4 also represents a reference case to the planet compositions that have been considered in previous formation models outside the water ice line (Venturini et al. 2020; Burn et al. 2024; Chakrabarty & Mulders 2024), especially those that considered chemical equilibration (Werlen et al. 2025a). For the rock composition, we follow Young et al. (2023) and use chondritic abundances for C, O, Na, Si, and fix the molar ratios of Mg/Si and Fe/Si to unity (see Grimm et al. 2026, for variable refractory element ratios). For soot material, we adapt the composition of C100H75−79O11−17N3−4S1−3 from Bergin et al. (2023) to a fixed composition of C100H77O14 for simplicity. Nitrogen and sulfur are not included in the chemical network used here, although their incorporation into global chemicalequilibrium models has been explored in Werlen et al. (2026a). A variable soot composition would be more realistic, however our results are only marginally sensitive to the exact soot composition. In addition to the planetary core, planets accrete primordial gas early during their formation before the dissipation of the protoplanetary disk. We assume the primordial gas to be composed of 99.9% H2 and 0.1% CO2 by mole, corresponding to a solar C/O ratio of 0.5 (by moles) (Suárez-Andrés et al. 2018). As the amount of accreted gas can vary mainly depending on planet mass and the temperature environment. We treat the accreted amount of gas as a free parameter and vary the accreted mass fraction from 1-9%. We show that our results (Section 3) are only weakly sensitive in this range. We use planets of 6 and 10 M⊕. For all planets, we assume the temperature at the atmosphere–magma ocean interface (TAMOI) to be equal to 3000 K and the temperature of the silicate-metal equilibration to be 500 K higher. As those temperatures are generally unconstrained, our results of atmospheric mass fraction and composition will, in part, depend on our choice of temperatures. We therefore test all cases also with a temperature of TAMOI = 2000 K and find that although absolute values may vary, the main compositional trends across the investigated core composition cases remain largely unchanged. We highlight differences in the main text whenever relevant.
2.1. Chemical Thermodynamics We use the open-source, CPU-based global chemical equilibrium (GCE) model described in (Grimm et al. 2026), which has highly improved computational efficiency (∼300x speedup) compared to the original framework developed by Schlichting & Young (2022). It also includes a key addition of carbon partitioning in the metal phase (Werlen et al. 2025b). The network adopted in this study includes 19 chemical and linearly independent reactions and involves 26 phase components distributed across three coexisting phases: metal, silicate, and gas (see Appendix A).