Effect of carbon on the thermo-chemical evolution of exo-ocean worlds

The elemental composition of exoplanets is intimately linked to that of their host stars. Understanding the internal structure and volatile content of small exoplanets is essential for constraining their formation environments and potential habitability. Recent studies on the composition of large icy moons suggest that carbon molecules (graphite and organics) represent an important fraction, on the order of 20 wt%, of their interior (Reynard and Sotin 2023). Using these bodies, in particular Titan, as analogs for exoplanets that would have accreted beyond the snow line, we investigate their thermo-chemical evolution. Some of these exoplanets could have migrated closer to their star and may exist in the present sample of detected exoplanets.

In this study, we investigate how stellar elemental abundances—particularly of C, O, Mg, Si, and Fe—influence the partitioning of silicates, carbon compounds, and ices within exoplanetary interiors. Using data from the Hypatia and APOGEE catalogs, we compile a sample of 23 confirmed small planets (R < 2 R🜨 and 0.1 < M < 10 M🜨) orbiting G-, K-, and M-type stars for which both stellar compositions and planetary parameters are available. We model their internal structures under two end-member scenarios: one assuming carbon as elemental graphite, and another in which carbon is stored as insoluble organic macromolecules (IOM). Ternary diagrams derived from these models reveal a strong compositional dependence on stellar metallicity: stars with low [Fe/H] tend to form volatile-rich planets with high water and organic content, while metal-rich stars favor carbon- and silicate-dominated interiors depleted in ices. A majority of stars align along a compositional trend defined by equal mass fractions of carbon-rich and silicate-bearing phases, which can be derived analytically from Mg/Si and C/Si ratios. This alignment is consistent across spectral types, suggesting that elemental ratios—not stellar classification—are the primary drivers of bulk planetary chemistry.

To contextualize these findings, we apply simple thermochemical models to carbon-rich silicate planets, assessing the fate of primordial organic matter and its role in shaping planetary outgassing. Our results suggest that the degradation of refractory organics leads to the formation of thick N₂,-CH₄ atmospheres, covering subsurface ocean. We further predict that carbon-rich planets that formed beyond the soot line and then migrated towards their star would have an atmospheric composition quite different from planets forming within the soot line. While the former retain methane- and nitrogen-rich envelopes, the latter are deprived of refractory carbon and more likely to outgas secondary atmospheres dominated by C0₂. Future observational campaigns with missions like JWST and ARIEL will be essential to test these predictions and to characterize the atmospheres of carbon- and ice-rich worlds beyond the Solar System.