Resolving core-envelope degeneracies in giant planets and sub-Neptunes: Constraining the equivalence in the presence of dilute gradients
Year: 2026
Type: article
Abstract: Context. The widespread recognition of dilute, ‘fuzzy’ cores in giant planets and massive volatile mantles in sub-Neptunes has transformed planetary interior modelling into a highly degenerate, multi-dimensional problem. Aims. To map the structural degeneracies introduced by these complex interiors, we present fuzzycore , an open-source static structural integrator with a unified layered architecture spanning rock-water-envelope sub-Neptunes through volatile-rich gas giants. The framework supports dense parameter sweeps over compositional gradients at fixed observed ( M p , R p , Z atm ), enabling consistent forwardmodel exploration of interior degeneracies across the radius valley and the warm-giant population. Methods. The framework solves the equations of hydrostatic equilibrium across phase-separated iron, rock, and water layers beneath a gaseous envelope. We employed a highly resolved adaptive grid to model arbitrary heavy-element gradients within the envelope, parameterising the dilution width to systematically evaluate the structural impact of varying core-envelope boundaries. Results. We benchmarked fuzzycore against an evolution-derived Jupiter profile, demonstrating that when envelope boundary metal-licities and integrated heavy-element budgets are matched, the macroscopic planetary radius is robust to the exact functional topology of the gradient to within 2.3%. This empirical robustness allowed us to use smooth parameterisations to densely map interior degeneracies. Applying this to the sub-Neptune regime, we generated a water-world degeneracy atlas, quantifying how the required envelope metallicity and dilution width trade off against assumed water-mass fractions to reproduce a fixed planetary radius. Conclusions. By decoupling structural profiling from time-dependent energy transport and convective mixing, fuzzycore enables the systematic mapping of static degeneracies across the sub-Neptune and warm-giant populations at resolutions that complement, rather than replace, fully evolutionary Henyey treatments. It serves as a forward-model backend for linking atmospheric metallicity priors from JWST and Ariel to deep interior architectures.
Source: Astronomy and Astrophysics
Authors: C. Wilkinson, B. Charnay, S. Mazevet, B. Perrier, A.-M. Lagrange, L. Delaye
Institutions: Centre National de la Recherche Scientifique, Université Paris Cité, Université Paris Sciences et Lettres, Observatoire de Paris, Sorbonne Université, CY Cergy Paris Université, LIRA - Laboratoire d'instrumentation et de recherche en astrophysique, Université Côte d'Azur, Observatoire de la Côte d’Azur
Language: English
Topic: Astro and Planetary Science
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7172007259