Composition gradients in sub-Neptunes: K2-18 b and TOI-270 d as case studies

Structure models of sub-Neptunes commonly assume purely adiabatic interiors with distinct layers of homogeneous composition. We assess how allowing for more complex interiors with composition gradients affects the inferred internal structure and bulk compositions of the sub-Neptunes K2-18 b and TOI-270 d. We compare purely adiabatic models with distinct layers against models that include composition gradients and stable non-convective regions. We present solutions that are consistent with the observed masses, radii, and atmospheric boundary conditions. We find that composition gradients significantly increase the range of viable interior structures: For example, the interior degeneracy remains substantial even for fixed mass and radius and the maximum hydrogen-helium (H-He) mass fraction can increase by up to a factor of five for K2-18 b. We further show that correlations, such as those between the H-He abundance and the ice-to-rock or rock-to-iron ratios, can weaken when composition gradients are introduced. For example, the Spearman rank correlation coefficient between the H-He and iron abundances decreases from about0.7 to about0.4 for TOI-270 d in our models. Purely adiabatic models underestimate the range of plausible compositions and overestimate the impact of more precise measurements, atmosphere models, and host star constraints on a more accurate characterization. We suggest that interior models of sub-Neptunes should by default include more complex interiors, such as composition gradients and non-convective regions, when using data for interpreting the planetary structure, formation, and evolution.