Posteriors for the main internal structure parameters of HIP 41378 b, namely the mass fractions of the planet’s inner core (far left), mantle (middle left), and envelope layers (middle right), as well as the mass fraction of water in the envelope layer (far right). We show models assuming the planet’s Si/Mg/Fe ratios match those of the host star exactly (purple), are Fe-enriched compared to the host star (pink), and are independent of the host star metallicity (blue). For all three options, we also use two water priors, favoring a water-rich (top row) and water-poor composition (bottom row), respectively. The vertical dashed lines show the medians of the inferred distributions and the dotted lines the chosen priors.