The bulk metal content of WASP-80 b from joint interior-atmosphere retrievals: Breaking degeneracies and exploring biases with panchromatic spectra

Abstract: WASP-80 b is an unusually low-density exoplanet in tension with the metal-rich composition expected for a planet of its mass. We aim to derive precise constraints on WASP-80 b's bulk metal mass fraction, atmospheric composition, and thermal structure. We conducted a suite of retrievals using three approaches: traditional interior-only, atmosphere-only, and joint interior-atmosphere retrievals. We coupled the open-source models GASTLI and petitRADTRANS, which describe planetary structure and thermal evolution, and atmospheric chemistry and clouds, respectively. Our retrievals combine mass and age with panchromatic spectra from JWST and HST in both transmission (0.5-4 $μ$m) and emission (1-12 $μ$m) as observational constraints. We identify two fiducial scenarios. In the first, WASP-80 b has an internal temperature consistent with its age in the absence of external heating sources, and its atmosphere is in chemical equilibrium, with an atmospheric metallicity M/H = 2.75$^{+0.88}_{-0.56}$x solar, a bulk metal mass fraction $Z_{planet}=0.12\pm0.02$, and a core mass $M_{core}=3.49^{+3.49}_{-1.59} \ M_{\oplus}$. In the second scenario, WASP-80 b may be inflated by an additional heat source - possibly induced by magnetic fields - with an atmospheric metallicity M/H = 10.00$^{+8.20}_{-4.75}$x solar, $Z_{planet}=0.28\pm0.11$, and $M_{core}=31.8^{+21.3}_{-17.5} \ M_{\oplus}$. The super-solar M/H and sub-solar C/O ratios in both scenarios suggest late pebble or planetesimal accretion, while additional heating is required to reconcile the data with the more massive core predicted by the core accretion paradigm. In general, joint retrievals are inherently affected by a degeneracy between atmospheric chemistry and internal structure. Together with flexible cloud treatment and an unweighted likelihood, this leads to larger uncertainties in bulk and atmospheric compositions than previously claimed.