The bulk metal content of WASP-80 b from joint interior-atmosphere retrievals

Breaking degeneracies and exploring biases with panchromatic spectra

Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

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Received: 21 July 2025
Accepted: 3 December 2025

Abstract

The atmospheres of warm gas giants can be readily characterized through transmission and emission spectroscopy. WASP-80 b is one such exoplanet, with an unusually low density that is in tension with the metal-rich composition expected for a planet of this mass. The aim of this work 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 model GASTLI to describe the planetary structure and thermal evolution and petitRADTRANS to describe the atmospheric chemistry and clouds. Our retrievals combined the mass and age with panchromatic spectra from JWST and HST in both transmission (0.5−4 μm) and emission (1–12 μm) as observational constraints. We identified two fiducial scenarios. In the first, WASP-80 b has an internal temperature consistent with its age in the absence of external heating sources; in addition, its atmosphere is in chemical equilibrium, with an atmospheric metallicity of M/H = 2.75_−0.56^+0.88× solar, a bulk metal mass fraction Z_planet = 0.12 ± 0.02, and a core mass M_core = 3.49_−1.59^+3.49 M_⊕. In the second scenario, WASP-80 b would be inflated by an additional heat source, possibly induced by magnetic fields, with an atmospheric metallicity of M/H = 10.00_−4.75^+8.20× solar, Z_planet = 0.28 ± 0 .11, and M_core = 31.8_−17.5^+21.3 M_⊕. 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. Taken together with flexible cloud treatment and an unweighted likelihood, this leads to larger uncertainties in bulk and atmospheric compositions than had previously been claimed.