Deep radiative zones affect giant planet cooling and internal structure: Implications for exoplanet characterisation

Abstract

The radiative opacity plays a critical role in shaping the thermal evolution and interior structure of giant planets. Near $\sim$ 2,000 K, a hydrogen-transparency region creates a window of reduced opacity that can give rise to detached, deep radiative zones between two convective zones. This local opacity minimum could be deepened further by alkali depletion. While such zones have been explored for Jupiter and Saturn, their influence on cold to warm giant exoplanets remains unstudied. We investigate how opacity windows and the resulting deep radiative zones affect the cooling, radius evolution, and characterisation of giant exoplanet interiors and atmospheres. We computed thermal evolution models for cold to warm Jupiters spanning masses of 0.3 to 4.0 M$_{\rm{J}}$, envelope metallicities from one to ten times solar, equilibrium temperatures of 100 to 800 K, and a parametrised reduction in the radiative opacity. Detached deep radiative zones develop in moderately irradiated Jupiters older than a few gigayears even with unmodified opacities, and earlier and more extensively when the opacity is reduced. The age and equilibrium temperature at which they appear depend on planetary mass, envelope metallicity, and opacity, with metal enrichment suppressing them at low equilibrium temperatures but promoting them at higher ones. A deep opacity window accelerates cooling, reducing predicted radii by up to 5% and interior temperatures by several tens of percent, which translates to a $\sim$ 10 percentage point difference in the inferred bulk metallicity. Deep radiative zones are likely common in warm giant exoplanets and could decouple atmospheric composition from bulk interior composition, complicating the interpretation of atmospheric observations. The opacity treatment therefore introduces significant uncertainties in atmospheric and interior characterisation.