Deep radiative zones affect the planetary cooling and internal structure: implications for exoplanet characterisation

TL;DR

This study explores how radiative opacity variations, especially due to alkali metal depletion, influence the thermal evolution, radius, and interior structure of warm giant exoplanets, affecting their characterization.

Contribution

It systematically investigates the impact of opacity windows and deep radiative zones on exoplanet evolution and characterization, highlighting their significance in planetary modeling.

Findings

Deep radiative zones develop in older, moderately irradiated Jupiters.

Opacity windows accelerate cooling, reducing planetary radii by up to 5%.

Inferred bulk metallicity can differ by about 10 percentage points due to opacity effects.

Abstract

The thermal evolution and interior structure of giant exoplanets are sensitive to the treatment of radiative opacity. At temperatures of ~2000 K, depletion of alkali metals can create a window of reduced opacity, potentially giving rise to deep radiative zones. While such zones have been discussed for Jupiter, their role in the evolution and characterisation of warm giant exoplanets has not been systematically investigated. We investigate how opacity windows and the resulting deep radiative zones affect the cooling, radius evolution, and the characterisation of interiors and atmospheres of giant exoplanets. We computed thermal evolution models for warm Jupiters spanning masses of 0.3 to 1.0 Jupiter masses with equilibrium temperatures of 200 to 800 K, with a parametrised reduction of the radiative opacity near ~2000 K. Deep radiative zones develop in moderately irradiated Jupiters older than ~4 Gyr even with unmodified opacities, and earlier and more extensively when the opacity is reduced. A deep opacity window accelerates the planetary cooling, reducing predicted radii by up to 5% and interior temperatures on the order of a few 10%. We show that this translates to a ~10 percentage point difference in 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. We suggest that the opacity treatment introduces significant uncertainties in atmospheric and interior characterisation.