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

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.

Figures (8)

• Figure 1: Radius evolution for different planetary masses (rows), equilibrium temperatures (columns), and opacity window factors (coloured lines; see legend). The radius difference between the various window factors increases with the equilibrium temperature and can reach a relative difference of up to about 5%.
• Figure 2: Kippenhahn diagram showing the evolution of convective (orange) and radiative zones (blue) with time for $M = 1.0 \,\rm{M}_{\rm{J}}$, $T_{\rm{eq}}$ = 400 K, and $w =$ 0, 0.50, and 0.90. All cases show that these planets develop two radiative zones: A first one at the planetary surface, and a second deep radiative zone at higher pressures whose thickness and time of appearance depends on the opacity.
• Figure 3: Top: Pressure-temperature profile at 10 Gyr for a planetary mass of 1 $\rm{M}_{\rm{J}}$, equilibrium temperature of 400 K, and four different opacity window factors $w = 0, 0.50, 0.70$ and $0.90$. The lines are coloured according to $w$, and convective regions are marked with thicker line widths. Bottom: The difference in the logarithmic temperature as a function of pressure. Deep opacity windows lead to significantly colder deep interiors, which relative differences of up to about 35%.
• Figure 4: Radius as a function of time for different values of $w$ and bulk metallicities (see legend). The radius and age of HATS-49 b are shown as grey error bars. To match the observations HATS-49 b can have either 40% ($w = 0$) or 30% ($w = 0.9$) of heavy elements by mass, depending on the depth of the opacity window.
• Figure 5: Planetary radius as a function of time for different bulk metallicities (coloured lines; see legend). The solid lines are calculated from evolution models. The dashed lines use the heuristic that each 1% change in radius can accommodate about a 2% change in bulk metallicity (see text for details).