Layered convection as the origin of Saturn's luminosity anomaly

TL;DR

This paper proposes that layered convection within Saturn's interior, caused by compositional gradients, can explain its unexpectedly high luminosity without additional energy sources, challenging traditional models of giant planet interiors.

Contribution

It introduces planetary evolution models showing layered convection as a key factor in Saturn's luminosity, revising the standard homogeneous interior paradigm.

Findings

Layered convection reduces Saturn's cooling rate.

It can account for Saturn's observed luminosity without extra energy.

Implications for understanding exoplanet radii.

Abstract

As they keep cooling and contracting, Solar System giant planets radiate more energy than they receive from the Sun. Applying the first and second principles of thermodynamics, one can determine their cooling rate, luminosity, and temperature at a given age. Measurements of Saturn's infrared intrinsic luminosity, however, reveal that this planet is significantly brighter than predicted for its age. This excess luminosity is usually attributed to the immiscibility of helium in the hydrogen-rich envelope, leading to "rains" of helium-rich droplets. Existing evolution calculations, however, suggest that the energy released by this sedimentation process may not be sufficient to resolve the puzzle. Here, we demonstrate using planetary evolution models that the presence of layered convection in Saturn's interior, generated, like in some parts of Earth oceans, by the presence of a compositional gradient, significantly reduces its cooling. It can explain the planet's present luminosity for a wide range of configurations without invoking any additional source of energy. This suggests a revision of the conventional homogeneous adiabatic interior paradigm for giant planets, and questions our ability to assess their heavy element content. This reinforces the possibility for layered convection to help explaining the anomalously large observed radii of extrasolar giant planets.