Layered convection as the origin of Saturn's luminosity anomaly

As the giant planets of our Solar System continue to cool and contract, they radiate more energy than they receive from the Sun. A giant planet's cooling rate, luminosity and temperature at a given age can be determined using the first and second principles of thermodynamics. Measurements of Saturn's infrared luminosity, however, reveal that Saturn is significantly brighter than predicted for its age. This excess luminosity has been attributed to the immiscibility of helium in Saturn's hydrogen-rich envelope, which leads to rains of helium-rich droplets. Existing calculations of Saturn's evolution, however, suggest that the energy released by helium rains might be insufficient to resolve the luminosity puzzle. Here we demonstrate, using semi-analytical models of planetary thermal evolution, that the cooling of Saturn's interior is significantly slower in the presence of layered convection generated--like in Earth's oceans--by a compositional gradient. We find that layered convection can explain Saturn's present luminosity for a wide range of initial energy configurations without invoking any additional energy source. Our findings suggest that the interior structure, composition and thermal evolution of giant planets in our Solar System and beyond may be more complex than the conventional approximation of giant planets as homogeneous adiabatic bodies.