A gas giant with a substantial H/He envelope. Intrinsic cooling, contraction and helium differentiation contribute to its thermal evolution.
Removing helium from a hydrogen-rich envelope lowers He/H and raises its reciprocal H/He. A helium-rain explanation of atmospheric depletion must keep those two ratio conventions distinct. Settling helium-rich material can release gravitational energy while changing atmospheric composition; excess luminosity alone does not uniquely determine the interior process.
