What is the primary heat source in the cores of giant planets?

The primary heat source in the cores of giant planets, such as Jupiter and Saturn, is believed to be the residual heat from their formation, which includes both gravitational contraction and differentiation. When these planets formed, the collapse of gas and dust under gravity created a lot of heat. As the planets continue to compact over time, they release this heat. Moreover, some of the heat is produced by the slow compression of the planet's interior, which is known as Kelvin-Helmholtz contraction.

Additionally, radioactive decay of elements can also contribute to the internal heat of a giant planet, but this is a much smaller effect compared to the heat from formation and contraction. In planets like Jupiter, helium rain (helium droplets falling through the hydrogen) could also release heat through friction and the process of differentiation (the separation of different elements and compounds within the planet's interior).