Describe the different processes that lead to substantial internal heat sources for Jupiter and Saturn. Since these two objects generate much of their energy internally, should they be called stars instead of planets?

Jupiter and Saturn, the gas giants of our solar system, generate substantial internal heat through two primary processes: gravitational contraction and radioactive decay.

1. Gravitational Contraction: This process, also known as Kelvin-Helmholtz mechanism, involves the gravitational energy of the planet being converted into thermal energy. As the planet contracts under its own gravity, the potential energy of the gas decreases and its kinetic energy increases, which raises the temperature of the gas. This is a significant source of heat for both Jupiter and Saturn.

2. Radioactive Decay: The decay of radioactive elements within the planet also generates heat. As these elements break down, they release energy in the form of heat. This is a more minor source of heat compared to gravitational contraction, but still contributes to the overall internal heat of these planets.

Despite these internal heat sources, Jupiter and Saturn should not be classified as stars. The key difference between stars and planets is the process of nuclear fusion. Stars generate their energy through the fusion of hydrogen atoms into helium in their cores, a process that requires extremely high temperatures and pressures that are not present in Jupiter or Saturn. While these planets are large and generate internal heat, they do not have the necessary conditions for hydrogen fusion, and thus are not stars.