Explanation:
The internal heat sources for Jupiter and Saturn derive from primordial heat resulting from the initial gravitational contraction of each planet. Jupiter also generates heat by slow contraction, which liberates substantial gravitational energy. A significant part of Saturn’s heat comes from the release of gravitational energy from helium separating from the lighter hydrogen and sinking to its core. What one considers to be a star is a matter of definition, as we discuss in more detail in the chapter on The Birth of Stars and the Discovery of Planets outside the Solar System. While both Jupiter and Saturn generate much of their energy internally, they are not large enough (by a significant factor) to support nuclear reactions in their interiors, and so are not considered to be stars.
