Final answer:
The excess heat of Jupiter and Saturn originates from retained primordial heat and ongoing internal processes, such as contraction and the separation of helium from hydrogen. Despite internal heat generation, their lack of nuclear fusion keeps them classified as planets rather than stars.
Step-by-step explanation:
The excess heat produced by Jupiter and Saturn is the result of several factors. Jupiter, being the largest planet, retained much of its primordial heat, which continues to contribute to its internal heat source. Furthermore, the contraction of its massive gaseous body adds additional heat, with internal energy equivalent to 4 million billion 100-watt lightbulbs. Saturn, although with less primordial heat, generates heat through the separation of helium from hydrogen in its interior, with the heavier helium forming droplets that release gravitational energy as they sink towards the core. Saturn's internal heat production is about half that of Jupiter but is significant given its smaller mass.
Despite their internal heat generation, Jupiter and Saturn are not considered stars because they lack the mass necessary for sustained nuclear fusion, the fundamental process powering stars. They are classified as gas giants, with the internal heat sources contributing to their atmospheric dynamics and high internal temperatures.