[Swordmaker]

Jupiter has insufficient mass to ignite. It would have to be 85 times more massive to be a red dwarf. The smallest red dwarf is about 0.075 solar masses.

Jupiter is not even big enough to be a brown dwarf, which may have some short lived fusion reactions burning deuterium.

Both Jupiter and Saturn emit more energy than each of them receive from the Sun. Ergo, some source of energy is driving them beyond Fusion energy the gravity driven fusion.

[Fred Hayek]

The energy output is due to the gas giant contracting due to gravity. But there is no fusion taking place. Look up “brown dwarf”. It’s the same reason why brown dwarves (with insufficient mass to start the proton-proton reaction) have “surface” temperatures” in the neighborhood of 1200 K. Eventually the contraction will end due to equilibrium and the body will cool due to thermal radiation.