Jupiter radiates roughly twice as much energy as it gets from the Sun. It fuels its planet-swallowing storms from the inside out, driven by a 43,000°F core and friction from "helium rain."
This energy comes from three primary sources, none of which involve nuclear fusion, the process that powers stars.
First, Jupiter is still cooling down from its birth. When the solar system formed 4.5 billion years ago, gravity pulled a colossal amount of gas and dust together to build the planet. As this material collapsed inward, its gravitational potential energy was converted into kinetic energy, and then into thermal energy as the particles collided. Because Jupiter is so massive and well-insulated by its thick atmosphere, it is still radiating this primordial heat away today.
Second, the planet is still shrinking. As Jupiter slowly loses its original heat, the gas cools and compresses. This ongoing gravitational contraction—known as the Kelvin-Helmholtz mechanism—causes the planet to shrink by roughly two centimeters per year. As the outer layers fall very slightly inward, that lost potential energy is transformed into additional heat.
Finally, deep within Jupiter’s interior, the planet generates energy through helium rain. At extreme depths, pressures are so high that hydrogen is squeezed into a liquid metal. The surrounding helium condenses into liquid droplets that are heavier than the hydrogen. These droplets rain down toward the core. The friction of countless helium droplets falling through the metallic hydrogen ocean generates a massive amount of thermal energy.