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Why does Neptune give off more energy than it receives from the Sun?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

Neptune emits more than twice the energy it receives from the Sun because it generates additional heat from within its interior. While the exact processes responsible for this phenomenon are not fully understood, this internal heat source distinguishes Neptune from Uranus, which has very little energy escaping from its interior.1

Although Neptune’s internal heat budget is significant, scientists note that this does not necessarily mean Neptune is hotter at its center than Uranus. Instead, it is possible that the deep atmosphere of Uranus contains multiple stratified layers that act as insulation, trapping radiation that would otherwise escape, whereas Neptune lacks such an insulating barrier. Consequently, the excess energy radiated by Neptune is a result of this internal heat escaping into space.21