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Why is Neptune slightly warmer than Uranus despite being farther 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 is warmer than would be expected based on its distance from the Sun because it radiates significantly more energy from its interior than it receives from solar radiation. While Neptune is more than 50 percent farther from the Sun than Uranus and receives less than half the sunlight, the two planets have nearly equal effective temperatures.1

The primary reason for this difference is that Neptune emits more than twice the energy it receives from the Sun, a surplus generated by its internal heat source. In contrast, Uranus radiates very little excess internal heat; its internal energy source supplies at most 14 percent more energy than it receives from the Sun.12

Scientists have proposed that multiple stratified layers in the deep atmosphere of Uranus may act as insulation, trapping internal heat that more readily escapes from Neptune. Additionally, differences in the planets' internal structures—potentially resulting from unique events during their formation, such as collisions with large bodies—may explain why Uranus exhibits such an anomalous, low internal heat output compared to the other giant planets.32