If you plunged into the darkness of Neptune's night side, the temperature wouldn't drop a single degree. The entire planet is locked in a uniform −328 °F (−200 °C) deep freeze.
This thermal consistency is driven by Neptune’s internal heat engine. At a distance of 2.8 billion miles from the Sun, solar radiation is extremely weak—about 900 times dimmer than on Earth. If Neptune relied solely on the Sun for warmth, it would be much colder than it actually is. Instead, the ice giant radiates 2.61 times more energy into space than it absorbs from sunlight. This vast reservoir of internal heat, likely left over from the planet’s formation and generated by the slow contraction of its core, warms the atmosphere from below, completely overpowering any meager day-night fluctuations caused by the distant Sun.
Neptune's extreme weather ensures this heat is distributed perfectly. Despite its 16-hour rotation rate, the planet boasts the fastest winds in the solar system, whipping through the upper atmosphere at up to 1,300 mph (2,100 km/h). These supersonic winds act like a colossal atmospheric blender. Combined with the sheer mass and thermal inertia of an ice giant's deep atmosphere, the weather systems circulate internal heat globally long before any localized cooling can occur on the night side.
While the day-night cycle does not alter Neptune’s temperature, there is one place on the planet that breaks the uniform freeze. Because Neptune is tilted on its axis, its south pole has been locked in continuous, albeit faint, summer sunlight for over 40 years. This prolonged exposure makes the southern pole about 18 °F (10 °C) warmer than the rest of the planet—just enough to allow frozen methane to thaw, rise into the stratosphere, and escape into space.