Answer: Neptune

When you think of the planets far out on the edges of our Solar System, you probably think of them as icy cold, giant rocks frozen for all eternity barely within reach of the Sun’s warmth. While it’s true that distant planets are much colder than Earth—Neptune and Uranus have atmospheric temperatures hundreds of degrees lower than that of Earth—that doesn’t mean they are devoid of heat energy.

Neptune, despite its relative coldness, radiates 2.61 times more heat energy into space than it receives from the Sun (despite being the farthest planet from the Sun). Why Neptune is such a toasty little outer planet remains a mystery to astronomers, as it shares many similarities with Uranus, but is significantly warmer. The current explanation is that thermal energy left over from the creation of Neptune combined with changes (conversion) in internal core and mantle elements creates the difference in temperature seen between the two neighboring planets.