The two largest planets in the solar system share many similarities, but Jupiter and Saturn radiate more energy than they receive for two different reasons. Jupiter radiates more energy than it receives because of its great size, while Saturn’s atmosphere causes it to radiate more heat than it gets.
When planets form, they are extremely hot and radiate a lot of energy. Over time, they cool and give off less heat. Because Jupiter is so large, it radiates more energy than it gets from the Sun. By contrast, Saturn is smaller, so it formed at a lower temperature than Jupiter and cooled more quickly due to its higher surface to volume ratio. This lower temperature results in a net gain in energy radiation. According to the University of Oregon, the helium in Saturn’s atmosphere began to rain as the temperature fell. As the liquid helium rubs against the hydrogen in the atmosphere, it causes friction, which ultimately raises the temperature of the planet and, therefore, the amount of energy it radiates.
Being gaseous planets, Jupiter and Saturn (as well as the other gas giants, Uranus and Neptune) exhibit much different behavior than rocky planets, such as the Earth or Mars. While they are covered in a blanket of gases, scientists believe that both Jupiter and Saturn possess rocky cores. Each of these cores is hypothesized to be five to 20 times the mass of the Earth.