Jupiter, Saturn, and Neptune radiate more energy into space than they receive from the Sun. What is the source of the additional energy?

Gravitational contraction is a fascinating process. Imagine a planet slowly squeezing itself over millions of years. This compression generates heat, similar to how squeezing a rubber ball warms it up. This process is known as the Kelvin-Helmholtz mechanism. When a planet like Jupiter contracts, its gravitational potential energy gets converted into thermal energy. This heat then travels from the planet's core to its surface, eventually radiating into space. This slow contraction means that even without energy from the Sun, the planet can still emit heat due to its own internal changes. It’s a significant reason why giant planets like Jupiter, Saturn, and Neptune radiate more energy than they receive from the Sun.

Another internal process contributing to the extra energy radiated by Jupiter, Saturn, and Neptune is radioactive decay. Within the core of these planets, there are elements that are unstable. These elements constantly transform into more stable ones, a process that releases energy. In simpler terms, think of it as tiny fireworks going off inside the planet, generating heat. This heat from radioactive decay adds to the planet's total energy output. Therefore, between the combined effects of gravitational contraction and radioactive decay, these planets can glow with more energy than they get from the Sun.

Jupiter is the largest planet in our solar system and radiates a significant amount of energy. It emits about 1.6 times the energy it receives from the Sun. One reason is its ongoing gravitational contraction, which we discussed. Additionally, its large size means there’s more material undergoing radioactive decay at its core. These processes working together ensure that Jupiter glows brighter and is warmer than it would be if it only relied on sunlight. This emission is a key factor that astronomers use to understand how large gas giant planets generate and maintain their energy.

Saturn, another gas giant, similarly emits more energy than it receives from the Sun. It radiates about 1.8 times the energy that it absorbs from sunlight. The source of this additional energy is much like Jupiter's: gravitational contraction and radioactive decay. However, recent research suggests that there might be an additional factor. Saturn's internal structure may allow helium to precipitate out of its hydrogen-rich atmosphere. As helium droplets rain down, they release gravitational energy, adding to the planet's heat. So, while Saturn’s primary energy sources remain contraction and decay, this helium precipitation provides an intriguing extra source of energy.

Neptune is a bit different from Jupiter and Saturn but follows the same general principles. Neptune emits about 2.6 times the energy it receives from the Sun. It's this high energy output, despite its distance and limited sunlight, that points strongly toward significant internal heat sources. Gravitational contraction and radioactive decay are again prime contributors. Moreover, Neptune's internal heat might be due to a slower rate of cooling. Whatever the exact mix of mechanisms, it's clear that like its fellow gas giants, Neptune's additional energy primarily comes from its own complex internal processes.