For a time, scientists were puzzled at how the larger planets in the solar system, namely Jupiter and Saturn, managed to give off more energy than they received from the sun. They didn't have nearly enough mass to spark nuclear fusion, but somehow they were radiating more heat and light than was expected. In the late 1800s, Lord Kelvin and Hermann von Helmholtz surmised that the gravitational contraction of a gas giant would create a source of energy that would radiate away from the planet.

As heat is released into the atmosphere of a gas giant, the gas pressure at the surface drops enough to allow gravity to gain the upper hand. The core of the giant then contracts, which releases energy in the form of heat and light. This energy radiates to the surface, where it is dispelled into the atmosphere, continuing the cycle. The sun itself could continue to shine for 100 million years even without nuclear fusion, solely because of the energy radiated due to the Kelvin-Helmholtz mechanism.

It is thought that the convection of heat deep within Jupiter's liquid layers due to the Kelvin-Helmholtz mechanism is the reason for the complex weather patterns observed on the surface. Oddly enough though, this still doesn't explain all of the energy emitted by Saturn. The planet's luminosity is still too high, even after taking into account the mechanism, meaning that Saturn is tapping some still unknown energy source.