The Kelvin-Helmholtz mechanism is an astronomical event that occurs when the surface of a star or a planet cools. As a result of this cooling, the pressure drops, and the star or planet compresses to compensate. This compression, in turn, heats up the core of the star/planet. This mechanism is evident on Jupiter and Saturn. It is estimated that Jupiter and Saturn each radiate more energy through this mechanism than each receives from the Sun.
The mechanism was originally proposed by Kelvin and Helmholtz in the late 1800s to explain the source of energy of the sun. It was recognized by Sir Arthur Eddington among others that the amount of energy generated by the Kelvin-Helmholtz mechanism is far too low to power the sun, and the true source of the Sun's energy remained uncertain until it was shown to be nuclear fusion by Hans Bethe.