The
Kelvin–Helmholtz mechanism is an
astronomical process that occurs when the surface of a
star or a
planet cools. The cooling causes the pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats up the core of the star/planet. This mechanism is evident on
Jupiter and
Saturn and on
brown dwarfs whose central temperatures are not high enough to undergo
nuclear fusion. It is estimated that Jupiter radiates more energy through this mechanism than it receives from the Sun, but Saturn might not. The latter process causes Jupiter to shrink at a rate of two centimetres each year.