What is the name of the mechanism that causes Jupiter to emit more heat than it receives from the Sun?

The mechanism that causes Jupiter to emit more heat than it receives from the Sun is the Kelvin–Helmholtz mechanism.

As Jupiter slowly cools, its immense gravity allows the planet to contract. That contraction releases gravitational potential energy, converting it into heat that is radiated from the atmosphere. Jupiter is not producing this excess energy through nuclear fusion; its interior is far too cool and diffuse for sustained hydrogen fusion.

The mechanism was proposed in the nineteenth century by William Thomson, later Lord Kelvin, and Hermann von Helmholtz. They originally used gravitational contraction to explain how the Sun might shine. That idea could not account for the Sun’s geological age, but it remains important for giant planets, brown dwarfs and contracting stars.

Jupiter’s internal heat helps power atmospheric activity and contributes to its effective temperature. The planet is estimated to contract by roughly 1 millimetre per year, producing an internal heat flux of about 7.5 watts per square metre. Radioactive decay and tidal heating are real heat sources elsewhere, but they are not the standard explanation for Jupiter’s excess emission.