Which planets emit more energy than they receive from the Sun? What are the possible reasons for this?

Jupiter, Saturn and Neptune emit significantly more energy into space than they receive from the Sun.
Jupiter emits 2.7 times more heat than it receives, due to slow compression and, presumably, due to thermonuclear reactions.
Saturn emits 2.5 times the energy it receives from the Sun. The excess energy exceeds that which can be obtained through compression and radioactive decay. Interestingly, Uranus, located between Saturn and Neptune, emits as much energy as it receives from the Sun.