Core: At the center of the planet is a molten rock core which is many times bigger and more massive than the entire Earth. It is 20,000 °C, about three times hotter than the Earth's core.

Internal Heat: Jupiter is a heat source; it radiates 1.6 times a much energy as it receives from the Sun. This energy is produced by Jupiter's shrinking due to gravity, and this produces heat. Also, it is still cooling down, losing its initial energy (the energy it received as the Solar System formed).

Does Jupiter produce energy by nuclear fusion -- NO. Jupiter, the biggest of the gas giants, is too small to produce a core temperature that is hot enough to undergo fusion (you need about 3 million degrees to start the fusion of hydrogen). You'd need a body that was many times the mass of Jupiter to get nuclear fusion (the theoretical limit is about 8 percent of the mass of the Sun).

Magnetic Field: Jupiter has a very strong magnetic field. The magnetic field is probably generated as the planet spins its deep metallic-hydrogen layer with electrical currents .

Jupiter's magnetic field (Jupiter's magnetosphere) extends for millions of miles into space. The tail of this magnetic field (which is extended by the solar wind), extends into the orbit of Saturn! A tremendous amount of charged particles are trapped within this magnetosphere, especially in the inner parts of this field. This makes Jupiter the most deadly radiation environment of any of the planets.