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There are three scenarios I will like to discuss here.

1- Jupiter and other "failed star" planets. These are gas giant planets which are much much bigger than earth (100 times or more, by mass) but don't have enough gravitational crunch in the core to initiate fusion like a star. However, these massive planets still generate heat energy in their core due to outrageous pressures that exist there and the friction that takes place between the atoms.

So. How do these planets generate heat energy without compensating for it in terms of mass etc?

2- Same question about neutron stars. Their core is composed entirely of neutrons which do not disintegrate into electron-proton pair. Yet their core temperatures reach millions of degrees. How come these beasts maintain such horrific temperatures without converting any matter into energy (like a "normal" star does)?

3- They say there are active volcanoes on some of Jupiter's moons (Io to be precise). Now at such distances from the sun, one would expect dead, activity-less moons. They also state that jupiter's massive pull causes a tidal effect on the moon and initiates colossal friction in its core which results in volcanic action. So ... can we simply "generate" energy out of gravitational force? I mean ... it would make sense if Io was gradually spiralling in towards gravitational doom, decreasing it's distance toward Jupiter with each orbit. But they say the distance of Io from Jupiter is constant. Oh well.

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  • $\begingroup$ Jupiter isn't done with its gravitational collapse into equilibrium, yet, and it still has plenty of long lived radioactive elements in its core. Neutrons stars have a very large heat capacity and they can undergo phase transitions that keep them hot for some time. Your mental model of Io's orbit and interaction with Jupiter and it's other moons is simply wrong. $\endgroup$ – CuriousOne Sep 8 '15 at 13:50
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The Earth also radiates more energy than it receives from the Sun. There are many mechanisms by which an astronomical body can generate heat, and nuclear fusion is just one. In the case of the Earth the extra heat is generated by radioactive decay in the Earth's core.

There is an excellent discussion of the source of Jupiter's excess heat in this article from the Nanjing University web site. The main source of Jupiter's heat is probably simply the energy from the gravitational collapse that is still working its way out. At first glance it might seem improbably that Jupiter is still losing its original collapse energy, but Jupiter is big and heat transport from the core is extremely slow.

Radiactive heating will be happening in Jupiter's core just as in Earth's core, but the outer planets contain relatively lower percentages of the heavy elements and therefore lower levels of radionuclides. There isn't enough radioactive decay in Jupiter's core to make much difference.

The same applies to neutron stars. They aren't generating much in the way of heat, but they will take a long time to cool.

Finally, tidal losses cause a satellite to move outwards not inwards. This is a result of the conservation of angular momentum. The Moon is moving away from the Earth for precisely this reason. You say the Jupiter-Io distance is not changing, but I'm not sure we can measure it accurately enough to say that. Although the volcanos may look spectacular the energy involved in tidal heating is relatively small and any changes in the Jupiter-Io distance will also be small.

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  • $\begingroup$ Excellent explanation!! Also answers my questions about white dwarfs and several other astronomical phenomena. $\endgroup$ – Youstay Igo Sep 8 '15 at 16:22

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