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.