Because the solar system was formed from matter that was already hot when it coalesced into the sun and planets. About 4 1/2 billion years ago, the earth started as one big molten mass of rock and other materials — the surface being too hot for life to exist. The first simple life didn't show up until about a billion years later. read more
It does not. Geothermal is from nuclear decay and various forms of energy originating from kinetic energy of objects that formed earth. read more
You may not think that energy to be much when compared to the sun but you would be wrong in some aspects. The Earth’s core alone is 11,000 degrees. That is hotter than the surface of the sun. The Earth’s geothermal energy is created by the decay of radioactie materials in the core and in the surrounding layers of rock. read more
[Geothermal energy comes from the heat in the core and mantle of the earth. Disintegrating radioisotopes and the gravitational effects on Earth's core generate heat.] Ocean thermal energy conversion (OTEC) is not being used for energy generation anywhere right now. read more