Radioactivity is a natural phenomenon ionizing present everywhere, which occurs in the nucleus, in the depths of the atoms. Unstable radioactive elements of the earth radiate much energy per disintegration.
Disintegration is the transformation of matter which naturally ejects particles and produces radioactive neutron radiation and energy. Uranium, thorium and potassium are the main radioactive elements responsible for the heat of the Earth. Although terrestrial radioactivity steadily decreased because radioactive atoms become stable elements, it remain still very important because uranium 238, thorium-232 and potassium 40 are long-lived elements (several billion years). Other radioactive elements those which have a very short period relative to the age of the Earth disappeared. And since its formation there is 4.2 billion years, the Earth gradually loses its intrinsic heat and cools down.
Earth drains continuously, a total power of ≈ 44 terawatts equivalent of 44,000 nuclear reactors. What is remarkable about observing our solar system, is the extraordinary diversity of objects connected to it. Yet they are all formed from the same cloud at the same place in the universe, from the same materials at the same time there are about 5 billion years.
How objects have been able to evolve so differently from identical initial conditions?
Astrophysicists have long sought to understand this phenomenon. And the most amazing is that an object has been able to create the conditions for the emergence of life and to have them preserved until today. If Astrophysics explains quite well the evolution of stars, it is far to explain the evolution of planets.
What are the conditions that led to the Earth so much complexity?
Is this the most complex object in the universe?
How planets absorbed energies so different?
From the same initial state, all objects over time will evolve differently, they will gradually acquire an internal energy and will slowly lose it, depending of their mass. Indeed, the size of the object has a major importance in the accumulation of internal energy, astronomical objects are like energy tanks which empty gradually as the radioactive decay of its elements. The main active planets engine is the internal radioactivity which is converted into heat which rises from the center to the surface and maintains a certain activity (magnetism, volcanism, continental drift, recycle the atmosphere,...) on the planet.
The enormous solar energy is not sufficient to maintain our planet active because this energy is blocked at the earth's surface and does not penetrate the center of the Earth. The energy which do that the Earth is alive is trapped inside the planet, it is that of the radioactivity of uranium, thorium and potassium. If there was no radioactive decay, the Earth would be a dead planet. In summary, more the tank is bigger and more it will store energy. When the object is large it cools slowly. Thus small asteroids and comets have frozen there 5 billion years, the large asteroids have frozen there 4 billion years, the Moon has frozen there 3 billion years, Mars has frozen there to 1 billion years, the Earth after 4.2 billion years ago is still an active planet. More solar system objects are small (asteroids, comets) and more they interest to scientists because they spread all their energy into space and kept intact the materials at the time of their "dead", in particular organic molecules.
NB: natural radioisotopes: Americium 241, antimony 125, carbon-14, cesium-134, 135 and 137, chlorine-36, cobalt-60, curium 242 and 244, iodine-129, 131 and 133, krypton-85 and 89, phosphorus-32, plutonium-239 and 241, polonium-210, potassium-40, radium 226 and 242, ruthenium-106, selenium-75, sulfur-35, strontium-90, thorium-234, tritium 3, uranium 235 and 238.
Image: The Earth is a huge reservoir of energy. This energy depths up to the surface by convection. The heat released by the natural radioactivity of deep rocks as uranium, thorium and potassium maintains a mild temperature on the surface since 4 billion years. The different layers of the Earth's are crust solid on the surface which has a thickness of 30-65 km, the upper mantle viscosity of 670 km, the lower elastic mantle of 2180 km, the liquid outer core 2270 km and the solid inner core 1220 km.