The Earth is about 4.6 billion years old (Anderson, 2007). In terms of its thermal regime, the planet is in the process of cooling. However, to have reached its current state, the Earth and the other objects making up the Solar System went through several stages, such as the accretion of the planet from the dust of the solar nebula, the formation of the magma-ocean, stratification of matter by density, solidification of the magma-ocean, formation of the lithosphere which is taking place today, periods of increased volcanic and metamorphic activity, numerous tectonic processes with global and regional significance (obduction, subduction, orogeny, etc.), heat production by short-lived and long-lived radioisotopes, and numerous other features and processes related to thermodynamic and temperature conditions. Many features and processes prevent the Earth from cooling quickly. These include the Sun's radiation, heat production by long-lived radioactive isotopes, and different chemical reactions. All of these features and processes are related to heat: some led to the accumulation of heat (accretion, the heat produced by radioactive decay, stratification of the magma-ocean, etc.), whereas others were involved in the transfer of heat energy to the surface (volcanism, obduction, formation of orogeny, hot springs, etc.) or the transfer of the colder matter to greater depths (subduction, immersion, penetration of water through fractures to deeper layers within the crust and upper mantle). All these processes are tightly related, have been influenced by the ever-changing Earth's thermal regime at every step of its evolution, and are associated with or represent essential components of geothermics. Therefore, all these features and processes warrant special attention and analysis to paint a complete picture of planet Earth's thermal evolution and conditions.
Throughout the nineteenth century, an increasing amount of geological data began to conflict with religious beliefs concerning the origin of the Earth. The main controversies had to do with (1) the formation of the Solar System and Earth, (2) the age of the Earth, and (3) the composition and conditions of the Earth at the time of its formation. All these factors dictate the Earth's thermal regime, thermal gradient, the depth of the Curie discontinuity, and many other geothermal factors. The first scientific theory of planetary formation was the "vortex theory" put forward in 1644 by Descartes (Brandner, 2006). Descartes believed that the universe was filled with vortices of swirling particles and that the Sun was condensed from a gigantic vortex that somehow contracted. At the same time, the planets and satellites were formed from smaller vortices.
The nebular hypothesis was introduced in the eighteenth century. It was first proposed in 1734 by Swedenborg (1734) but later apparently presented independently as a complete model by both Kant (1755) and Laplace (1799–1825). This hypothesis, known as the Kant-Laplace theory, constituted a turning point in our scientific understanding of the formation of the Solar System and the Universe. The nebular hypothesis proposed by Kant (1755) is qualitative. It includes such details as the slow rotation of nebulae, their gradual condensing and flattening due to gravity, and, eventually, the formation of stars and planets. Laplace (1799–1825) based his hypothesis on a solid mathematical and physical foundation. In his work, Laplace also proved the dynamic stability of the Solar System.