Earth Development of Earth's structure and compositionplanet

The origin of Earth in its present form has long been the subject of intellectual interest, but since the mid-20th century scientists have made particularly significant advances both in concepts and in measurements. Analysis of the isotopes in meteorites and, in particular, of rocks brought back from the Moon by U.S. Apollo astronauts have produced some of the major contributions. Other gains have come from geochemical research on terrestrial samples combined with the new understanding of internal processes brought on by the recognition of plate tectonics, study of the terrestrial planets as a group, and advances in numerical modeling of the physical processes that lead to planetary formation.

The starting point in tracing planetary evolution is nucleosynthesis, the formation of the chemical elements on a cosmic scale. This includes the nuclear processes by which the lightest elements—mostly hydrogen and helium—were produced at the explosive birth of the universe (see big-bang model), some 14 billion years ago, and the subsequent formation of the heavier elements within stars (see chemical element: Origin of the elements). By analogy with what astronomers presently observe to happen in regions of star formation, it is thought that the solar system began as a cloud of gas and dust comprising such preexisting elements. Under its own gravitational attraction, the cloud collapsed into a rotating disk of matter, called the solar nebula. The collapse could have been initiated by a shock wave emanating from a nearby supernova, a violently exploding star, or by random density fluctuations in the cloud itself. Once sufficiently high pressures and densities were achieved in the compacted nebular core, nuclear fusion reactions within it could begin, giving birth to a star. The outer part of the rotating disk—the matter not incorporated into the new Sun—became the raw material for the planets and other orbiting bodies of the solar system. The birth of the Sun, which makes up more than 99.9 percent of the mass of the entire solar system, is taken to be the time at which the planets started to form, approximately 4.56 billion years ago.