cosmogony

The study of how celestial systems and objects in general are formed but, in particular, the study of the origin of the solar system. Various theories to account for the formation of the solar system have been proposed since Descartes first attempted in 1644 to apply scientific reasoning to what would now be called cosmology. His "vortex" theory, and more modern versions of it, suppose the existence of turbulence from which planets form but, since there is no known mechanism to drive turbulence, such theories have been rejected. Tidal theories invoke the close approach of another star to the Sun, resulting in material being torn out of the Sun, which would then condense into planets, but these too are now regarded as unlikely explanations. Tidal mechanisms in which stars interact with giant molecular clouds are more probable. The modern view is that the solar system formed from a slowly rotating cloud of gas. As the cloud collapsed, a dense opaque core formed - ultimately to become the Sun - surrounded by a disc of gas and dust. The first such nebular theories were suggested by Immanuel Kant (1724-1804) in 1755 and Laplace in 1796. Ideas on how the planets actually formed within the disc have changed greatly in recent years. Current thinking is that they gradually accumulated by accretion. The effect of heating by the Sun, which diminishes with distance, accounts for the difference between the inner rocky planets and the outer gas giants.