In cosmogony, the solar nebula is the gaseous cloud (or accretion disc) from which our solar system is believed to have formed. This nebular hypothesis was first proposed in 1755 by Immanuel Kant, who argued that nebulae slowly rotate, gradually condensing and flattening due to gravity, eventually forming stars and planets. A similar model was proposed in 1796 by Pierre-Simon Laplace.

Formation and evolution

The solar nebula is believed to have had an initial diameter of 100 AU and a mass believed to be 2-3 times the Sun's current mass. Over time, gravity caused the cloud to condense and, as density and pressure increased, a protostar emerged at the centre of the nebula. The early system was heated by friction; fusion would not occur for some time. Due to the conservation of angular momentum, the nebula did not fully collapse upon itself, instead forming a disk. Protoplanetary discs emerged in orbit around the protosun.

Within this system, lighter elements such as hydrogen and helium were driven out of the central regions by solar wind and radiation pressure during a highly active T Tauri phase, leaving behind heavier elements and dust particles, which clumped into planetesimals and protoplanets. In the outer part of the solar nebula, ice and volatile gases were able to survive. As a result, the inner planets are formed of minerals, while the outer planets are more gaseous or icy.

After about 100 million years, the heat within the protosun reached such a level that thermonuclear reactions began to occur - the protosun became the Sun. At about the same time, the innermost planets formed, approximately 4.6 billion years ago.

The moons of the gas giants are believed to have formed in a roughly analogous process, coalescing from an accretion disk which formed as the giant planets themselves were forming. In contrast, Earth's moon is believed to have formed in a collision with a Mars-sized object.

The Kant-Laplace and Near-Collision Theories

During the late-19th century the Kant-Laplace views were criticized by James Clerk Maxwell, who showed that if matter of the known planets had once been distributed around the Sun in the form of a disc, forces of differential rotation would have prevented the condensation of individual planets. Another objection was that the Sun possesses less angular momentum than the Kant-Laplace theory indicated. For several decades, most astronomers preferred the near-collision theory, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets.

Objections to the near-collision theory were also raised and, during the 1940s, the Kant-Laplace theory was modified such that it became accepted. In the modified version, the mass of the original protoplanet was assumed to be larger, and the angular momentum discrepancy was attributed to magnetic forces.