Solar nebula

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

The Sol Nebula

The "life cycle" of the Sol Nebula is more or less similar to that of other solar nebulae.

Early solar nebulae (in the history of the universe) were formed of hydrogen, helium and lithium; while later stars were formed of heavier elements. As the Sol system is comparatively rich in these heavier elements, it can be argued that this system did not emerge directly from the "Big Bang".

This nebula had an initial diameter of 100AU and a mass of ~200-300% that of Sol's current mass. Over time, gravity caused the cloud to condense and, as density and pressure increased, a protostar emerged. The early system was heated, not by fusion, but by friction. Due to the conservation of angular momentum, the nebula did not fully collapse upon itself, and thus protoplanetary discs emerged, in orbit, around the protosun.

Within this system, heavier elements tended to fall more towards the center (clumping into planetesimals and protoplanets). In addition, the outer part of the solar nebula cooled off (if it was ever hot to begin with) and, thus, ice and combustible gases were able to "survive". As a result, the inner planets are formed of minerals, whilst, the outer planets are more gaseous/icy.

At some point, the heat within the protosun reached such a level that thermonuclear reactions began to occur. At this point, a "true" star was been "born". The protostar lasted for ~100M years and the cycle was completed at about the same time the innermost planets had developed; this was ~4.6B years ago. Although the moonss existed, they were not yet orbiting planets; this would occur over the next 800M years.

The Kant-Laplace and Near-Collision Theories

During the late-19th century the Kant-Laplace views were criticized by Maxwell, who showed, if matter of the known planets had once been distributed (around the Sun) in the form of a disk; forces of differential rotation would have prevented the condensation of individual planets. Another objection was that the Sun possesses less angular momentum that 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 Sol.

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.