Formation And Evolution Of The Solar System

Main article: Formation and evolution of the Solar System The most widely accepted theory of planetary formation, known as the nebular hypothesis, maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud which was light years across. Several stars, including the Sun, formed within the collapsing cloud. The gas that formed the Solar System was slightly more massive than the Sun itself. Most of the mass collected in the centre, forming the Sun; the rest of the mass flattened into a protoplanetary disc, out of which the planets and other bodies in the Solar System formed. Laplace refuted this idea in 1796, showing that any planets formed in such a way would eventually crash into the Sun. Laplace felt that the near-circular orbits of the planets were a necessary consequence of their formation.[4] Today, comets are known to be far too small to have created the Solar System in this way. In 1755, Immanuel Kant speculated that observed nebulae may in fact be regions of star and planet formation. In 1796, Laplace elaborated by arguing that the nebula collapsed into a star, and, as it did so, the remaining material gradually spun outward into a flat disc, which then formed the planets. Alternative theories The nebular hypothesis initially faced the obstacle of angular momentum; if the Sun had indeed formed from the collapse of such a cloud, the planets should be orbiting around it far more slowly. The Sun, though it contains almost 99.9 percent of the system's mass, contains just 1 percent of its angular momentum. Attempts to resolve the angular momentum problem led to the temporary abandonment of the nebular hypothesis in favour of a return to "two-body" theories. Henry Norris Russell,showed that it ran into problems with angular momentum for the outer planets, with the planets struggling to avoid being reabsorbed by the Sun. Victor Safronov who showed that the amount of time required to form the planets from such a diffuse envelope would far exceed the Solar System's determined age. In 1960, W. H. McCrea proposed the protoplanet theory, in which the Sun and planets individually coalesced from matter within the same cloud, with the smaller planets later captured by the Sun's larger gravity.[4] This theory has a number of issues, such as explaining the fact that the planets all orbit the Sun in the same direction, which would appear highly unlikely if they were each individually captured. The capture theory, proposed by M. M. Woolfson in 1964, posits that the Solar System formed from tidal interactions between the Sun and a low-density protostar. The Sun's gravity would have drawn material from the diffuse atmosphere of the protostar, which would then have collapsed to form the planets.However, the capture theory predicts a different age for the Sun than for the planets,[citation needed] whereas the similar ages of the Sun and the rest of the Solar System indicate that they formed at roughly the same time. Fred Hoyle noted that, even while the distribution of elements was fairly uniform, different stars had varying amounts of each element. To Hoyle, this indicated that they must have originated within the stars themselves. The abundance of elements peaked around the atomic number for iron, an element that could only have been formed under intense pressures and temperatures. Hoyle concluded that iron must have formed within giant stars.From this, in 1945 and 1946, Hoyle constructed the final stages of a star's life cycle. As the star dies, it collapses under its own weight, leading to a stratified chain of fusion reactions: carbon-12 fuses with helium to form oxygen-16; oxygen-16 fuses with helium to produce neon-20, and so

on up to iron.There was, however, no known method by which carbon-12 could be produced. Isotopes of beryllium produced via fusion were too unstable to form carbon, and for three helium atoms to form carbon-12 was so unlikely as to have been impossible over the age of the universe.