Solar System, origin
Solar System, originThere have been various theories on the origin of the Solar System, many of which have been discarded or modified as observational data has slowly accumulated. The Sun and planets are now believed to have formed together 4.56 billion years ago by the contraction of a cloud of interstellar gas, mainly hydrogen and helium, and dust grains – the solar nebula. The contraction may have been triggered by shock waves from a nearby supernova, but continued under the mutual gravitational attraction of the contents of the nebula (see star formation).
Conservation of angular momentum dictated that as the cloud contracted its rotation rate increased, causing it to become disk-shaped around the developing Sun. Collisions between dust grains became frequent, leading to energy loss and a rapid concentration of particles in the plane of the disk, which lay close to the present plane of the ecliptic, and a gradual accretion of grains into pebbles, boulders, and larger bodies began. Only when these planetesimals had reached a few kilometers across could their gravity accelerate the accretion process by attracting more solid material. Eventually the protoplanets of the present planets and satellite systems were built up, most of them rotating and revolving in the same sense as the parent nebula. Possibly any protoplanets that formed between the orbits of Mars and Jupiter suffered gravitational perturbations by Jupiter and were fragmented by collisions to leave the asteroids of today.
Although the Sun had yet to begin nuclear fusion processes, temperatures in the inner Solar System were high enough to maintain such volatile substances as water, methane, and ammonia in a gaseous state; the planetesimals that gave rise to the inner planets were therefore formed mainly from the nonvolatile component of the nebula, such as iron and silicates. Farther from the embryo Sun, beyond the asteroid belt, lower temperatures allowed volatiles to become important constituents of the giant planets, their satellite retinues, and their ring systems. The condensation of dirty snowflakes in these regions accelerated the accretion process. The origin of the volatile-rich comets is more obscure, but they may have formed concurrently as planetesimals in the Saturn to Neptune region and near the outer edge of the Solar System's disk. When the planetesimals had reached Moon size, the low temperatures also made it possible for them to greatly augment their masses by attracting and retaining large quantities of the light elements hydrogen and helium from the surrounding nebula. This produced the huge atmospheres of the giant planets.
As the Sun continued to collapse, the density and temperature in its central core rose (see star formation). Once the core temperature reached some ten million kelvin, the Sun began to generate energy by the nuclear fusion of hydrogen. The onset of fusion processes in the Sun set up the solar wind, which drove uncollected gas and dust grains from the Solar System. Heating of the protoplanets by gravitational contraction and interior radioactivity caused partial melting that led to the present internal differentiated structure of the planets. Impacts by remaining planetesimals scarred the surfaces of the planets and their satellites, although the Earth's craters have been largely removed by erosion. Cratering events continue on a much reduced scale to the present day as errant asteroids or cometary nuclei collide with the planets or their satellites.
The suggestion that a contracting nebula formed the Solar System was the basis of the nebular hypothesis, popular during the 19th century. However this was unable to explain why most of the Solar System's angular velocity resides in the revolution of the planets, rather than in the rotation of the Sun, and was replaced by various encounter theories early in the 20th century. The renaissance of a nebular theory came with the realization that magnetohydrodynamic forces would transfer angular momentum from the early Sun to its surrounding nebula, and that the Sun would shed further angular momentum through the solar wind. In fact, the early Sun might have been twice its present mass and might have lost a large percentage of its angular momentum during its T Tauri phase when the solar T Tauri wind was a gale. Calculations incorporating these ideas also suggest that the solar nebula contained at least three solar masses of material, most of which was expelled again into interstellar space.