Formation

The Solar System is believed to have formed according to the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.[8] This theory holds that 4.6 billion years ago the Solar System formed from the gravitational collapse of a giant molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars.[9] Studies of ancient meteorites reveal traces of elements only formed in the hearts of very large exploding stars, indicating that the Sun formed within a star cluster, and in range of a number of nearby supernovae explosions. The shock wave from these supernovae may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, allowing gravitational forces to overcome internal gas pressures and cause collapse.[10]
The region that would become the Solar System, known as the pre-solar nebula,[11] had a diameter of between 7000 and 20,000 AU[9][12] and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).[13] As the nebula collapsed, conservation of angular momentum made it rotate faster. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency. The center, where most of the mass collected, became increasingly hotter than the surrounding disc.[9] As gravity, gas pressure, magnetic fields, and rotation acted on the contracting nebula, it began to flatten into a spinning protoplanetary disk with a diameter of roughly 200 AU[9] and a hot, dense protostar at the center.[14][15]
Studies of T Tauri stars, young, pre-fusing solar mass stars believed to be similar to the Sun at this point in its evolution, show that they are often accompanied by discs of pre-planetary matter.[13] These discs extend to several hundred AU and reach only a thousand kelvins at their hottest.[16]

Hubble image of protoplanetary disks in the Orion Nebula, a light years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed
After 100 million years, the pressure and density of hydrogen in the center of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion. This increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged star.[17]
From the remaining cloud of gas and dust (the "solar nebula"), the various planets formed. They are believed to have formed by accretion: the planets began as dust grains in orbit around the central protostar; then gathered by direct contact into clumps between one and ten kilometres in diameter; then collided to form larger bodies (planetesimals) of roughly 5 km in size; then gradually increased by further collisions at roughly 15 cm per year over the course of the next few million years.[18]
The inner Solar System was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc)[9] and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt.[19]
Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn became the gas giants. Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).[20][21]
Once the young Sun began producing energy, the solar wind (see below) blew the gas and dust in the protoplanetary disk into interstellar space and ended the growth of the planets. T Tauri stars have far stronger stellar winds than more stable, older stars