Our solar system began as a huge cloud of gas, dust and soot, called a nebula. This cloud was mostly hydrogen, but also contained traces of other elements, such as carbon, silicon, oxygen, iron, and so on. The cloud was likely the result of the death of another, nearby star. The cloud was likely about 2 light years across, or perhaps more. The cloud was very sparse, with a density many thousand times less than the earth's atmosphere.
Despite the low density, there was still a large mass of gas and dust, and all mass has gravity. As a result of this gravity, and perhaps a shockwave from another star, the cloud began to collapse.
A clump began to form at the centre, which was denser than the surrounding gas. This would eventually become the sun. As the gas and dust moved inward, it began to spin like water going down a drain. The smaller it got, the faster the rate of spin. As a result of the spin, the cloud flattened to a thick disk. This process took over a million years.
This process continued, with more and more gas getting sucked into the proto-sun. By about 10 million years, denser regions in the swirling cloud also attracted material, and began to clump into planetessimals -- lumpy chunks of rock, dust and ice, such as asteroids and comets.
At an age of 50 million years, the planetessimals also clumped together, growing larger and larger, gaining more gravity, and attracting even more material. As the proto-sun and proto-planets grew bigger, the pressure from gravity caused them to heat up. The central region of this disk was hot enough to prevent water, ammonia and methane from solidifying, but cool enough that metals and silicates could.
By about 100 million years, the proto-sun had grown so big, and the pressure and temperature became so high in the core that nuclear fusion occurred, and the sun ignited. Pressure from the solar wind and radiation drove off most of the remaining dust and gas, leaving mostly empty space between the planets.
For many millions of years, planetessimals continued to collide with the planets and moons, until most of the bodies in orbits near the planets were scooped up. Comet- like bodies with ice and other volatiles probably provided the source for earth's early atmosphere.
Evidence for this model of solar system formation:
There are several pieces of evidence to support the Nebular Hypothesis:
1. All planets lie on the same plane, and orbit the sun in the same direction
2. The outer planets are composed mostly of volatiles, ices and gases, while the inner planets are rocky
3. The solar system contains rocky asteroids and icy comets
4. disks of gas and dust, called "protoplanetary disks" have been observed around young stars
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