About 5 to 6 billion years ago. One of these stars began to run out of Hydrogen fuel. It expanded to a red giant and then collapsed on itself and exploded in a supernova. In this supernova, like billions that have occurred elsewhere in our Universe, all of the other elements were created.
II. The mass of new matter again collapsed into a disk shape mass of dust and gas (a). The center became superheated and formed a new star, our sun (b). From this disk of matter the planets began to condense (c), according to the widely supported nebular hypothesis of Immanuel Kant and Pierre-Simon Laplace. The two strongest points in favor of this idea are: 1) that the disk began by rotating in one direction and the rotation of all of the planets around the sun follows the original disk; and 2) that because the disk flattened out as time progresses, all of the orbits of the planets (except Pluto) lie more or less in the same plane (d). Pluto is possibly a captured giant asteroid.
III. The earth condensed in four basic steps. 1) It began to accrete from the nebular cloud as particles smashed into each other forming so-called planetesimals. These in turn collided with each other and as their mass grew began to gather material from the nebular disk. 2) As the mass of the Earth grew so did it's gravitational force and the Earth began to compress itself into a smaller and denser body. This happened about 4.5 billion years ago. 3) In the third step the compression itself began to heat the interior of the Earth; also there was heat generated by radioactive decay. The interior of the earth began to melt. Because iron is the heaviest of the common elements that make up the Earth, as the Earth began to melt droplets of melted iron began to sink towards the center of the earth, where they condensed. 4) Proceeding slowly at first it sped up to catastrophic proportions - hence it is called the iron catastrophe. Note that 3 and 4 in the figures to the right are cross sections.
It was the iron catastrophe that set up the overall structure of the Earth.
All during this time the earth was still being bombarded by asteroids and comets, a process that still occurs but at a much lower (although still significant!) rate.
At some point however, the Moon formed. Exactly how that happened is a major subject of debate. One theory claims that the Moon is a tiny planet captured by the Earth's gravity. The other is that the moon was literally splashed out of the Earth by the impact of a Mars-sized planet. The latter theory is favored now because it explains some odd but important features of the Earth's and Moon's chemical composition.
The Earth was reborn during the iron catastrophe and maybe again by the formation of the Moon. Any trace of surface structure was wiped out by the melting.
IV. However, the crust finally solidified by about 3.7 billion years ago. Gasses pouring out of volcanoes and fissures, along with lava, began to accumulate, perhaps added to by the impact of a few giant comets (which are mostly gas).
The gases that accumulated were those we still find coming out of volcanoes:
Water vapor (H2O)
Hydrogen chloride (HCl)
Carbon Monoxide (CO)
Carbon Dioxide (CO2)
Nitrogen (N2)
These gases combined to form:
Methane (CH4)
Ammonia (NH4)
Hydrogen Cyanide (HCN)
This atmosphere would be quickly fatal to us.
V. As the crust cooled water would condense and accumulate as oceans. This happened very soon after the crust solidified.
