Earth formed as part of the birth of the solar system: what eventually became the solar system initially existed as a large, rotating cloud of dust and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements produced by stars long gone. Then, about 4.6 billion years ago, a nearby star probably became a supernova. The explosion sent a shock wave toward the solar nebula and caused it to contract. As the cloud continued to rotate, gravity and inertia flattened the cloud into a proto-planetary disc, perpendicular to its axis of rotation. Most of the mass concentrated in the middle and began to heat up.

The impossibility of kinetic heat, produced by the infall of matter escaping caused the centre to heat up sufficiently to enable the centre of the concentration to produce its own internal heat source through nuclear fusion of hydrogen into helium, starting as a T Tauri star, our early sun. Meanwhile, as gravity caused matter to condense around dust particles, the rest of the disc started to break up into rings. Small fragments collided and became larger fragments. These included one collection approximately 150 million kilometers from the center: Earth. As the Sun condensed and heated, fusion began, and the resulting T Tauri solar wind cleared out most of the material in the disc that had not already condensed into larger bodies.

The Moon

The origin of the Moon is still uncertain, although much evidence exists for the giant impact hypothesis. Earth may not have been the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from both the Sun and the Earth, at the fourth or fifth Lagrangian point. This planet, named Theia, is thought to have been smaller than the current Earth, probably about the size and mass of Mars. Its orbit may at first have been stable but destabilized as Earth increased its mass by the accretion of more and more material.

Theia swung back and forth relative to Earth until, finally, an estimated 4.533 billion years ago, it collided at a low, oblique angle. The low speed and angle were not enough to destroy Earth, but a large portion of its crust was ejected.

Heavier elements from Theia sank to Earth’s core, while the remaining material and ejecta condensed into a single body within a couple of weeks. Under the influence of its own gravity, this became a more spherical body: the Moon. The impact is also thought to have changed Earth’s axis to produce the large 23.5° axial tilt that is responsible for Earth’s seasons. (A simple, ideal model of the planets’ origins would have axial tilts of 0° with no recognizable seasons.) It may also have sped up Earth’s rotation and initiated the planet’s plate tectonics.

Early Earth

The prevailing view of early Earth is that it was utter hell, a fiery environment unsuitable for life. Scientist even named it the Hadean eon, for the ancient Greek word for the down under. But the planet may have been suitable for life just 200 million years after the solar system formed, new research suggests. This new view "contrasts with the hot, violent environment envisioned for our young planet by most researchers and opens up the possibility that life got a very early foothold," Bruce Watson of Rensselaer Polytechnic Institute said today.

Prevailing View

The popular belief among scientists is that Earth during the Hadean eon – all Earth time prior to 3.8 billion years ago – was a hot, volcanic environment in which no living thing could survive. There were no oceans and no oxygen in the atmosphere. It was bombarded by planetoids and other material left over from the formation of the solar system. This bombardment, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction, caused the planet at this stage to be fully molten. Heavier elements sank to the center while lighter ones rose to the surface, producing Earth's various layers. Earth's early atmosphere would have comprised surrounding material from the solar nebula, especially light gases such as hydrogen and helium, but the solar wind and Earth's own heat would have driven off this atmosphere.

This changed when Earth was about 40% its present radius, and gravitational attraction allowed the retention of an atmosphere which included water. Temperatures plummeted and the crust of the planet was accumulated on a solid surface, with areas melted by large impacts on the scale of decades to hundreds of years between impact. Large impacts would have caused localized melting and partial differentiation, with some lighter elements on the surface or released to the moist atmosphere. The surface cooled quickly, forming the solid crust within 150 million years.

From 4 to 3.8 billion years ago, Earth underwent a period of heavy asteroidal bombardment. Steam escaped from the crust while more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity.

The planet cooled. Clouds formed. Rain gave rise to the oceans within 750 million years (3.8 billion years ago).The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its entry, ultraviolet radiation flooded the surface.

Picture This

A new study concludes Earth had continents and oceans 4.3 billion years ago, which is just a geological eyeblink after the planet is thought to have formed, in the wake of the Sun's birth 4.6 billion years ago.

A separate study reported in May came to a similar conclusion, also suggesting that notions of a fiery, hellish planet back then have been overblown. Here's why it matters: A world with water and land and somewhat moderate temperatures and volcanic conditions would have been habitable. That does not mean there was life, but the conditions were in place.

University of Colorado researcher Stephen Mojzsis explained what our world might have looked like back then:

"Before 4 billion years ago, the Earth would not be recognizable for the Pale Blue World that we are familiar with today. Indeed, although we now understand that there were significant landmasses already present by that time, the denser carbon dioxide-rich atmosphere would have given the sky a reddish-tinge.

The oceans, with a much higher concentration of iron than our contemporary oceans, would look a dark greenish-blue and these oceans would have bathed hundreds of small continents akin to New Zealand or the Japan arc," he said.

The conclusion is based on an analysis of hafnium, a rare element in ancient minerals from the Jack Hills in Western Australia. The rocks are thought to be among the oldest on Earth, dated to 4.4 billion years ago.

"The evidence indicates that there was substantial continental crust on Earth within its first 100 million years of existence," Mojzsis said.

Perfect for Microbes