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When the sun began to form, clouds of gas formed in rings around the sun. They eventually coalesced into gaseous balls orbitting the sun. These proto-planets consisted mainly of hydrogen and helium, and thus could not have formed into the rocky planets. However, they did have gravitational fields and they were able to sweep other matter up from interstellar space.

The fusion process in stars, such as our sun, will eventually produce abundant light elements, but very little of the heavy elements. However, from time to time a supernova star explodes at such high temperatures as to produce the heavy elements of our planetary system. Each supernova scatters its matter into interstellar space, to be attracted towards other stars and solar systems such as our own. Over the eons, some of this was accumulated by the planets, including our own, Earth.

These balls cooled, to become the planets we know today. But they still had massive atmospheres consisting mainly of hydrogen and helium. The larger planets, such as Jupiter, with their greater gravity, were able to retain those lighter gases and are today the "gas giants". Intermediate planets, such as Earth and Venus, gradually lost most of the gaseous hydrogen and helium, but retained the heavier gases, such as oxygen and nitrogen. Smaller planets, with their weaker gravitational fields, gradually lost nearly all their atmospheres.

Thus, the Earth formed, as one of the planets in our solar system, at first into a barren planet, hostile to life forms.

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10y ago

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