The universe contains huge clouds made up of very large amounts
of dustand gas. About 6,000,000,000 (billion) years ago, one of
these clouds began to condense. Gravitation–the pull that all
objects in the universe have for one another–pulled the gas and
dust particles together. As the dust cloud condensed, it began
to spin. It spun faster and faster and flattened as it spun. It
became shaped like a pancake that is thick at the centre and thin
at the edges.
The slowly spinning centre condensed to make the sun. But the
outer parts of the pancake, or disk, were spinning too fast to
condense in one piece. They broke up into smaller swirls, or
eddies, which condensed separately to make the planets.
The forming sun and planets were made up mostly of gas. They
contained much more gas than dust. The earth was far bigger than
it is now and probably weighed 500 times as much.
The large body of dust and gas forming the sun collapsed rapidly
to a much smaller size. The pressure that resulted from the
collapse caused the sun to become very hot and to glow brightly.
The newly born sun began to heat up the swirling eddy of gas and
dust that was to become the earth. The gas expanded, and some of
it flowed away into space. The dust that remained behind then
collected together because of gravity. Although the shrinking
earth generated a lot of heat, most of this heat was lost into
space. Therefore, the original earth was most likely solid, not
molten.
This hypothesis was developed by a scientest, Harold C. Urey in
1952. It is also known as the Urey’s hypothesis. He showed that
methane, ammonia, and water are the stable forms of carbon,
nitrogen, and oxygen if an excess of hydrogen is present. Cosmic
dust clouds, from which the earth formed, contained a great
excess of hydrogen.
