The Dust-Cloud Hypothesis
The universe contains huge clouds made up of very large
amounts of dust and 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 center and thin
at the edges.
The slowly spinning center 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 scientist, 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.