The Dust-Cloud Hypothesis

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.