The universe contains huge clouds made up of very large amountsof dustand gas. About 6,000,000,000 (billion) years ago, one ofthese clouds began to condense. Gravitation–the pull that allobjects in the universe have for one another–pulled the gas anddust particles together. As the dust cloud condensed, it beganto spin. It spun faster and faster and flattened as it spun. Itbecame shaped like a pancake that is thick at the centre and thinat the edges.
The slowly spinning centre condensed to make the sun. But theouter parts of the pancake, or disk, were spinning too fast tocondense in one piece. They broke up into smaller swirls, oreddies, which condensed separately to make the planets.
The forming sun and planets were made up mostly of gas. Theycontained much more gas than dust. The earth was far bigger thanit is now and probably weighed 500 times as much.
The large body of dust and gas forming the sun collapsed rapidlyto a much smaller size. The pressure that resulted from thecollapse caused the sun to become very hot and to glow brightly.
The newly born sun began to heat up the swirling eddy of gas anddust that was to become the earth. The gas expanded, and some ofit flowed away into space. The dust that remained behind thencollected together because of gravity. Although the shrinkingearth generated a lot of heat, most of this heat was lost intospace. Therefore, the original earth was most likely solid, notmolten.
This hypothesis was developed by a scientest, Harold C. Urey in1952. It is also known as the Urey’s hypothesis. He showed thatmethane, ammonia, and water are the stable forms of carbon,nitrogen, and oxygen if an excess of hydrogen is present. Cosmicdust clouds, from which the earth formed, contained a greatexcess of hydrogen.