The earth, the planet on we are living, is the third planet from the sun. It came from the shrinking (收缩) of a vast cloud of dust and gas in outer space. Many scientist believe that the earth, as well as the sun and the other planets in our solar system, (form) at least five billion years ago. The earth is slightly flattened at the North and South Poles, it would look like a perfect ball if you were far out in space.
