This huge cloud of dust and gas is the orion Nebula. Inside it stars are forming. About 4,600 million years ago the sun and the planets were forming inside a nebula like this.
Most of the scientists believe that the sun and the other planets are formed about 4,600 million years ago from a huge cloud of tiny, solid particles and gases called a NEBULA. The solid particles and some of the gas had been thrown out of earlier stars that had died. The nebula began to shrink and spin, collapsing inwards because of its own gravity. Soon, material near the centre was colliding at tremendous speeds and giving out so much heat that a glowing star, the SUN, was born.
The rest of nebula formed into a ring around the sun and collision inside this ring built up the planets. For a time the planets were very hot, but they never become hot enough to shine like stars. All the planets were bombard by other much smaller bodies, so that their surfaces became covered with crates like the one still seen on the Moon today.
On the earth, however, wind and rain have gradually worn most of the crates away. The earth original atmosphere probably contained large amounts of carbon dioxide . As plants developed on earth they used the carbon dioxide to make food and released the oxygen on which all animal life depends.