This is a schematic showing the forming solar system. Click on image for full size version (9K GIF)

Scientists think that the solar system formed out of a cloud of hydrogen and helium gas, which was spinning.

Because the cloud was spinning, it flattened into a frisbee shape, just like a ball of pizza dough becomes flat when a chef spins it in the air. The planets and the sun started to form after the cloud became a flat disc.

As the cloud flattened, the gaseous material inside was forced to begin changing into solid form.

These little particles of solid material were soft and sticky, and further clumped together to form larger balls of solid material whenever they touched each other, somewhat the way "silly putty" does. Eventually only a few large clumps of this material remained in the forming solar system, and they became the core of "protoplanets".