1. Begin with a big ball of gas in outer space. If it is big enough, it will undergo gravitational collapse and fragmentation ("Jeans collapse").

2. As the fragmenting gas cores get dense and hot enough, nuclear reactions begin in the center. The result? Star Formation!

Star Forming Gas Clouds

3. As these gas clumps collapse into stars, they also form surround disks of gas. Why? Rotation. Because of centrifugal forces, collapse is retarded along the equator of the rotating cloud. A flattened disk forms.

We see this in Orion!

4. The gas disk cools, forms dust and rocks (and ices). These particles collide with one another and grow in size.

5. Runaway growth: As these particles grow in size, their own gravity takes over and accelerates their growth ("rich get richer"). "Planetesimals" form in a few million years.

6. "Winds" from the young star blow excess material out of the solar system.

7. Over the next hundred millions years, existing planetesimals continue to collide and grow in mass, eventually forming the young solar system.