It is likely that a shock wave from the explosion of a nearby star caused the solar nebula to collapse. The nebula most likely began with a slight overall rotation. As the cloud contracted, it began to spin faster. The gravitational attraction between particles became stronger as the solar nebula shrank, accelerating the process.
Formation of the Protoplanetary Disk
As the solar nebula rotated faster, it began to flatten out. Over about 100,000 years, a large disk-shaped cloud of dust and gas called a protoplanetary disk formed. Looking like a giant fried egg rotating in space, the disk was densest in the center and thinner toward the edges. The planets would eventually form from the outer parts of the disk.
Nearly all of the mass of the solar nebula, about 99.9 percent, became concentrated near the center. The sun would eventually form in this region. The enormous density at the center of the disk increased until the temperature reached a few million degrees. Then, about 10 million years after the formation of the solar nebula, nuclear reactions began to fuse hydrogen into helium. Our sun was born.
Planetesimals and Protoplanets
The nebular theory also explains how the planets could have formed. Within the contracting protoplanetary disk, dust grains collided frequently. Weak surface forces held these dust grains together, forming loose balls of dust. As these balls of dust collided, they grew larger and larger.
Scientists estimate that, after a few million years, there were about a billion planetesimals revolving around the sun. Planetesimals were asteroid-like bodies that eventually combined to form planets. Planetesimals grew by accretion, the process of adding mass by colliding with other planetesimals. Once planetesimals became larger than about a kilometer in diameter, they began to exert a significant gravitational attraction on nearby objects. The planetesimals attracted more material, causing them to grow even faster. The result was the accretion of planetesimals into a much smaller number of moon-sized protoplanets. These protoplanets eventually joined to form the current planets in a series of immense collisions.The nebular theory provides an explanation for the current motions of the sun, planets, and most moons. Nearly all are now revolving in the direction that the protoplanetary disk was spinning.
What were planetesimals?
Quick Lab
Forming Planets
Procedure 
Cut two strips of newspaper, each 2.5 cm by 30 cm. Tape the ends of each strip together to make two equal loops.
Cross the loops with one nested in the other to make a sphere shape. Tape the loops together as shown.
Gently push the pencil through the crossing points. The end with more tape should be fixed in place and the other end should slide freely on the pencil.
Move the free end about 8 cm from the fixed end. Hold the pencil horizontally and spin it. Observe the movement of the loops.
Analyze and Conclude
Observing How did rotation affect the loops?
Using Models How does the model help explain the fact that the planets' orbits are close to the ecliptic plane?
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