Where did the nebula go?
Solar wind, heat, and light pressure drove the gas away.
What about the left over planetesimals?
- Most of the rocky ones in the inner solar system eventually collided with planets. (That’s why the rate of impacts was high 4 Gya, but is low now.)
- There’s about 20,000 left over mostly between Mars and Jupiter (Asteroids!)
- Jupiter’s gravity prevented a planet from forming there.
- Encounters with the giant Jovian planets kicked most of the remaining icy ones into the outer solar system or interstellar space
- These are comets!
- The encounters would kick them in any direction. (This explains why comets aren’t concentrated in the plane of the solar system.)