Short period comets have orbits that take them into the inner solar system. The orbits are usually close to the plane of the solar system. Source in Kuiper belt lies beyond the orbit of Neptune. Planitesimals collide and stick together to form planets and moons but coalescence of asteroids is prevented by influence of Jupiter's gravity. Terrestrial planets lose whatever hydrogen and helium they have.
Solar wind blows away remaining gases.
Some problems that still need to be worked out . Angular momentum of sun is too low to agree with this theory. Problem appears to be solved by the theory that Pluto belongs to Kuiper belt. No explanation of how planets condense. Also doesn't explain difference between Terrestrial and Jovian planets.