The interesting thing is that different kinds of things condense at different distances from the protostar . Near the star, the temperature is higher, so only metals and rock can condense out it remains too hot for water or other gases to condense. Far away from the star, everything can condense, including water and other volatile gasses. Terrestrial planets are rocky and have higher densities, basically because at the distance of the terrestrial plants from the sun, it is so hot that only rocks and metals could condense. In contrast, at the distance of the Jovian planets, everything condenses because it is cooler so the average density is lower.
Particles accrete .
This means that the initial condensates run into each other, sticking together, and slowly building up into large clumps. Once the clumps become as large as 15 times the mass of earth, their own gravity will become important , and condensates will start to fall onto those large clumps because of their gravity. It also explains their low density , because their gravity allows them to hold on to very light gasses including hydrogen and helium that basically escape from terrestrial planets. Next time we will finish talking about the formation of the solar system, discuss planets around other stars, and start our discussion of the terrestrial planets.