A nebula will stay hotter toward the center because of the sun, and radiation from the sun would cause less dense gases and molecules to be pushed towards the outer solar system. The core accretion model (Lissauer 1993; Pollack et al. Scientists have two competing but viable theories about planet formation. The core-accretion and disk instability models have so far been used to explain planetary formation. The core of the sun is so important to because with out the core will be hot. Since snow tends to stick to itself, the ball will get bigger as more and more layers of snow are added. The discovery of exoplanets revealed that, around other stars at least, some worlds moved from their natal neighborhood. 5. Another great example that you may have seen is the formation of cotton candy. //-->,