Can the theory of formation of the solar system explain all the observables?
This is explained by the Lewis Model. In the early Solar System, which was a cloud of gasses, the inner parts were warmer than the outer parts. In the inner region, only things like metal or rock could condense, so the inner planets (Mercury, Venus, Earth, and Mars) are composed chiefly of metal and rock. As you move out to the cooler outer regions, it gets cool enough for things like water ice, and then ammonia and methane ice to condense.
The reason why the outer layers of the gas giants (Jupiter, Saturn, Uranus, and Neptune) are composed of lighter elements is that these planets grew larger than the Earth, quickly. There are two reasons why. One is that, in the outer regions, it was cool enough for a larger range of materials to condense -- not only rock and metal, but also things that condense at cooler temperatures such as water ice and ammonia ice, so there was more "raw materials" for the planets to be made of. The other reason is that ice sticks together better than rocks and metals, so when the ice that had condensed in small pieces ran into other pieces of ice, it tended to make bigger pieces, rather than bounce off or fragment as pieces of rock do. The outer planets originated as big planets made of ice and rock. The were massive enough that their gravity allowed them to accummulate hydrogen and helium, which the inner planets did not have enough mass to hold on to, and grow to their current titanic proportions.
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