How does the solar nebula theory explain……?
The inner terrestrial planets have similar mean densities; none of them accreted much low density material, which was still vaporized in the hot inner Solar System. As we move to Mars and the Asteriods, more volatile condensates like water were available, lowering the mean density. But these bodies were not able to grow large enough to gravitationally accrete any gasses. Jupiter and Saturn have low densities due to their high proportion of gas. So far so good. However, we now see that the densities of the outermost bodies increase again. This is due to a somewhat subtle chemistry effect. In the much colder outer Solar System, carbon is able to bind with oxygen to form carbon monoxide gas, which mostly gets blown away. So, most of the carbon gets tied up in this gas and is not available to form low density methane condensates. Ordinarily, this low density methane ice would be accreted onto the planet; since it’s not, the planet has a higher proportion of heavy materials, like water and
