Page 57 - Curriculum Visions Dynamic Book

after a while all of the silicon atoms would have reacted. That limits the amount of rock that can form. There are a thousand hydrogen atoms for every oxygen atom. The reaction of these two elements makes water the most common molecule in the universe but still leaves an immense amount of hydrogen that has not reacted.
Close to the Sun it is too hot for water to freeze, but from Jupiter outward it is too cold for it to stay liquid. So, objects in the cool regions of the Solar System have much more material to build with (that is, they have rock and water) than those close to the Sun (which have only rock).
As planets grow, their gravity becomes stronger.
When they are ten times the mass of the Earth, they have a sufficiently powerful gravity to attract helium and hydrogen atoms, and so they can continue to grow using the most plentiful material left in space. That allows ice-cored gas planets to become more and more massive—surrounded by hydrogen and helium.
In the outer regions of the Solar System the planets grew to such a size that they attracted material from around them, which then formed a disk like a miniature Solar System. It is the material from which the present rings of the giant planets were formed.
Planets like Jupiter would also have been so hot as they grew to full size that ice could not freeze close to them. As a result, the kinds of moons found away from the giant planets change, just as the nature of the planets changes away from the Sun. In the case of Jupiter the inner moons are rocky and have a high density, while the outer ones have a much lower density.
This explanation suggests why the inner planets are rocky and of high density and have matching moons, while the outer planets have a lower density and have some high- density moons and some low-density moons.
A timeline for the Solar System
How long would all of this take? Present estimates suggest that the planets formed in a very short time. A body
10 kilometres across could form in a thousand years, a planet in 10 million years.
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