from the collapse of a molecular cloud
consisting mostly of hydrogen, but also
containing heavier elements. As the cloud
collapsed, its central region began to heat
up until eventually the hot ‘proto-Sun’ was
formed. The planets formed by the
accretion of material from within this cloud.
Due to their high boiling points, only metals
and silicates could exist in solid form in the
hot, inner regions of the early Solar System.
More volatile elements either remained
gaseous or were blown away by the Sun’s
energy. Since this heavy material existed
mainly in the inner regions of the Solar
System, the planets that formed there are
small and rocky. In the cold outer reaches
of the Solar System, compounds with low
melting points could remain in solid form.
These ices were also much more abundant
than the heavier elements. The outer
planets were therefore able to grow much
larger, and held onto large atmospheres of
hydrogen and helium gas.
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