Why are the Inner Planets composed mostly of rock and the Outer Planets composed mostly of
gas?

When the solar system was forming, all planets, both inner and outer planets, in the beginning had nearly identical proportions of hydrogen and helium gases as well as rocks in their cores. The proto-planets that would become Mercury, Venus, Earth, and Mars probably had the same thick atmospheres as the Jovian gas giants.

However, when fusion started up in the Sun, it is thought that a massive "seismic" explosion started from the Sun's core and blew outwards. This formed a wave of concussion that blew away the thick gas atmospheres of the inner planets much like a candle's flame is blown off its wick when you make a sudden puff.

The inner planets were stripped bare down to their rocky cores and only formed a secondary atmosphere once radioactive heat and volcanic activity ignited their cores to force volatile gases to the surface. Jupiter, Saturn, Uranus, and Neptune were too far away for this concussion wave to blow away their thick atmospheres so they still have them today. Some of the escaping gases could have been captured by the outer planets, making them slightly bigger than they originally were.

I know it's a lot but I didn't know how to summarize it in the best way...Good Luck!

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