Why are the inner planets solid, and the outer planets made of gas?
1 Answer
A massive concussion wave from the proto-Sun's initial ignition is thought to have blown away the thick atmospheres of the inner protoplanets but left the outer planets untouched.
Explanation:
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.
