Why are the inner planets solid, and the outer planets made of gas?

The proto-planets that would become Mercury, Venus, Earth, and Mars most likely shared the thick atmospheres of the Jovian gas giants during the solar system's formation, with nearly equal amounts of rocks and hydrogen and helium gases in their cores for all planets, inner and outer.

But when fusion began inside the Sun, a massive "seismic" explosion is thought to have originated at the core of the Sun and blown outward, creating a concussion wave that blew away the dense gas atmospheres of the inner planets, much like a sudden puff blowing out of a candle.

The outer planets may have absorbed some of the escaping gases, causing them to become slightly larger than they were originally, but the inner planets were stripped bare 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 retained them.

The inner planets, Mercury, Venus, Earth, and Mars, are solid because they formed closer to the Sun where temperatures were high enough for rocky materials to condense and solidify. The outer planets, Jupiter, Saturn, Uranus, and Neptune, are made of gas because they formed farther from the Sun where temperatures were lower, allowing gases like hydrogen and helium to remain in gaseous form.