If we discount dwarf planets after the asteroid belt all planets are gas, is there a specific reason or is it just coincidence
In: 2019
The solar wind is strong enough closer in to clear out any large regions of gas and force it out further into the solar system. Which just leaves the heavy/rocky parts remaining. Or the inner planets initially formed as gas giants too but the solar wind stripped away most of their atmosphere. Gas giants may have rocky parts too, but small compared with their overall size.
Our solar system formed as a massive protosolar disk of spinning gas and dust. Gravity pulled light gasses into the center – eventually, the gravitational pressure and gas density was enough to initiate hydrogen fusion, and the protostar became the sun. Dust was slowly accumulating into rocks and asteroids, and eventually the core of the rocky planets, and the gas giants. Meanwhile, the solar wind from the new star was now pushing light elements and molecules (hydrogen and water) away from the sun. This left the materials for primarily rocky planets in the inner solar system, and pushed the materials for gas giants out beyond the orbit of Mars. Earth was later bombarded with icy comets, which is how we have so much water.
This is the commonly accepted model of our solar systems formation. There are other proposed models that have the gas giants forming close in and migrating to the outer solar system later.
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My question would be, why are gas planets even a thing? What constitutes a planet – with no solid mass – a planet?
Most of the heaviest elements on earth are located in the core of the planet. For the same reason that gasses are in the atmosphere and rocks are on the surface and that ice floats on water. It’s all about density.
When you have a giant gas cloud at the birth of the solar system then the heavier elements like rocks and metals are generally drawn closer to the largest source of gravity which means you will find more rocks and metals closer to what became the suns and then so the materials forming bodies at a close distance to the sun have more metals and rocks where as those further away are primarily lighter elements like gasses.