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
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.
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?
Random luck. Most solar systems is gas planets sometimes even closer to it’s sun than Mercury is to ours then maybe rocky planets.
Now it might only seen like this because of our technology only able to detect massive planets Neptune size and bigger. Even the super earths are close to Neptune’s size. So until we get even better cameras or have a new detection method other than checking for the star wobble that massive mostly gas planets can do, it’s going to look like most solar systems we detect that has planets will have gas planets near their sun.
The random chance aspect would have been more apparent if you included the dwarf planets and re-thought the ‘rocky’ planets.
We recently (kinda arbitrarily) redefined the definition of a planet to be more strict, and that is what lead to the rock->gas ordering, when a slightly different definition would have lead to rock->gas->rock or rock->gas->rock->gas->rock ordering.
And the idea of gas vs rocky is more a function of size. There aren’t going to be giant rocky or tiny gas because amount of gravity vs amount of wind from the stars is what allows those compositions to exist.
So, as has been observed in other star systems, the ordering on size (and therefore, composition) can also be random.
This blog has a series of posts on the Solar System’s story. These two posts are relevant to your question and are worth a read:
https://planetplanet.net/2022/06/28/growth-and-migration-of-the-giant-planets/
https://planetplanet.net/2022/06/29/formation-of-the-rocky-planets-choose-your-own-adventure/
Solar wind shoos lighter things away, so the rocky things stayed closer and formed the rock planets while the lighter things went farther away and formed the gas giants
Trained Geologist here. I’m noticing a lot of misinformation in this thread. You’re describing the concept known as the “ice line.”
A temperature gradient exists in our solar system. It’s hotter closer to the sun, and colder farther away. The elements that comprise the rocky terrestrial planets have freezing points much much higher than the gas giants. Naturally, these were able to condense into solids much closer to the sun.
Similarly, the gas giants were only able to form once it was cool enough for relatively volatile gasses in the solar system to freeze.
In both instances, these frozen grains were slamming into one other as they orbited the sun and accreted into planetesimals. Over time these planetesimals continued to slurp up any surrounding material within their growing gravitational fields, becoming mature planets.
So in short, this is principally a consequence of the different freezing points of different materials in our solar system.
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