Sorry Pluto
In: Physics
The solar wind from the early sun blew all of the lighter elements like hydrogen and helium away from the inner planets, leaving them only rocks to build from. Once you pass Martian orbit, the solar wind had spread out enough to allow planetoids to capture the hydrogen and helium in the solar system accretion disk. Those planetoids eventually became the gas giants that we see.
They are formed from the heavier elements which make them rocky planets and the heavy elements are a lot rarer than hydrogen and helium which make up a lot of the gas giants. At a distance from the Sun there was more material to hoover up as the outer planets were forming especially the gases.
People are saying that it is because the sun’s photonic energy pushed lighter elements farther away where they accumulated into the outer bands. But this is bull crap. There have been many many solar systems found that have huge gas giants closer to their sun. Therefore this theory is bogus.
I think the real answer is that it is just coincidental. What is clear is that some planets form earlier, larger, and faster than others and so they tend to suck up a lot of the material in the early solar system (jupiter, saturn).
And depending where they happen to form, their gravity intermingles with the gravity of the sun to stabilize/destabilize rest of the planets that might form. This is why the inner belt right next to jupiter (between mars/jupiter) is full of asteroids that never formed into a planet. Jupiter is destabilizing it.
Basically it was a combination of two things.
In the early years of our solar system you could think of it as a giant cloud of hot dust and gas spinning very fast.
As the solar system spun, lighter elements such as the gasses were pushed outwards to the edge of the solar system and heavier elements like metals were not pushed as far. Think of how when you spin a ball on a string and if you let go it will fly away. Same concept.
In addition to this the sun helped push these elements as well throguh what’s called solar wind. This is basically the light from the sun impacting the elements and pushing them. This act the same way, pushing lighter elements more than heavier elements.
After the universe was separated like this, dust and gas started to coalesce into larger and larger bodies until we got the planet we know today with mostly gas in the outer region and mostly rocks in the inner region.
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You would think the answer should be simple, but it could actually be quite complicated. The real answer is that we don’t know for sure, but there are some pretty good guesses. The guesses take the form of different models that try and explain what we see today, based on observations. Some of these observations are made right here in our backyard, while other observations are made by looking at stars and planets that formed far away, in other solar systems.
Our solar system began as a cloud of gas and material floating in space, that slowly began to condense. Eventually, the material falling in on itself was under so much pressure that it created the Sun. The Sun sprang into existence, but left behind a lot of swirling gas and dust known as a [protoplanetary disk.](https://en.wikipedia.org/wiki/Protoplanetary_disk) Just as the sun formed through small molecules smashing into each other over and over again over a long period of time, other objects in the disk had been, and would continue to collide. These were the beginnings of the planets, known as [Planetesimals.](https://en.wikipedia.org/wiki/Planetesimal)
This early period would have been a very chaotic time for the solar system, planetesimals would have been crashing in to each other, forming larger and larger bodies, while others were being ejected from our solar system entirely.
The inner solar system, the region closest to the sun, was so hot that it prevented most volatile materials such as water or methane from condensing and forming planetesimals, this is one reason why even today most of the water in our solar system exists in the outer regions. This contributed to the inner solar system planetesimals being comprised mostly of silicate materials.
Meanwhile, in the outer solar system, planets were forming beyond the [frost line](https://en.wikipedia.org/wiki/Frost_line_(astrophysics)) which let icy compounds to remain as solids. Now a few other things were happening: as other comments mentioned, the planetary disk was spinning around the sun. The disk was spinning faster close to the sun, and slower further away from the sun. As the sun’s solar wind pushed material into the outer solar system, the slow movement of this outer disk prevented more material from falling back into the inner solar system—effectively moving more and more material into the outer solar system. With more resources to work with and collide into, the proto-planets in the outer system slowly began to amass more and more material. Much of this material was left over gas, which allowed the gas giants Jupiter and Saturn to collect such large amounts of material.
There are two complicating factors with this model: The first is that Jupiter appears to have moved at some point into it’s current position, and the other is than Uranus and Neptune were likely *pushed* into their current positions, but did not form there. These two ice giants are believed to have formed near where Jupiter and Saturn did, and moved outwards.
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