Sorry Pluto
In: Physics
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.
Latest Answers