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Asked by kiand to Heather, Helen, Hugh, Jane, Julian on 16 Nov 2014. This question was also asked by #Niamh #wow.
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Julian Onions answered on 16 Nov 2014:
They form from the gas and test left over from the star forming.
They start of by dust grains sticking together, and slowly getting bigger. Then we don’t really understand the next step, but eventually the grains get to big rock sizes, and start attracting other rocks under gravity.
These get bigger and bigger collecting more rocks, ice, and gas – depending on how close they are to the sun.
Then eventually they’ll have picked up most of the debris left over, and stop growing more or less, and we’ll have planets!
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Helen Johnson answered on 17 Nov 2014:
We think solar systems form out of large clouds of gas and dust called a nebula. As the cloud comes together under gravity, the centre collapses and heats up to form a star. What’s left is a rotating cloud of material, which gets thinner the faster it rotates. This is called a ‘protoplanetary disk’. We saw one of these in action just recently, a pretty fascinating picture: http://apod.nasa.gov/apod/ap141110.html
Within the disk, some regions are denser than others, and the matter starts to clump together. As the clumps orbit around the star they attract more material because of their gravity. These are the beginnings of planets! Planetesimals towards the centre are rocky, because most of the gas has already been cleared out by the star (so we get planets like Earth), but in the outer parts there is lots of ice and gas left over (to form the ‘gas giant’ planets like Jupiter).
These ‘baby’ planets continue to grow and will collide with each other. Eventually there will be fewer, larger planets which have cleared out their orbits by either absorbing material or tossing it out of the way with gravitational interactions. Now we have a solar system!
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