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Asked by 424spaa48 to Heather, Helen, Hugh, Jane, Julian on 18 Nov 2014.
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Julian Onions answered on 18 Nov 2014:
We have planets (we think) because when the sun forms, out of a spiralling cloud of gas and dust, not all of it gets sucked into the centre, but some of it sticks together in orbits, and attracts other nearby rocks, eventually getting big enough for a planet. These then tend to stay where they are after some initial shuffling around. Earth will stay in its orbit for the foreseeable future for instance.
Occasionally during all this shuffling around early on, combinations of planets will come very close to each other, and either end up in collisions (which is how we think the moon was formed), or sometimes kicked out of the solar system altogether, to wander between stars.
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Helen Johnson answered on 19 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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