How are planets formed?
At the beginning there is a cloud of dusk and gas, as theory goes. The cloud collapses to form a star in the centre surrounded by a rotating disk. (Think of a spinning clump of dough flattening into the shape of a pizza.) Eventually small bits of the material in the disk begin to stick together somewhat like dust bunnies are formed. If they get larger they attract more material thanks to gravity. Planetesimals are formed that grow into planet embryos by collisions. How the embryos turn into big gas giants like Jupiter or small rocky planets like Earth is still discussed. Since the discovery of the big exoplanets that are very near to their host stars most astronomers think that the planets probably migrate through their planetary system during formation. With computer models they try to simulate the process.