Planet formation begins at distances in excess of 50 AU from the mother star, when random variations in the density of the protoplanetary gas cloud begin to attract more gas and so grow under the force of gravity.
Inside these loose clumps, called giant planet embryos, any rocky material aggregates at the centre forming a rocky core. These cores all rotate in the same direction as the original gas cloud because they from by the gravitational collapse of the cloud rather than by random collisions.
As the cores are forming, the embryonic planets interact with the mother star’s gas cloud causing them to spiral inwards.
When the embryonic planets get closer than this critical radius, they loose their gas envelopes leaving behind terrestrial rocky planets like ours.
Incidentally, at the critical radius, the inspiralling planets discard not only gas but any solids still mixed up in their outer atmospheres. This radius corresponds to the asteroid belt in our system. This new thinking explains for the first time how the belt formed and why it separates the gas giants from the terrestrial planets.
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One impressive feature of this model is that it naturally accounts for the structure of the Solar System, with the distant gas giants separated from the inner rocky planets by an asteroid belt. No other model does this so elegantly.
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