This drawing shows how forming planets drew gas to themselves.
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Image from: The New Solar System
How a proto-planet sweeps up nearby material
As shown in this picture, while they were forming in the solar nebula, the nuclei of the planets-to-be (called protoplanets) drew material to themselves from the cloud of gas and dust around them. The bigger protoplanets were able to attract even more gaseous material unto themselves.
Big protoplanets such as Neptune were positioned far out in the original nebula where there was less gas, and so these protoplanets were not able to sweep up as much gas as other giant planets.
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