This drawing depicts a position where the formation of ice became important.
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The position of Neptune when gas changed to ice
The position of the planets in the solar nebula affected how big they became and what they were made of. The blue line in the picture shows where it became so cold that ice began to form.
Planets that formed beyond the blue line drew ice to them, plus molecules of rocks and gas. Keeping ice resulted in these forming planets becoming big and full of gas. The planets that formed close to the sun became small and full of rock.
But, as far away as Neptune was in the nebula, the cloud was thin and there wasn't as much gas, so proto-Neptune was not able to sweep up as much gas as the proto-Jupiter. Thus Neptune is much smaller than Jupiter.
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