This drawing shows a position where ice became important.
Click on image for full size
The position of Uranus 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 Uranus was in the nebula, the cloud was thin and there wasn't as much gas, so proto-Uranus was not able to sweep up as much gas as the proto-Jupiter. Thus Uranus is much smaller than Jupiter.
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