Exploratour - Evolution of the Solar System
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, beginning with the forming Jupiter, drew ice to them, plus molecules of rock 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.
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