This drawing depicts a position where the formation of ice became important.
Click on image for full size
The position of Neptune when gas changed to ice
The position of the planets in the solar nebula greatly affected their 1. size and 2. composition. This is because of the effect of how cold it was in the nebula.
1. The nebula was a lot warmer close to the proto-sun. The blue line shown in the picture shows the point at which the temperature became cold enough for gases to become ice. At this point and further out, the materials that forming planets (proto-planets) began to extract from the cloud were ice, as well as rocky material and gas molecules. Retention of ice resulted in these proto-planets becoming giant, massive planets. (The same thing happens to raindrops when it becomes cold enough for the raindrops to become slushy snow drops, the drops are more massive). Planets which formed closer to the proto-sun were smaller, and more rocky.
Neptune was very far out in the nebula, however, where the cloud was not only becoming colder, but becoming thinner as well. This meant that there was less gas to sweep up, so proto-Neptune was not able to sweep up as much gas as the proto-Jupiter. Thus Neptune is much smaller than Jupiter.
2. Neptune (unlike Jupiter) is made of complex molecules, and not a simple molecule like hydrogen. That is because near the proto-Neptune, the temperature was so cold that even methane and ammonia, which normally would drift away, solidified into ice and were drawn to the forming planet.
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