Why are the four inner planets small and rocky while the four outer ones are giant balls of gas? The border is the frost line, an invisible circle in the gas-and-dust disc inside which only rock and metal can condense, while beyond it water ice, CO₂ and ammonia freeze solid too, leaving far more building material. Before that the molecular cloud had to collapse, and conservation of angular momentum spun it faster and flattened it into a disc, exactly as a skater pulls in their arms. The whole chain wrapped up around 4.6 billion years ago.
Why are the four inner planets small and rocky while the four outer ones are giant balls of gas? The border is the frost line, an invisible circle in the gas-and-dust disc inside which only rock and metal can condense, while beyond it water ice, CO₂ and ammonia freeze solid too, leaving far more building material. Before that the molecular cloud had to collapse, and conservation of angular momentum spun it faster and flattened it into a disc, exactly as a skater pulls in their arms. The whole chain wrapped up around 4.6 billion years ago.
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