Why do gas giants have clearly delineated surfaces, whereas the Earth's atmosphere fades into space?
Why do the solar system planets go rock-gas-ice instead of rock-ice-gas when moving away from the sun?
Why do the gas giants in the Solar System have comparatively large orbits compared to the inner planets?
What's the critical mass of a rocky planet before its gravity traps enough gas in its atmosphere to start becoming a gassy planet
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