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Originally Posted by jehane
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THe important thing here is that size is *not* the only factor determining mass. The density, which is determined by the composition of the planet, is a huge factor. Hence the differences between the rocky planets (inner 4) and the gas giants (outer 4).
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Even within the group of rocky planets the impact of density shows. Mercury, which is rather dense, has about the same surface gravity as the distinctly larger Mars, which is less dense.