A few million years after the solar system was born, a young Jupiter may have journeyed inward, with an early Saturn following (chart shows masses and distances of outer planets from the sun over time). This trip could explain Mars’ small size (top right, in comparison with Earth); as Jupiter moved, it swept away much of the material that would have otherwise come together to form Mars.
Source: (graph): K.J. Walsh et al/Nature 2011 Credit: (Earth and Mars): Walter Myers/Photo Researchers, Inc.
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