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In Brief

14 January 2009

Were Mercury and Mars separated at birth?

When the four terrestrial planets - Mercury, Venus, Earth and Mars - are placed side by side, it's clear their size distribution is close to symmetrical, with the two largest lying between the two smallest (Illustration: NASA)

LINE up Mercury, Venus, Earth and Mars according to their distance from the sun and you’ll see their size distribution is close to symmetrical, with the two largest planets between the two smallest. That would be no coincidence – if the pattern emerged from a debris ring around the sun.

Brad Hansen of the University of California, Los Angeles, built a numerical simulation to explore how a ring of rocky material in the early solar system could have evolved into the planets. He found that two larger planets typically form near the inner and outer edges of the ring, corresponding to Venus and Earth. A number of smaller bodies also form within the ring. These are typically scattered away by the larger two, but if they experience collisions on the way, they can end up in stable orbits similar to those of Mercury and Mars. Once beyond the ring, they cannot acquire mass and so remain pint-sized.

“This is nicely consistent with various properties of Mercury and Mars,” says Hansen, who presented the work at the American Astronomical Society meeting in Long Beach, California. Both small planets have features that could have been caused by giant impacts. In one run of Hansen’s simulation, Earth received a smash, too, much like the one thought to have created the Moon.


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