by JOHN MATSON
The wandering orbit of Jupiter at the dawn of the solar system may have had wide-ranging effects
Drifters: In this artist’s conception Jupiter and Saturn lurk close to the inner solar system. Their forays close to the sun may have guided the formation of the terrestrial planets. PHOTO/Dan Durda/SwRI
The planets of our solar system follow nice, predictable orbits, but it was not always so. In the chaotic early days of the solar system, Jupiter and its fellow giant planets seem to have migrated from their birthplaces into the stable orbits that we observe today.
The migration of giant planets has been invoked to explain a number of features of planetary systems, such as the uneven spacing among the objects of the Asteroid Belt in our solar system. Migration would also explain the huge planets in other planetary systems known as “hot Jupiters” that orbit extremely close to their host stars, far closer than where they could have plausibly formed.
Now a new study, published online June 5 in Nature, demonstrates that a peculiar migration of Jupiter—first inward, then outward—could account for Mars‘s relatively small size. (Scientific American is part of Nature Publishing Group.)
Researchers have built a number of numerical simulations to try to trace how the planets formed in the tens of millions of years following the solar system’s birth. But often, Mars has posed a problem. “As things would slowly build up, essentially what always would happen is you’d end up with planets of about the size of Earth and Venus where they should be,” says lead study author Kevin Walsh, a planetary scientist at the Southwest Research Institute in Boulder, Colo. “But the object that ended up around the location of Mars was the size of Earth as well.” In actuality, compared with Earth, Mars is only about half its diameter, and about one tenth its mass.
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