Magazine article Natural History

Rock Collections

Magazine article Natural History

Rock Collections

Article excerpt

In order to form gas-giant planets such as Jupiter and Saturn, consisting primarily of hydrogen and helium, it has been widely held that first a solid core had to form-one that was roughly ten times the size of Earth, and one that would form before the solar nebula dissipated in about one to ten million years. Modelling such an event has been a challenge. Recent models have shown how centimeter-to-meter-sized pieces of stone, or "pebbles," are first attracted to each other by aerodynamic drag, and then gravity binds them together to form "planetisimals," objects 100 to 1,000 kilometers in size. Like a rolling snowball, these planetesimals gather leftover pebbles and form cores ten times the size of Earth in only a few thousand years.

When computer simulations were run, however, these models did not reproduce the structure of the Solar System, according to Harold F. Levison of the Southwest Research Institute and NASA Solar System Exploration Research Virtual Institute in Boulder, Colorado. …

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