Washington - December 8, 1999 - In our Solar System there are two 'gas giants' (Jupiter and Saturn), having rocky cores surrounded by a much more massive hydrogen and helium atmosphere, and two `ice giants' (Uranus and Neptune), which have icy mantles around their cores and only a thin outermost gas layer.
Theorists have struggled to explain how Uranus and Neptune formed at their present locations in the Solar System, because at such distances the disk density would have been low, and the time required for these planets to grow is thought to be longer than the age of the Solar System.
Astronomers Edward Thommes and Martin Duncan of Queen's University, with Harold Levison of the Southwest Research Institute in Boulder Colorado, propose a solution to this problem in the December 9th issue of Nature magazine.
Their computer simulations begin with the assumption that Uranus and Neptune started out as rocky cores (just like the Jovian and Saturnian cores) in the Jupiter-Saturn region, and were violently tossed out by gravitational scattering from Jupiter and Saturn when one or both accreted large amounts of gas.
The ejected planets remained in highly chaotic orbits for a short period of time (a few hundred thousand years), after which they settled down (by means of gravitational interaction with the small bodies in the disk beyond Saturn) and gradually migrated out to their present nearly circular orbits.
The authors start their simulations with four rocky cores embedded in a narrow zone of the solar nebula and find that in 50% of their trials, the giant planets end up more or less where they are now.
The model is unusual in suggesting that all four giant planets originated in the same region of the solar nebula - within a ring 5 to 10 astronomical units in distance from the Sun (1 AU = Sun-Earth distance).
This is substantially narrower than previous estimates of 10 to 20 AU for the birthplace of Uranus and Neptune, and far inward from their present locations of 19 and 30 AU, respectively.
Because the nebular disk density and orbital rotation is much higher at 5 AU than at 30 AU, the timescale for growing planetary cores is much reduced in this model, and so provides a solution to part of the Uranus and Neptune formation mystery.
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