The April 27 issue of Nature has an article on how the outer planets got their tilts. The Planetary Society did a writeup on this article. You can find it here:
http://www.planetary.org/blog/article/00000553/
from the Planetary Society article:
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...Uranus and Neptune begin much closer to the Sun than their current positions, at about 13 and 14 AU. They stay pretty comfortably in those positions for about 100,000 years...
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This doesn't make sense to me. Uranus and Neptune at 13 & 14 AU would very heavily perturb themselves. Their comfortably round orbits would last only until their 1st conjunction, and at those distances, they would have a conjunction about every 500 years. After passing conjunction, they pull each other out of round orbits, into orbits with high eccentricity, sometimes entering orbits that cross each other, and sometimes switching places. This is all within a few hundred years.
The Nature paper gives a diagram, showing that Uranus and Neptune peacefully co-exist in round orbits for about 100,000 years. Then it shows their orbits in utter chaos after Saturn and Jupiter migrate into a 2:1 orbital resonance. But my simulation shows the utter chaos beginning immediately, without the help of Jupiter or Saturn.
If they were less massive at that time, that might help explain things, but according to the Nature paper:
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...After a short phase in which the giant planets acquired most of their mass from the surrounding gas, they became immersed in a residual disk of small planetesimals. The exchange of angular momentum with these planetesimals caused the outer planets to migrate far from their birthplaces...
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So it seems to me like the author of the Nature paper is considering fully or nearly-fully formed planets. But I find that even with half their present masses, Neptune and Uranus can not peacefully co-exist at 13 & 14 AU.
Any thoughts?
Here's some screen shots showing the paths of a fully-formed Uranus & Neptune starting in circular orbits at 13 & 14 AU from the Sun. Within a few hundred years, they've crossed each other's orbits and swapped places. This completely contradicts what the Nature paper diagram shows.
