# The Solar System explosion in the Nice model

This video depicts one variant of the Nice model (pronounced "neese", like the city in France). I'll briefly describe it in case the link ever dies. Here is the initial configuration:

The four coloured circles indicate the orbits of the four Solar System (SS) gas giants, red for Jupiter, yellow for Saturn, blue for Neptune, purple for Uranus. The green dots represent a hypothesized disk of planetesimals. Initially, Neptune has an orbit smaller than Uranus. In the Nice model, the outer gas giants occasionally scatter small objects inward, and by conservation of $E$ and $L$, move outward. The inward-scattered objects then often scatter off of Jupiter and are ejected (or nearly so) from the SS, so Jupiter slowly migrates inward. Eventually Jupiter and Saturn cross their 2:1 mean-motion resonance, which pushes Saturn out into encounters with Uranus and Neptune, scattering the two outer giants into the planetesimals. The result of the system re-stabilizing is an explosive disruption of the planetesimals, and Neptune "switching places" with Uranus, giving this configuration at later times:

The whole model seems to hang together rather nicely (ahem, no pun intended), which is presumably why it's been fairly successful. My question is what prevents one of the two ice giants (Uranus, Neptune) from being ejected in that big shake-up. With their orbits swapping places, my intuition is nagging me that one of the orbits (probably Neptune's) shouldn't survive the process. Is there something that is forcing the two planets back to a stable configuration? Or is this just one lucky scenario, where more generically there's a substantial probability that one of the giants is ejected?

• Perhaps you could clarify why this hurts your intuition? I guess if you picture this process happening in a continuous manner, you are thinking at some point during the orbit swap, Neptune and Uranus will have a close approach. Is that it, or is there something else which doesn't seem right to you? – levitopher Feb 17 '15 at 17:40
• @levitopher I guess I am worried about a close approach. It's not guaranteed of course, but the odds for/against are one of the things I'm interested in. – Kyle Oman Feb 17 '15 at 19:44
• The video depicts the original Nice model, and not the Nice 2 model, right? – HDE 226868 Feb 18 '15 at 1:19
• @HDE226868 I believe that this is the original Nice model, yes. If anyone knows otherwise, let me know. – Kyle Oman Feb 18 '15 at 1:25
• Not just pronounced like the city in France, it is the city in France! – Rob Jeffries Jan 24 '16 at 21:05