The five-planet Nice model is a numerical model of the early Solar System that is a revised variation of the Nice model. It begins with five giant planets, the four that exist today plus an additional ice giant between Saturn and Uranus in a chain of mean-motion resonances.
After the resonance chain is broken, the five giant planets undergo a period of planetesimal-driven migration, followed by a period of orbital instability with gravitational encounters between planets similar to that in the original Nice model. During the instability the additional giant planet is scattered inward onto a Jupiter-crossing orbit and is ejected from the Solar System following an encounter with Jupiter. The model was first formally proposed in 2011 after simulations indicated that it was more likely to reproduce the current Solar System than a four-planet Nice model.[1]