The Nice 2 model is a model of the early evolution of the Solar System. The Nice 2 model resembles the original Nice model in that a late instability of the outer Solar System results in gravitational encounters between planets, the disruption of an outer planetesimal disk, and the migrations of the outer planets to new orbits. However, the Nice 2 model differs in its initial conditions and in the mechanism for triggering the late instability. These changes reflect the analysis of the orbital evolution of the outer Solar System during the gas disk phase and the inclusion of gravitational interactions between planetesimals in the outer disk into the model.
Description The Nice 2 model begins with the outer planets in a stable quadruple resonance with each planet in resonance with its nearest neighbors. One example among several potential stable quadruple resonance configurations is Jupiter and Saturn in a 3:2 resonance, Saturn and Uranus in a 3:2 resonance, and Uranus and Neptune in a 4:3 resonance. Interactions with an outer planetesimal disk that is gravitationally stirred by Pluto-sized objects cause the planets to migrate inward while remaining in resonance. During this migration the eccentricity of the inner ice giant increases, leading to secular-resonance crossings. After several hundred million years, the resonant configuration is destabilized during one of these secular-resonance crossings. Gravitational encounters between the planets similar to those in the original Nice model begin shortly thereafter.
Development The Nice 2 model addresses some weaknesses of the original Nice model. The first weakness is the artificial selection of the initial orbits of the outer planets to produce an instability that matches the timing of the Late Heavy Bombardment. The second weakness is the sensitivity of the timing of the instability to the location of the inner edge of the planetesimal disk. The Nice 2 model uses particular initial conditions, derived from the examination of the orbital evolution of giant planets orbiting in a gas disk, which may occur under appropriate circumstances. An instability trigger with no apparent correlation between the timing of the instability and the position of the inner edge of the planetesimal disk is the result of the incorporation of the interactions between planetesimals into the Nice 2 model.
Initial conditions The initial orbits of the giant planets in the Nice 2 model correspond to a predicted orbital structure of the outer Solar System at the end of the gas disk phase. Models of giant planets orbiting in a gas disk predict that they would migrate toward the central star at a rate dependent on the mass of the planet and characteristics of the disk. In a system with multiple planets this migration can result in the convergence of the planet's orbits and their capture into mean-motion resonances. Investigations focusing on Jupiter and Saturn demonstrated that they can be captured in a 3:2 or 2:1 resonance depending on the characteristics of the protoplanetary disk. After capture into resonance, the gaps that Jupiter and Saturn formed in the disk's density distribution may overlap and their inward migration may be halted or reversed. When Uranus and Neptune are added in turn to the model they are captured into further resonances with the capture of the outer ice giant resulting in the inner ice giant having a higher eccentricity than the other planets. The result is a system in a quadruple resonance. A number of stable configurations have been identified with the particular final configuration depending on the starting locations of the planets.
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