In astronomy, the Nice (/ˈniːs/) model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary disk. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies such as the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.

The original core of the Nice model is a triplet of papers published in the general science journal Nature in 2005 by an international collaboration of scientists. In these publications, the four authors proposed that after the dissipation of the gas and dust of the primordial Solar System disk, the four giant planets (Jupiter, Saturn, Uranus, and Neptune) were originally found on near-circular orbits with radii between 5.5~17 astronomical units (AU), closer to the sun than the present distance of Uranus, and much more closely spaced and compact than in the present. A large, dense disk of small rock and "ice" planetesimals totalling about 35 Earth masses extended from the orbit of the outermost giant planet to some 35 AU from the sun.

According to the Nice model, the planetary system evolved in the following manner: Planetesimals at the disk's inner edge occasionally pass through gravitational encounters with the outermost giant planet (Uranus or Neptune), which change the planetesimals' orbits. The planet scatters inward the majority of the small icy bodies that it encounters, which in turn moves the planet outwards in response as it acquires angular momentum from the scattered objects. The inward-deflected planetesimals successively encounter Uranus, Neptune, and Saturn (or Neptune, then Uranus, then Saturn), moving each outwards in turn by the same process. Despite the minute orbit change each exchange of momentum produces, cumulatively these planetesimal encounters shift (migrate) the orbits of the planets by significant amounts. This process continues until the planetesimals interact with the innermost and most massive giant planet, Jupiter, whose immense gravity sends them into highly elliptical orbits or even ejects them outright from the Solar system. This, by contrast, causes Jupiter to move slightly inward.

The low rate of orbital encounters governs the rate at which planetesimals are lost from the disk, and the corresponding rate of migration. After several hundreds of millions of years of slow, gradual migration, Jupiter and Saturn, the two inmost giant planets, reach their mutual 1:2 mean-motion resonance, meaning that the period of Saturn is twice that of Jupiter. This resonance increases their orbital eccentricities, destabilizing the entire planetary system. The arrangement of the giant planets alters quickly and dramatically. Jupiter shifts Saturn out towards its present position, and this relocation causes mutual gravitational encounters between Saturn and the two ice giants, which propel Neptune and Uranus onto much more eccentric orbits. These ice giants then plough into the planetesimal disk, scattering tens of thousands of planetesimals from their formerly stable orbits in the outer Solar System. This disruption almost entirely scatters the primordial disk, removing 99% of its mass. Although the scenario explains the absence of a dense trans-Neptunian population, alternative models that achieve the same depletion of trans-Saturnian asteroids, but without planet migration or chaotic resonances, have been proposed.

The details of the calculations of the Nice model are sensitive to chaotic interactions between planets and asteroids. Such calculations are notoriously plagued by numerical errors, in particular round-off and time discretisation errors. Originally it was thought that the model would cause some of the planetesimals to be thrown into the inner Solar System, producing a sudden influx of impacts on the terrestrial planets: the Late Heavy Bombardment (LHB). However, it has since been demonstrated that the LHB is inconsistent with the age and abundance of craters on the asteroid Vesta, and that the original lunar observations were the result of statistical aberrations in crater age determination.

Following the Nice model, the giant planets eventually reach their final orbital semi-major axes, and dynamical friction with the remaining planetesimal disc damps their eccentricities and makes the orbits of Uranus and Neptune circular again.

In some 50% of the initial models of Tsiganis and colleagues, Neptune and Uranus also exchange places. Such statistics, however, cannot be interpreted as a probability in a dynamically chaotic system. Although, an exchange of Uranus and Neptune would be consistent with models of their formation in a disk that had a surface density that declined with distance from the Sun, there is no compelling argument why planet mass should follow the disc's density profile.

Running dynamical models of the Solar System with different initial conditions for the simulated length of the history of the Solar System produce various distributions of minor bodies in the Solar System. In order to explain the wide variety of object families in their respective observed abundances, a wide range of initial conditions for the solar system are necessary. This diversity in initial conditions renders the model inpractical and suspect, because there can only be one realization of the early Solar System: that realization should explain all the families of minor bodies in their observed abundances.