Nice model

The Nice (/ˈniːs/) model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Observatoire de la Côte d'Azur, 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 including the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune. Its success at reproducing many of the observed features of the Solar System means that it is widely accepted as the current most realistic model of the Solar System's early evolution, although it is not universally favoured among planetary scientists. Later research revealed a number of differences between the original Nice model's predictions and observations of the current Solar System, for example the orbits of the terrestrial planets and the asteroids, leading to its modification. The Nice (/ˈniːs/) model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Observatoire de la Côte d'Azur, 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 including the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune. Its success at reproducing many of the observed features of the Solar System means that it is widely accepted as the current most realistic model of the Solar System's early evolution, although it is not universally favoured among planetary scientists. Later research revealed a number of differences between the original Nice model's predictions and observations of the current Solar System, for example the orbits of the terrestrial planets and the asteroids, leading to its modification. 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: Rodney Gomes, Hal Levison, Alessandro Morbidelli, and Kleomenis Tsiganis. 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 between ~5.5 and ~17 astronomical units (AU), 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. Scientists understand so little about the formation of Uranus and Neptune that Levison states, '...the possibilities concerning the formation of Uranus and Neptune are almost endless.'However, it is suggested that this planetary system evolved in the following manner: Planetesimals at the disk's inner edge occasionally pass through gravitational encounters with the outermost giant planet, which change the planetesimals' orbits. The planets scatter the majority of the small icy bodies that they encounter inward, exchanging angular momentum with the scattered objects so that the planets move outwards in response, preserving the angular momentum of the system. These planetesimals then similarly scatter off the next planet they encounter, successively moving the orbits of Uranus, Neptune, and Saturn outwards. Despite the minute movement each exchange of momentum can produce, 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, in 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, cross their mutual 1:2 mean-motion resonance. 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, a scenario which explains the modern-day absence of a dense trans-Neptunian population. Some of the planetesimals are thrown into the inner Solar System, producing a sudden influx of impacts on the terrestrial planets: the Late Heavy Bombardment. Eventually, the giant planets reach their current 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. 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, which predicts that the masses of the planets should also decline with distance from the Sun. Running dynamical models of the Solar System with different initial conditions for the simulated length of the history of the Solar System will produce the various populations of objects within the Solar System. As the initial conditions of the model are allowed to vary, each population will be more or less numerous, and will have particular orbital properties. Proving a model of the evolution of the early Solar System is difficult, since the evolution cannot be directly observed. However, the success of any dynamical model can be judged by comparing the population predictions from the simulations to astronomical observations of these populations. At the present time, computer models of the Solar System that are begun with the initial conditions of the Nice scenario best match many aspects of the observed Solar System. The crater record on the Moon and on the terrestrial planets is part of the main evidence for the Late Heavy Bombardment (LHB): an intensification in the number of impactors, at about 600 million years after the Solar System's formation. In the Nice model icy planetesimals are scattered onto planet-crossing orbits when the outer disc is disrupted by Uranus and Neptune causing a sharp spike of impacts by icy objects. The migration of outer planets also causes mean-motion and secular resonances to sweep through the inner Solar System. In the asteroid belt these excite the eccentricities of the asteroids driving them onto orbits that intersect those of the terrestrial planets causing a more extended period of impacts by stony objects and removing roughly 90% of its mass. The number of planetesimals that would reach the Moon is consistent with the crater record from the LHB. However, the orbital distribution of the remaining asteroids does not match observations. In the outer Solar System the impacts onto Jupiter's moons are sufficient to trigger Ganymede's differentiation but not Callisto's. The impacts of icy planetesimals onto Saturn's inner moons are excessive, however, resulting in the vaporization of their ice. After Jupiter and Saturn cross the 2:1 resonance their combined gravitational influence destabilizes the Trojan co-orbital region allowing existing Trojan groups in the L4 and L5 Lagrange points of Jupiter and Neptune to escape and new objects from the outer planetesimal disk to be captured. Objects in the trojan co-orbital region undergo libration, drifting cyclically relative to the L4 and L5 points. When Jupiter and Saturn are near but not in resonance the location where Jupiter passes Saturn relative to their perihelia circulates slowly. If the period of this circulation falls into resonance with the period that the trojans librate the range of their librations can increase until they escape. When this occurs the trojan co-orbital region is 'dynamically open' and objects can both escape and enter the region. Primordial trojans escape and a fraction of the numerous objects from the disrupted planetesimal disk temporarily inhabit it. Later when Jupiter and Saturn orbits are farther apart the Trojan region becomes 'dynamically closed', and the planetesimals in the trojan region are captured, with many remaining today. The captured trojans have a wide range of inclinations, which had not previously been understood, due to their repeated encounters with the giant planets. The libration angle and eccentricity of the simulated population also matches observations of the orbits of the Jupiter trojans. This mechanism of the Nice model similarly generates the Neptune trojans.