In astronomy, the Nice () 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.
Description
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 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 hundred million 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.
Solar System features 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.
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, there is no satisfactory computer model that explains the current Solar System's architecture.
The Late Heavy Bombardment
The main motivation for the introduction of the Nice model is to explain the Late Heavy Bombardment (LHB), a hypothetical surge in asteroid impacts and crater formation on the lunar surface and the terrestrial planets at about 600 million years after the Solar System's formation. However, newer studies on the age of lunar craters show no peak in the cratering record, but rather an exponential decay of the number of craters with time. The surge may be a statistical artifact, with a finite uncertainty on the determination of a crater´s age combining with the cutoff age of the moon to create an apparent peak in the inferred age distribution, the LHB. Also recent measurements by laser ablation microprobe of the argon 40 to argon 39 isotope ratio on the surface of Vesta are in considerable tension with the LHB.
The Nice model would explain the LHB as follows.
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 predicted orbital distribution of the remaining asteroids does not match observations. In the outer Solar System the impacts onto Jupiter's moons are sufficient to melt material on Ganymede, causing separation of the metals, rocks, and ices, but not Callisto, being further away from Jupiter and therefore being hit at lower speeds. The impacts of icy planetesimals onto Saturn's inner moons are excessive, however, resulting in the vaporization of their ices.
The strong doubts about the LHB as a unique phase in the Solar System's early evolution also weaken the credibility of the Nice Model.
Trojans and the asteroid belt 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 at which Jupiter passes Saturn relative to their perihelia circulates slowly. If the period of this circulation falls into resonance with the period at which the trojans librate, then the libration range can increase until they escape. When this phenomenon occurs, the trojan co-orbital region is "dynamically open" and objects can both escape and enter it. Primordial trojans escape and a fraction of the numerous objects from the disrupted planetesimal disk temporarily inhabit it. Later when the separation of the Jupiter and Saturn orbits increases, 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 due to their repeated encounters with the giant planets which had not previously been understood. The libration amplitudes and eccentricities 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.
A large number of planetesimals would have also been captured in Jupiter's mean motion resonances as Jupiter migrated inward. Those that remained in a 3:2 resonance with Jupiter form the Hilda family. The eccentricity of other objects declined while they were in a resonance and escaped onto stable orbits in the outer asteroid belt, at distances greater than as the resonances moved inward. These captured objects would then have undergone collisional erosion, grinding the population away into progressively smaller fragments that can then be subject to the Yarkovsky effect, which causes small objects to drift into unstable resonances, and to the Poynting–Robertson drag which causes smaller grains to drift toward the sun. These processes may have removed >90% of the original mass implanted into the asteroid belt. The size-frequency distribution of this simulated population following this erosion are in excellent agreement with observations. This agreement suggests that the Jupiter trojans, Hildas, and spectral D-type asteroids such as some objects in the outer asteroid belt, are remnant planetesimals from this capture and erosion process. The dwarf planet may be a Kuiper-belt object that was captured by this process. A few recently discovered D-type asteroids have semi-major axes which is closer than those that would be captured in the original Nice Model.
Outer-system satellites