Nice model

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.

[4][5][6] 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.

Despite the minute movement 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.

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.

Although the scenario explains the absence of a dense trans-Neptunian population,[5] alternative models that achieve the same depletion of trans-Saturnian asteroids, but without planet migration or chaotic resonances, have been proposed.

[8] 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).

[11] Also recent measurements of laser ablation microprobe of the 40 to 39 Argon isotope ratio on the surface of (4)Vesta are in considerable tension with the LHB.

[15] The strong doubts of the LHB as a unique phase in the Solar System's early evolution also weaken the credibility of the Nice model.

[6] The captured Trojans have a wide range of inclinations, which had not previously been understood, due to their repeated encounters with the giant planets.

[19] A few recently discovered D-type asteroids have semi-major axes <2.5 au, which is closer than those that would be captured in the original Nice model.

[20] Any original populations of irregular satellites captured by traditional mechanisms, such as drag or impacts from the accretion disks,[21] would be lost during the encounters between the planets at the time of global system instability.

[26] However, Triton's capture would be more likely in the early Solar System when the gas disk would damp relative velocities, and binary exchange reactions would not in general have supplied the large number of small irregulars.

[4][10] Gravitational encounters between the planets scatter Neptune outward into the planetesimal disk with a semi-major axis of ~28 au and an eccentricity as high as 0.4.

These two populations are dynamically hot, with higher inclinations and eccentricities; due to their being scattered outward and the longer period these objects interact with Neptune.

The excess of low-inclination plutinos in other models is avoided due to Neptune being scattered outward, leaving its 3:2 resonance beyond the original edge of the planetesimal disk.

The differing initial locations, with the cold classical objects originating primarily from the outer disk, and capture processes, offer explanations for the bi-modal inclination distribution and its correlation with compositions.

[27][28] The cold population also includes a large number of binary objects with loosely bound orbits that would be unlikely to survive close encounter with Neptune.

[29] If the cold population formed at its current location, preserving it would require that Neptune's eccentricity remained small,[30] or that its perihelion precessed rapidly due to a strong interaction between it and Uranus.

[10] Objects scattered outward by Uranus and Neptune onto larger orbits (roughly 5,000 au) can have their perihelion raised by the galactic tide detaching them from the influence of the planets forming the inner Oort cloud with moderate inclinations.

Hydrodynamical models of the early Solar System indicate that the orbits of the giant planets would converge resulting in their capture into a series of resonances.

The combination of resonant planetary orbits and the late instability triggered by these long distant interactions was referred to as the Nice 2 model.

[35] The step-wise separation of the orbits of Jupiter and Saturn avoids the slow sweeping of secular resonances across the inner solar System that increases the eccentricities of the terrestrial planets[35] and leaves the asteroid belt with an excessive ratio of high- to low-inclination objects.

Most of the rocky impactors of the Late Heavy Bombardment instead originate from an inner extension that is disrupted when the giant planets reach their current positions, with a remnant remaining as the Hungaria asteroids.

[43] Following the breaking of the resonant chain Neptune first migrates outward into the planetesimal disk reaching 28 au before encounters between planets begin.

[46] Neptune's eccentricity can remain small during the instability since it only encounters the ejected ice giant, allowing an in situ cold-classical belt to be preserved.

[44] The lower mass planetesimal belt in combination with the excitation of inclinations and eccentricities by the Pluto-massed objects also significantly reduce the loss of ice by Saturn's inner moons.

[48] A recent study found that the five-planet Nice model has a statistically small likelihood of reproducing the orbits of the terrestrial planets.