Nice model

• 1. Introduction

  ›

  The Nice model is a scientific scenario describing how the giant planets of our Solar System reached their current positions long after the Solar System first formed. It takes its name from Nice, France, where the Côte d'Azur Observatory first developed it in 2005. At its heart, the model asks a question most of us have never thought to ask: why are Jupiter, Saturn, Uranus, and Neptune where they are today, and were they always there? The answer, it turns out, involves a period of slow drift, a sudden gravitational crisis, and a cascade of destruction that reshaped the entire Solar System. The Nice model was introduced to the world in a triplet of papers published in the journal Nature, and it has been debated, revised, and extended ever since.

• 2. A Compact Beginning

  ›

  When the four authors of the original 2005 papers set out their proposal, they described a Solar System that would look alien to us today. Jupiter, Saturn, Uranus, and Neptune occupied near-circular orbits with radii between 5.5 and 17 astronomical units from the Sun. That places even the outermost giant planet closer to the Sun than Uranus sits today. Beyond those planets stretched a vast, dense disk of rocky and icy planetesimals with a combined mass of roughly 35 Earth masses, reaching out to about 35 astronomical units. This disk was the engine that would drive everything that followed. Planetesimals along its inner edge drifted into gravitational encounters with the outermost planets, and those brief interactions carried enormous long-term consequences. Each exchange of momentum nudged the planets outward by only a tiny amount, but over several hundred million years the accumulated effect shifted the orbits of the outer planets by significant distances. Jupiter, receiving planetesimals flung inward by its neighbors, actually moved slightly closer to the Sun.

• 3. The Resonance Crisis

  ›

  After several hundred million years of this slow migration, Jupiter and Saturn reached a critical alignment known as their 1:2 mean-motion resonance, in which Saturn's orbital period became exactly twice that of Jupiter. That resonance amplified the orbital eccentricities of both planets and destabilized the entire planetary system. What followed was rapid and dramatic. Jupiter flung Saturn outward toward its present position, and Saturn's relocation triggered gravitational encounters between Saturn and the two ice giants, Neptune and Uranus. Those encounters sent both ice giants onto much more eccentric orbits, which drove them plowing into the outer planetesimal disk. The disruption scattered tens of thousands of planetesimals and stripped the disk of roughly 99% of its mass. In about half of the initial simulations run by Tsiganis and colleagues, Neptune and Uranus also exchanged places during this upheaval. The model's authors noted that such a statistic cannot be treated as a probability in a dynamically chaotic system, but it does highlight how violent the rearrangement was.

• 4. The Late Heavy Bombardment

  ›

  The main motivation for building the Nice model was to explain a hypothetical event known as the Late Heavy Bombardment, a proposed surge of asteroid impacts and crater formation on the Moon and the inner planets roughly 600 million years after the Solar System formed. In the model's original framing, icy planetesimals scattered by Uranus and Neptune crossed into the inner Solar System, and sweeping mean-motion resonances excited the eccentricities of asteroids in the asteroid belt, driving them onto orbits that intersected the terrestrial planets. That process would have removed roughly 90% of the asteroid belt's mass. The number of planetesimals that would have reached the Moon was stated to be consistent with the lunar crater record. However, more recent studies complicate the picture considerably. Newer analyses of lunar craters show no peak in the cratering record but rather an exponential decay in the number of craters over time, suggesting the bombardment spike may be a statistical artifact. Laser ablation microprobe measurements of the argon-40 to argon-39 isotope ratio on the surface of the asteroid Vesta are also in considerable tension with the Late Heavy Bombardment. These doubts about the bombardment as a real event weaken one of the Nice model's central pillars.

• 5. Trojans and Small Bodies

  ›

  Jupiter and Saturn crossing the 2:1 resonance opened what researchers describe as a "dynamically open" trojan co-orbital region around Jupiter's L4 and L5 Lagrange points, allowing primordial trojans to escape and new objects from the disrupted disk to stream in. When the separation between Jupiter's and Saturn's orbits eventually increased, the trojan region became "dynamically closed" again, trapping a fraction of the newcomers. The captured trojans display a wide range of orbital inclinations, a feature that had not previously been understood, and the libration amplitudes and eccentricities of simulated populations match real observations of Jupiter trojans. A similar mechanism generates Neptune's trojan population. Objects captured in Jupiter's 3:2 mean-motion resonance as Jupiter migrated inward form the Hilda family of asteroids. A much larger population escaped onto stable orbits in the outer asteroid belt beyond 2.6 astronomical units. Those objects then underwent collisional erosion, with the Yarkovsky effect and Poynting-Robertson drag removing material over time. That process may have eliminated more than 90% of the original mass implanted into the belt. The remaining spectral D-type asteroids in the outer belt, along with the Jupiter trojans and the Hildas, are thought to be remnant planetesimals from this capture and erosion sequence. A few recently discovered D-type asteroids with semi-major axes less than 2.5 astronomical units sit closer to the Sun than the original Nice model would predict.

• 6. The Kuiper Belt and Outer Reservoirs

  ›

  Before the instability, the Kuiper belt was much denser and closer to the Sun, with an outer edge at approximately 30 astronomical units. Uranus and Neptune formed most likely in the 15-20 astronomical unit range and in opposite locations, with Uranus sitting farther from the Sun than Neptune. Gravitational encounters with other planets scattered Neptune outward to a semi-major axis of roughly 28 astronomical units with an eccentricity as high as 0.4. That high eccentricity caused Neptune's mean-motion resonances to overlap, creating chaotic orbits in the surrounding region. Objects trapped in resonance as Neptune migrated outward had their eccentricities reduced and inclinations increased through the Kozai mechanism, allowing some to escape onto stable higher-inclination orbits. Others remained in resonance, forming the plutinos and other resonant populations. The model does correctly produce both resonant and non-resonant populations and an outer edge at Neptune's 2:1 resonance, but it predicts a greater average eccentricity in classical Kuiper belt objects than is actually observed: 0.10-0.13 in simulations versus 0.07 in observations. Objects scattered by Uranus and Neptune onto orbits with semi-major axes of roughly 5,000 astronomical units could have their perihelia lifted by the galactic tide, forming the inner Oort cloud. Those reaching even larger orbits could be perturbed by nearby stars to form the outer Oort cloud with its nearly isotropic distribution of inclinations. Objects scattered by Jupiter and Saturn were typically ejected from the Solar System entirely. Several percent of the initial planetesimal disk can be deposited across all these reservoirs.

• 7. Revisions and the Five-Planet Hypothesis

  ›

  The Nice model has been modified repeatedly since 2005. Hydrodynamical models of the early Solar System showed that the giant planets would have migrated toward the Sun in the gas disk, potentially ending up as close-orbiting hot Jupiters had Saturn not been captured into resonance with Jupiter first. That resonance eventually locked all four giant planets into a quadruple resonant configuration with Jupiter and Saturn in their 3:2 resonance, a setup known as the Nice 2 model. A second major revision introduced what became known as the jumping-Jupiter scenario, in which an ice giant encounters Saturn, is scattered inward onto a Jupiter-crossing orbit, and then is scattered outward after encountering Jupiter. That step-wise orbital separation avoids the slow sweeping of secular resonances that would otherwise leave the terrestrial planets with overly large eccentricities. The frequent ejection of an ice giant in simulations led David Nesvorný and others to propose a five-planet Nice model, in which the early Solar System contained five giant planets rather than four, with one being ejected during the instability. In that model, Neptune first migrates outward to 28 astronomical units before encounters between planets begin, and its eccentricity can remain small during the instability because it encounters only the ejected planet. A 2015 study found that the five-planet Nice model has a statistically small likelihood of reproducing the orbits of the terrestrial planets, and that finding suggests the instability may have occurred before those planets fully formed. At the present time, no satisfactory computer model fully explains all observed features of the Solar System's current architecture.