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In recent years, many thousands of exoplanets have been discovered orbiting distant stars (see Chapter 14). Most of these planetary systems are very different from our Solar System. Instead of all their planets traveling in near‐circular orbits that lie in the same plane, they are made up of planets traveling around the central stars in unusual orbits that are difficult to explain with the traditionally accepted planetary formation process.

These remote systems include exoplanets whose paths lie in completely different planes, and worlds with extremely eccentric orbits that take millennia to complete.

At the other extreme are the “hot Jupiters,” which orbit extremely close to their stars – much nearer than Mercury's distance from the Sun. At such close proximity to a star, temperatures would be too high for a massive planet to retain its gaseous envelope during formation.

If these worlds cannot have formed at their current locations, the obvious implication is that they formed further out and then their orbits were greatly modified. This would mean that the system evolved as the result of planetary migration.

The possibility of planetary migration has been considered for several decades, but the first detailed analysis of how this could occur came in 2005 with the introduction of the Nice Model (named after the French city).

Papers published in Nature by an international collaboration of scientists (Rodney Gomes, Hal Levison, Alessandro Morbidelli, and Kleomenis Tsiganis) suggested that planetary migration may have occurred as the result of an exchange of orbital momentum between the giant planets and innumerable planetesimals, over the course of a billion years.

The Nice Model envisaged an early Solar System in which the major planets were much more closely spaced and compact than at present, following near‐circular orbits between ∼5.5 and ∼17 astronomical units (AU).

Jupiter may have been a little farther out in the Solar System than it is today, whereas the other giants were closer to the infant Sun than at present. Beyond the planets was a region swarming with leftover planetesimals (comets and Pluto‐like objects).

The outermost planet, Neptune, began interacting with comets located at the inner edge of the young Kuiper Belt, so that some were scattered outward to interstellar space and others were sent inwards. Some of these entered the gravitational sphere of Uranus and were scattered again. This process of gravitational scattering was repeated with Saturn.

Whenever Saturn, Uranus, or Neptune decelerated a nearby planetesimal, causing the object to move closer to the Sun, the planet gained a tiny amount of momentum and accelerated. The overall result was the gradual outward migration of Neptune, Uranus, Saturn and the Kuiper Belt.

Unlike the three smaller giants, Jupiter was massive enough to eject large numbers of planetesimals to the outer reaches of the Solar System or out of the System altogether. Over time, after billions of such gravitational interactions, Jupiter spiraled inward a modest distance, at the same time as Saturn was drifting outward.

When Jupiter reached a distance of 5.3 AU and Saturn arrived at 8.3 AU, the two planets were in a 2:1 orbital resonance, so that one orbit of Saturn lasted precisely two Jupiter orbits. This would have occurred around 600–700 million years after they began to form.

The repeated gravitational pull of Jupiter caused Saturn's orbit to become much more elongated. This resulted in Saturn passing closer to Uranus and Neptune, so their orbits were also made more elliptical.

As the outer planets interacted chaotically with each other, it seems that Neptune and Uranus may have sometimes swapped places. One or both ice giants also plunged into the outer reservoir of planetesimals, scattering billions of them in all directions.

By the time the planets had cleared most of the intruders from their vicinities and the system had settled down again, Saturn had migrated out to about 9.5 AU. The effect on the outer planetary pair was even more extreme. Uranus had moved from about 13 to 19 AU, while Neptune had been catapulted from 15 to 30 AU.

Another consequence of this 500‐million‐year long planetary reshuffle was that the remaining planetesimals, perhaps 0.1% of the original population, were relocated beyond 30 AU, where they now reside as Kuiper Belt objects (KBOs).

The inward flux of planetesimals during the phase of dynamical instability also allows for chaotic capture of Jupiter's and Neptune's Trojan asteroid populations.

Furthermore, the asteroid belt was also strongly perturbed during Jupiter's migration, adding to the sudden, massive delivery of planetesimals to the inner Solar System. As their pockmarked surfaces show, the Moon and terrestrial planets appear to have suffered heavily during this Late Heavy Bombardment, around 3.9 billion years ago.

A modified version, the Nice 2 Model, suggests that the gradual scattering of planetesimals caused Jupiter and Saturn to fall into a 3:2 orbital resonance (not the originally proposed 2:1). This favours the development of a stable inner Solar System, where the rocky, terrestrial planets could form.

It also suggests that the mass of planetesimals hitting the regular satellites of Jupiter, Saturn etc. is smaller than assumed in studies based on the classic Nice Model by a factor of between 3 and 6. The impact rate is smaller in the Nice 2 Model because (at least in part) encounters with the planets cause the orbits of KBOs to become highly eccentric, resulting in less gravitational focusing by the planets.

Figure 1.17 According to the Nice model, the outward migration of Saturn's orbit changed the orbits of Uranus and Neptune. In the scenario shown here, the orbits of Uranus and Neptune cross over. Meanwhile, Neptune plows into the cloud of icy planetesimals that make up the young Kuiper Belt. The gravitational interaction of Saturn, Uranus, and Neptune with the planetesimals sent billions of these objects inward, toward the Sun. As a result, these three planets migrated outward to their present orbits. This may also account for a possible period of heavy bombardment in the inner Solar System about 4 billion years ago. Some planetesimals, such as Pluto, were locked into orbital resonances with Neptune.

(Nature)

Figure 1.18 One version of the Nice Model. (1) The giant planets surrounded by a cloud of planetesimals. Neptune's orbit (blue) is closer to the Sun than that of Uranus (green). (2) As the giant planets scatter comets into deep space, Jupiter migrates inward, while the other three planets migrate outward. (3) After Jupiter and Saturn briefly enter a 2:1 orbital resonance, they change the orbits of Uranus and Neptune, causing them to scatter billions of planetesimals inward or outward. (4) The giant planets settle into their final orbits and the outer belt of planetesimals is heavily depleted.

(Gomes, Levison, Morbidelli, and Tsiganis)

Suddenly, Saturn began to create havoc with the orbits of Uranus and Neptune, causing them to become more elliptical. They began to plow through the outer swarm of icy planetesimals, scattering billions of them in all directions. By the time the planets had cleared most of the intruders from their vicinities and the system had settled down again, Saturn had migrated out to about 9.5 AU. The effect on the outer planetary pair was even more extreme. Uranus had moved from about 13 to 19 AU, while Neptune had been catapulted from 15 to 30 AU.

Another consequence of this 500‐million‐year long planetary reshuffle was that the remaining planetesimals, perhaps 0.1% of the original planet‐building population, were relocated beyond 30 AU, where they now reside as Kuiper Belt objects.

Figure 1.19 The main stages of the Grand Tack Model. Panel (a) shows the initial state, where Jupiter (black circle labeled J) and a not‐complete Saturn (black circle labeled S) lie between an inner, warmish region of the Solar System populated by differentiated assorted kinds of asteroids (S‐type), and an outer region of C‐type asteroids containing water, carbon, etc. In panel (b), as Jupiter and proto‐Saturn move toward the young Sun, S‐type asteroids are scattered outward. Panel (c) shows the position of Jupiter and the fully‐grown Saturn after they “tacked,” or reversed course. The large circles in the center depict asteroids from the inner and outer Solar System orbiting mostly in separate regions of the asteroid belt. Eccentricity is a measure of how elliptical an orbit is; semi‐major axis is the mean distance from the Sun.

(Kevin Walsh)

Furthermore, the asteroid belt was also strongly perturbed during this burst of migration, adding to the sudden, massive delivery of planetesimals to the inner Solar System. As their pockmarked surfaces show, the Moon and terrestrial planets suffered heavily during this Late Heavy Bombardment, around 4 billion years ago.

According to a more recent theory, known as the Grand Tack Model, planetary migration was also a key factor during the first 5 million years of Solar System evolution (Figure 1.19). The model suggests that, directly after its formation out of the early solar nebula at about 3.5 AU, Jupiter migrated toward the Sun, as the dense gas in the nebula dragged it toward the Sun. Then, due to the growing gravitational influence of the newly formed Saturn, Jupiter halted its migration, then “tacked” (reversed direction) like a sailboat tacking around a buoy, when it reached about 1.5 AU. It then migrated outward toward its current position at 5.2 AU.

The migrating Jupiter depleted the mass concentrated at the current locations of Mars and the asteroid belt, thereby preventing Mars from growing bigger. The model also successfully explains the inclinations of asteroids and the transition from water‐poor to water‐rich asteroids in the middle of the main asteroid belt. The whole process took about 500,000 years.