Computer simulations are used to study how the giant planets formed and gravitationally interacted early in the solar system. These simulations demonstrate that the giant planets can migrate their orbits somewhat during their formation. This was initially a big surprise, but this was first explained by what is called the Nice model.
The Beginning Configurations in the Nice Model
We don’t really know what the original configuration of the giant planets was just after they formed, but we can run simulations with different possibilities and see which ones eventually evolve into the system we see today. One model that does a very nice job is the so-called Nice model. Nice here refers to the city of Nice in France.
The Nice model begins with the 4 giant planets, Jupiter, Saturn, Uranus, and Neptune situated more closely in space than they are today. Their orbits were more compact, fitting all 4 planets between about 5 and 17 astronomical units. Compare that to today where Jupiter resides at 5, but Saturn is at 9, Uranus is at 19, and Neptune is all the way out at 30 astronomical units.
During this early time, a disk of icy planetesimals, with a total mass of tens of Earth masses would have existed beyond 17 astronomical units out to about 30 astronomical units.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Inward Scattering of the Planetesimals
Out there the orbital speeds were too slow and the material too sparse to form a giant planet. Every once in a while, an icy planetesimal from this disk would be perturbed into the region of the giant planets.
So now, picture one of these planetesimals first encountering Neptune on its way in, pushing Neptune outward a bit, as the planetesimal moves inward. Then, the planetesimal gets to Uranus and scatters inward even more, moving Uranus out a bit. And the same thing happens yet again when the planetesimal encounters Saturn.
Now admittedly, these outward perturbations of the giant planets would be extremely small. But if successive planetesimals are getting scattered inward, over and over again, then the combined effect can be to move the planets’ orbits outward.
Jupiter Moves In
But something different happens when the inward-scattered planetesimal finally reaches Jupiter. Jupiter is so massive that it’s more likely, on average, to fling the planetesimal into a highly elliptical orbit, taking the object further outward in the solar system. But every time a planetesimal is flung outward, Jupiter migrates inwards a bit. So Jupiter moves inward while Saturn, Uranus, and Neptune each move outward.
And all this continues to go on for some time, until Jupiter and Saturn end up in a very special configuration. Eventually, they reach a state where Jupiter completes exactly 2 whole orbits every time Saturn completes 1. They are now in a 2:1 orbit-orbit resonance. This resonance means that Jupiter and Saturn can gravitationally influence each other more, in a quite regular way. And what happens is they perturb each other into more elliptical orbits.
Learn more about Uranus: A water world on its side.
Saturn’s Effect on Uranus and Neptune
Saturn quickly moves to a position closer to where we find it today, out at around 10 astronomical units. But this has implications for Uranus and Neptune. Gravitational interactions with Saturn move them onto more elliptical orbits, pushing them to cross farther out in the solar system. But there were still some icy planetesimals at these far locations.
So Uranus’s and Neptune’s orbital migration destabilized the disk of small bodies out there. Some of those planetesimals would be thrown into the inner solar system, producing a spike in bombardment of the inner planets. But these gravitational interactions between the ice giants and icy planetesimals also changes the ice giant orbits, reducing their ellipticities. The ice giant orbits then stabilize close to where we find them today: Uranus near 19 astronomical units and Neptune at 30 astronomical units.
Destabilization and Migration
So, the end scenario is the giant planets where we see them today, along with a period of increased bombardment in the early solar system very early on. We don’t know exactly where the giant planets started, but many possibilities lead to this giant destabilization and migration of the giant planets flinging planetesimals into the inner solar system.
It turns out that this whole process would have done more than fling icy planetesimals inward from the outer solar system. As Jupiter and Saturn migrate, resonances between orbits in the asteroid belt and Jupiter also cause asteroids to be perturbed into elliptical orbits and flung into the inner solar system as well. So, the bombardment was by icy planetesimals from the outer solar system, and by asteroids.
Learn more about Jupiter’s planet-like system of moons.
The Kuiper Belt and Captured Satellites
The Nice model can explain some other features of the solar system, too. For example, the Kuiper belt has a cold population of objects, cold here meaning the orbital ellipticities and inclinations are fairly low. But there is also a hot population with more elliptical and inclined orbits. The hot population would have been the planetesimals most disturbed by the migration of the outer planets, giving them the high inclinations and ellipticities we see today.
All of these orbital perturbations between the icy planetesimals and the giant planets may also explain the many captured irregular satellites in the outer solar system. Triton around Neptune has a retrograde, inclined orbit telling us it was a captured object.
In addition, the fact that Jupiter’s Trojan asteroids look like Kuiper belt objects might be explained if they actually originated in the Kuiper belt and were kicked in during the outward migration of the ice giant planets.
And then there’s the ice giants themselves. It’s easier for the ice giants to grow to their current sizes in simulations if they formed much closer in. Not as close as Saturn, mind you, which grew fast enough to attract hydrogen and helium gas. They grow faster closer-in because that’s where the density of the proto-planetary disk is greater. So, the basic Nice model of planet migration easily accounts for the solar system as it appears to us now.
Common Questions about the Original Nice Model
The Nice model begins with the 4 giant planets, Jupiter, Saturn, Uranus and Neptune, situated more closely in space than they are today. Their orbits were more compact, fitting all 4 planets between about 5 and 17 astronomical units. During this early time, a disk of icy planetesimals, with a total mass of tens of Earth masses would have existed beyond 17 astronomical units out to about 30 astronomical units.
According to the Nice model, the icy planetesimals would be perturbed by the giant planets and would fall inward toward the Sun. The cumulative effect of many planetesimals traveling toward the Sun was to push the Saturn, Uranus and Neptune outward, and to push Jupiter inward.
The original Nice model accounts for the Kuiper Belt’s hot and cold populations, as well as the many captured irregular satellites of the outer planets. In addition, Jupiter’s Trojan satellites, and the formation of the icy planets are also explained.