The Nice model premiered in 2005 in its original form, but the model has been tweaked over the years. Novel scenarios have been offered, such as the Jumping Jupiter idea and the Grand Tack theory. But how do these models explain the solar system?
Jupiter and Saturn Jump Around
One scenario is called Jumping Jupiter. In this case, one of the ice giants—Uranus or Neptune—first encounters Saturn. The encounter perturbs the orbit of Neptune or Uranus inward toward Jupiter, and that causes Saturn to get pushed outward. When the ice giant reaches Jupiter, that ice giant then gets scattered outward again, while Jupiter moves inward.
This results in Jupiter’s and Saturn’s orbits evolving in a quick jumping fashion, rather than a smooth continuous movement. Basically, Jupiter experiences a rapid change in orbital distance, and then stays put in a stable location afterward. Scientists like the Jumping Jupiter scenario because it avoids Jupiter and Saturn creating destabilizing orbital resonances for planets in the inner solar system, contrary to the original Nice model.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.
Where’s the Missing Planet?
But the Jumping Jupiter scenario sometimes has an unwanted side effect of its own: the ice giant that gravitationally scatters off of Jupiter can get ejected from the solar system, leaving only 3 giant planets, while we have 4 now. To fix that, scientists have suggested there were originally 5 giant planets, not 4. In this view, the giant planet we don’t see got ejected, allowed Jupiter to jump, while minimizing change to orbits in the inner solar system.
So, maybe there’s a lost solar system planet somewhere out there, floating amongst the stars. Or, maybe it’s still in our solar system and we just haven’t seen it yet. So, maybe there’s a “nether Neptune,” located about 20 times as far away as Neptune itself. And possibly that unconfirmed world was thrown way out there during a close encounter with Jumping Jupiter.
Learn more about mighty Jupiter, the ruling gas giant.
Grand Tack: Migrating Planets in the Gaseous Disk
The giant planets may have been migrating even earlier, while they were forming in the gaseous protoplanetary disk. One proposed model of this migration, called the Grand Tack model, may even explain some features in the inner solar system.
Here’s how it works. Consider what would happen if Jupiter began forming before Saturn. As Jupiter grows by collecting gas from the surrounding disk, it would have migrated inward to about 1.5 astronomical units due to tidal interactions with the gaseous disk.
Saturn starts forming a bit later, but it eventually grows big enough, moves inward even faster than Jupiter. Eventually, Jupiter and Saturn enter an orbital resonance of 3:2. And this resonance changes everything. Jupiter and Saturn now interact more with each other rather than tidally influencing the gaseous disk, and they start moving outward together.
This sudden change in direction of motion from inward to outward is why this model is called the Grand Tack model. This name borrows the idea of how sailors tack first one way, then another way, in order to arrive somewhere that would not have been possible without both movements.
A Small Mars and a Mixed Asteroid Belt
But why is this scenario appealing? The Grand Tack model may be able to explain why Mars is so small. Most computer simulations of the origin of the solar system tend to produce an Earth-sized planet, or larger, at Mars’s location. But what we have is Mars—a little planet, barely larger than Jupiter’s moon Ganymede.
Use the Grand Tack scenario. If Jupiter migrates significantly inward, near where proto-Mars is forming, then Jupiter can deplete the planetesimals near the proto-Mars orbit. So, Mars had less material available when forming, and this resulted in a smaller Mars.
The Grand Tack model also explains an unusual feature of the asteroid belt: it is more mixed than it should be. If the asteroid belt were comprised of planetesimals that formed at today’s orbital distances from the Sun, then you would expect there to be a compositional gradient in the asteroid belt: rocky asteroids would be more common in the inner belt, icy asteroids would dominate in the outer regions. But we see that asteroid types are mixed in more complex ways across the belt.
Learn more about water on Mars and prospects for life.
Did Asteroids Bring Water to Earth?
What about Earth? As you might imagine, a migrating Jupiter that destabilizes the asteroid belt and shrinks the planetesimals available to form Mars may also have affected early Earth.
Some of the destabilized asteroids would have been flung into Earth-crossing orbits. Impacts by icy asteroids could have delivered water and other volatiles to Earth, ultimately forming our oceans. And the later migration of the giant planets may also have consequences for early Earth. Is it a coincidence that the signs of earliest life occur around 4 billion years ago, around the same time as the Late Heavy Bombardment?
Did Asteroids Kill Mars ?
The big consequences for early Mars were quite different. Four billion years ago is around the time that Mars lost its atmosphere. This led to the loss of stability for liquid water on its surface. It is also when the Martian magnetic dynamo ceased to operate. Are these events related to the Late Heavy Bombardment? It is possible.
Here’s a plausible scenario. Large impacts can cause portions of the atmosphere of a planet to blow off. The greater the impacts, the greater the possible loss of atmosphere, especially for a small planet.
As for loss of the magnetic dynamo, impacts could also heat the mantle of Mars to the point where it insulates the liquid core by making the core-mantle boundary hotter than the interior part of the core. If the core can’t cool, then it won’t convect to generate the dynamo.
So, a large impact may be responsible for shutting down the magnetic field of Mars. And shutting down the magnetic field could also affect the atmosphere! Without a protective shield, high-energy particles from the solar wind might have more easily blown off more of the Martian atmosphere.
The Nice model’s variations open our eyes to possibilities. The orbits of the giant planets dance around the solar system as the giants migrate. And the model is able to explain some previously taken-for-granted features we see in the solar system. However, we don’t know for sure that the model describes what actually happened.
Common Questions about the Jumping Jupiter and the Grand Tack Models
In the Jumping Jupiter model, an ice giant first encounters Saturn. The encounter perturbs the orbit of the ice giant inward toward Jupiter, and that causes Saturn to get pushed outward. When the ice giant reaches Jupiter, it gets scattered outward again, while Jupiter moves inward. In most cases, the ice giant is ejected out of the solar system.
In the Grand Tack model, Jupiter begins migrating inward through the gaseous protoplanetary disk, followed a little later by Saturn. Once they reach orbital resonance, they migrate outward together.
In the Grand Tack model, as Jupiter moves inward, it depletes the material in the region around Mars, thus leading to the small Mars that we see today, rather than the bigger planet predicted by other models.