This is chapter 5 in the Solar System’s story. We’ll explore the growth and migration pathways of Jupiter and Saturn and their little cousins the ice giants.
Let’s take a look inside the giant planets.
Jupiter and Saturn are “gas giants” because they are mostly made of gas — that is, hydrogen and helium — with solid cores of about 20-ish Earth masses. The ice giants have cores that are about the same mass as Jupiter and Saturn’s, just with a lot less gas on top.
Let’s jump into how these planets form There are two ideas: a bottom-up model and a top-down one.
Growth of giant planets by “core-accretion”
The most widely-accepted model for the growth of gas giants is “core accretion”. First, a large core grows. Then gas is gravitationally captured from the disk and piled on top of the large core. Sounds pretty simple.
It’s easy to imagine that the difference between the gas- and ice giants is related to how fast their cores grew. Fast-growing cores accumulated all the gas they could but slow-growing cores ran out of time, only gathering a small amount of gas before the gas disk itself dissipated. It’s like group pizza lunch — the first to show up get all the pizza they want, but latecomers are often left hungry.
Jupiter and Saturn became true gas giants by gravitationally capturing gas directly from the disk. The process of gas capture is quite slow at first, and then becomes very fast when the planet reaches a critical mass (when the mass of the captured atmosphere is about the same as the mass of the core).
The cores of the giant planets grew by pebble accretion (see chapter 3). The biggest planetesimals grabbed on to the pebbles drifting by in the disk and grew really fast. But just how fast?
This part of the story has a strong connection with the meteorite dichotomy introduced in chapter 2. There are two classes of meteorites with different chemical properties, called CC (carbonaceous) and NC (non-carbonaceous). Their parent planetesimals must have grown in different parts of the Sun’s disk at the same time, even though pebbles can zip through the disk very fast.
A possible reason that there are NC and CC meteorites but none in between is that Jupiter’s core blocked the mixing of NC and CC pebbles. Once a core grows to 10-20 Earth masses, it blocks the pebbles from crossing its orbit — this is called “pebble isolation” (see chapter 3). The core chops the planetary system in half, not allowing any mixing between pebbles closer-in and those farther out.
Put together, this part of the story is illustrated in the image below. First, the difference between CC and NC material was produced as a result of gas with slightly different chemistry falling onto the Sun within the Sun’s birth cluster of stars at different times (see chapter 1). Then Jupiter’s core (“proto-Jupiter” in the image) blocked the inner (NC) and outer (CC) pebbles from mixing. Finally, when Jupiter grew to its full size, it gravitationally scattered some CC planetesimals inward, implanting many in the asteroid belt and delivering water to the growing Earth (see this post for details).
This is one of those scientific models that is very appealing but controversial. Some scientists argue that Jupiter’s core could not have blocked the pebbles, or that the Sun’s disk had a more complex structure or evolution that separated CC and NC pebbles without the need for Jupiter’s core. A lot of researchers are working on this, so this part of the story may change (or be strengthened) in the next few years.
Core-Migration-Accretion
Migration is a key part of the giant planets’ story, in three places. First, the growing cores of the giant planets must have migrated. Second, the fully-formed gas giants also migrated, but perhaps in a different mode (and maybe even in a different direction). Third, the fully-grown gas giants and the growing ice giants were all migrating around at the same time. Let’s take these one at a time.
As we saw in chapter 4, orbital migration is unavoidable. Any massive planet (or core) launches density waves within the gaseous disk, and the back-reaction of those density waves causes the planet’s orbit to shrink or grow.
Giant planet cores tend to migrate inward, and fast. According to most models, any core that formed around Jupiter’s present-day orbit would have migrated inward into the rocky planet region. So, why is the actual Jupiter so far out from the Sun? There are (at least) two possible solutions: either Jupiter’s core actually started even farther out than it is today, or migration was not directed inward or as fast as we might think.
One school of thought is that migration really did shrink down the Solar System (technical references here and here). Jupiter’s present-day orbit is at about 5 AU (astronomical units, the Earth-sun distance). This model proposes that Jupiter’s core really did form at 10 or even 20 AU from the Sun and migrate inward as it grew. Saturn and the ice giants formed even farther away (or later) and migrated inward as well. This model can explain a lot, but leaves a big question: what happened to everything else that formed closer than Jupiter’s core? It should have survived, so why was there so little of it?
The other idea is that Jupiter’s core’s migration was slowed down. The heat produced by pebble accretion can push planets to migrate outward, possibly a long distance (one crazy idea even proposed that Jupiter’s core originated close to the Sun). But this heating would have stopped once Jupiter grew massive enough to block the pebbles, so would this mechanism really prevent Jupiter from migrating too far in?
After Jupiter and Saturn were fully-grown, they continued to migrate but in a slower mode that is locked to the evolution of the disk (“type 2”; see chapter 4). Again, there is some debate about exactly what this migration looked like.
Depending on the properties of the Sun’s disk and how fast Saturn grew relative to Jupiter, the gas giants may have either migrated inward or outward together, or even remained on roughly fixed orbits (technical details here). In the Grand Tack model (which is coming in chapter 6), Jupiter migrated inward on its own, then Saturn caught up with it and the two planets migrated outward together, as illustrated in this image:
But it’s totally legit to imagine that the gas giants’ orbital histories did not involve any outward migration, and instead were either dominated by inward migration, with the planets simply starting quite a bit farther from the Sun. Maybe the weirdest outcome is that the gas giants didn’t really migrate that much at all — that is a real possibility, but only after Saturn caught up to Jupiter and the two planets shared a common gap. If the disk conditions are within a certain range, there is a balance of inward and outward forces such that the planets’ orbits are basically stationary. Inward migration is prevented, so this basically amounts to a weaker version of the evolution that causes outward migration.
What about the ice giants? After the gas giants were fully-formed (or at least Jupiter was), the ice giants were probably large cores in the outer Solar System. They must have tried to migrate inward, but they were stopped by Jupiter (and perhaps also Saturn). Exactly where and when this happened depends on the gas giants’ migration history, but it may have looked something like this:
Jupiter acted as a barrier to the migration of the ice giants (and possibly even Saturn’s core). Instead of migrating into the inner Solar System (maybe to become close-in super-Earths), they were stuck in the outer Solar System. All hail Jupiter, protector of the rocky planets!
Regardless of their exact migration pathway, the giant planets almost certainly ended up in a chain of orbital resonances. In such a chain, the period ratios of neighboring planets form simple ratios — for instance, Saturn may have completed 2 orbits for every 3 orbits of Jupiter. The giant planets’ resonant chain likely survived until the dispersal of the gaseous disk — this will serve as the starting conditions to understand their later evolution, their dynamical instability (coming in a few chapters).
Giant planet formation by gravitational collapse
I can’t finish this chapter without presenting the alternate model for giant planet formation. This is a top-down process that involves gravitational collapse of a small part of the disk to directly produce a gas giant. Here is a simulation:
Gravitational instability works really well in very massive planet-forming disks. It tends to form very massive gas giants far from their stars — in the ice giant realm and beyond. The current consensus is that it does not form planets in the Jupiter-Saturn region. However, there are some newly-discovered gas giants on wide-enough orbits that they could have formed by gravitational instability (for instance, the newly-discovered planet candidate around AB Aurigae).
For most gas giants — including our own Jupiter and Saturn, as well as the ice giants — we think core-accretion (with migration) is the likely formation pathway.
Wrap-up
The TL;DR version of this post: Most giant planets form in a bottom-up way, by first growing large cores and then piling gas on top, migrating all the while. During its growth, Jupiter may have blocked both inward-drifting pebbles and inward-migrating ice giants.
Additional Resources
- The Solar System’s story (with links to all posts)
- Give thanks to Jupiter, our little planet’s big protector (Nautilus article)
- MOJO video 3: why is Jupiter so much bigger than Earth (by Alessandro Morbidelli):
PLANETPLANET 
How much of an on/off switch is a Jupiter to the stop of migration across its orbit? Could you achieve the same effect with two half Jupiters in close orbits, not sure how stable that arrangement would be, but lets put that aside for sake of argument for now.
Good question! Jupiter’s ability to block inward-migrating planets is not perfect — in simulations there can be what we called “jumpers”, planets that jump across Jupiter’s gap and enter the inner Solar System. (Clearly, this did not happen in the Solar System, and that tells us something about what could or could not have happened during this stage.)