8-6 Gases in the outer solar nebula formed the Jovian planets, and planetary migration reshaped the solar system

We have seen how the low abundance of solid material in the inner solar nebula led to the formation of the small, rocky terrestrial planets. To explain the very different properties of the Jovian planets, we need to consider the conditions that prevailed in the relatively cool outer regions of the solar nebula.

The Core Accretion Model

The large Jovian planets initially formed through a similar process as the terrestrial planets—through the accretion of planetesimals. As discussed in Section 8-5, a key difference is that ices—in addition to rock and metal grains—were able to survive in the cooler outer regions of the solar nebula (see Figure 8-10). The elements of which ices are made are much more abundant than those that form rocky grains. Thus, more solid material would have been available to form planetesimals in the outer solar nebula than in the inner part. As a result, solid objects larger than any of the terrestrial planets could have formed in the outer solar nebula. Each such object could have become the core of a Jovian planet and served as a “seed” around which the rest of the planet eventually grew. For example, the mass of Jupiter’s rock and metal core is estimated to equal about 10 Earth masses. However, while additional solid material beyond the snow line plays a role in forming the larger cores of Jovian planets, astronomers do not fully understand how their cores get as large as they do, and this is an active area of research.

For Jupiter, a large seed mass of rock, metal, and ice is only the beginning: Most of Jupiter’s mass is hydrogen and helium. Recall that for a planet, retaining a gaseous atmosphere depends on both the planet’s mass and on the gas temperature (see Box 7-2). Accordingly, due to the lower temperatures in the outer solar system, Jupiter’s large seed mass could capture and retain hydrogen and helium gas. This picture—where a Jovian protoplanet core captures gas and grows by accretion—is called the core accretion model.

As Jupiter grew, its gravitational pull increased, allowing it to capture more gases and grow even larger until most of the available gas in its region had been captured. Because hydrogen and helium were so abundant (they are 98% of the solar nebula), Jupiter quickly grew to more than 300 Earth masses. You can see Jupiter forming before the other planets in Figure 8-13, which summarizes the formation of the solar system. Farther out in the solar nebula, Saturn would have gone through a similar process. About one-third the mass of Jupiter, Saturn’s 95 Earth masses would also have taken longer to accumulate, forming a few million years after Jupiter. Uranus and Neptune formed well beyond the snow line, where temperatures were cold enough for additional ices of carbon dioxide, methane, and ammonia to form the bulk of these planets.

Figure 8-13: The Formation of the Solar System This sequence of drawings shows stages in the formation of the solar system.

Like the protosun, each Jovian planet would have been surrounded by a disk—a solar nebula in miniature (see Figure 8-6). The large moons of the Jovian planets, including those shown in Table 7-2, are thought to have formed from ice particles and dust grains within these disks. Furthermore, since the moons formed from a rotating disk, these large satellites all orbit in the same direction. However, the Jovian planets also have smaller bodies—called irregular satellites—that orbit in the opposite direction, and these were probably captured after the planets formed.

Observations of protoplanetary disks around other protostars (such as those in Figure 8-8b) suggest that high-energy photons would eventually disperse any remaining hydrogen and helium gas, and this gas ends up in the space between the stars. With hydrogen and helium gone from the disk, formation of the Jovian planets came to an end after a few million years. However, the accumulation of planetesimals that built the terrestrial planets did not stop when gas was expelled from the protoplanetary disk, and the terrestrial planets took much longer to form than the Jovian planets.

Early Migration of Jovian Planets Shapes the Inner Solar System

We now look at migration, which refers to changes in orbital distances of the planets. Migration of the planets results from interactions—gravitational tugs and pulls—between planets and different parts of the forming solar system. The details of planetary migration are far from certain, but the broad picture that follows is emerging from computer simulations.

Within the first few hundred thousand years, long before the terrestrial planets formed, Jupiter’s interaction with the gaseous disk of hydrogen and helium led to an inward migration of Jupiter, followed by an outward migration. At its closest distance to the protosun, Jupiter migrated inward to about 1.5 AU, which is near the current orbit of Mars. With its large mass, Jupiter gravitationally deflected many of the planetesimals near the current Martian orbit. As a result, when Mars eventually formed in this region, it ended up with a low mass of about one-tenth of Earth’s mass. In fact, it was the mysteriously low mass of Mars that prompted this analysis of Jupiter’s inward migration. Even more, inward-then-outward migration of all the Jovian planets—called the Grand Tack model—also solves a long-standing mystery involving the asteroid belt.

The asteroid belt (Section 7-5) contains rocky objects typical of the inner solar system, but, surprisingly, also contains icy objects expected to have formed well beyond the asteroid belt. In the Grand Tack model, as both Jupiter and the other Jovian planets migrate outward, they deflect planetesimals inward to form the asteroid belt. Some of these planetesimals come from the inner solar system, but some also come from much farther out beyond the snow line, providing a very natural explanation for the icy objects in the asteroid belt.

This early migration, along with Jovian planet formation, was over within the solar system’s first few million years or so. Another type of migration occurred over the next few hundred million years, and reshaped the outer solar system.

Late Migration of Jovian Planets Reshapes the Outer Solar System

Astronomers have long suspected that some planetary migration must have taken place in the outer solar system. To see why, consider Neptune. At Neptune’s current large orbital distance, the timescale necessary to build a planet of Neptune’s large size by core accretion is much longer than the time that the protoplanetary disk was around. However, planets can grow much faster closer to the protosun, where the protoplanetary disk is denser. Therefore, astronomers suspect that Neptune formed closer in, and then migrated outward.

The leading theory described here for late migration of the Jovian planets is based on the Nice (pronounced “niece”) model. This model was developed in Nice, France, around 2005 and results from detailed computer simulations. It is important to emphasize that the Nice model is a work in progress and has already evolved with new ideas and better simulations.

In the Nice model, the Jovian planets would have all originally formed within about 20 AU of the protosun, even though the outermost planet today, Neptune, is at 30 AU. Furthermore, the outermost planet during this early time could have been Uranus, at 20 AU. Gravitational interactions could have switched the orbital ordering of Neptune and Uranus; in the Nice model, both initial scenarios can lead to the arrangement of our present-day solar system.

There was also a disk of planetesimals beyond the outermost Jovian planet at 20 AU, and through gravitational encounters, these planetesimals, on average, were scattered inward. Furthermore, when a big planet knocks a small planetesimal inward, the planet itself is kicked slightly outward. Over a few hundred million years, as numerous planetesimals were knocked inward by Saturn, Neptune, and Uranus, these Jovian planets slowly migrated outward to their current locations.

Most of the inward-moving planetesimals even made it close to Jupiter. With its much greater mass, Jupiter did not migrate inward very much, but did gravitationally fling most of the planetesimals clear out of the solar system. However, a small fraction of these icy objects would not have made it all the way out of solar system and are thought to currently orbit at about 50,000 AU from the Sun—about 0.8 light-year away and one-fifth the distance to the nearest star. These planetesimals would be very loosely bound to our Sun and gravitational deflections from passing stars would spread many of their orbits into a spherical “halo.” We call this hypothesized distribution of icy planetesimals the Oort cloud (Figure 8-14a), which formed beyond the objects of our next topic—the Kuiper belt.

Figure 8-14: The Kuiper Belt and Oort Cloud (a) The classical Kuiper belt of comets spreads from Neptune out to 50 AU from the Sun. Most of the estimated 200 million belt comets are believed to orbit in or near the plane of the ecliptic. The spherical Oort cloud extends from beyond the Kuiper belt. (b) Orbits of bodies in the Kuiper belt.

(b: NASA and A. Field/Space Telescope Science Institute)

Unstable Orbits, the Kuiper Belt, and the Late Heavy Bombardment

During the slow outward migration of the Jovian planets, their orbits remain mostly circular; in other words, they have very low eccentricity. The slowly expanding Jovian orbits are also quite stable and allow the migration to continue for about 600 million years or so. However, gravitational interactions with the planetesimals eventually destabilize planetary orbits, leading to elongated, or eccentric, orbits.

With elongated orbits, the planets gravitationally interact with each other more strongly. In many computer simulations, this nearly doubles the orbital distance of Neptune, sending Neptune out to its current distance of about 30 AU. As Neptune moves outward, its gravity flings nearby planetesimals to greater distances as well, creating an orbiting collection of icy objects called the Kuiper belt (Figure 8-14b). (The images that open this chapter show two young stars surrounded by dusty disks that resemble the Kuiper belt, one seen edge-on and the other seen face-on.) Taken together, the Oort cloud and Kuiper belt contain icy planetesimals that could not have formed at these great distances where matter was sparse, but instead were gravitationally deflected outward by Jupiter and then by Neptune.

The Nice model seems to solve another mystery of the solar system. Many astronomers think there was a cataclysmic period when the planets and moons of the inner solar system were subjected to a short but intense period of large impacts; this hypothesis is referred to as the Late Heavy Bombardment. The evidence comes from radioactive dating of craters on the Moon, but more lunar samples are needed to firmly establish a spike in the timing of impact events. This bombardment is considered “late” because the frequent planetesimal collisions of planet-building had long since passed.

So, what could cause a big jump in impacts? There are several ideas, but here is the most favored scenario. In the Nice model, the destabilized and elongated planetary orbits also led to deflections of planetesimals throughout the inner solar system. Very significantly, it takes about 600 million years before widespread deflection of the planetesimals, which is consistent with the surprisingly late timing of the Late Heavy Bombardment.

To a much lesser extent, gravitational deflections continue today. Planetesimals from either the Kuiper belt or the Oort cloud are occasionally knocked into new orbits. If one of these icy objects is deflected on a path through the inner solar system, it begins to evaporate, producing a visible tail, and is seen as a comet (see Figure 7-9).

Prior to 1995 the only fully formed planetary system to which we could apply this model was our own. As we will see in the next section, astronomers can now further test this model on an ever-growing number of planets known to orbit other stars.