From the December 2006 issue

A solar-system fairy tale

Once upon a time, two giants roughed up the neighborhood.
By | Published: December 21, 2006
Planetary computer simulation
There’s something wrong with the solar system.

First, theories about how planets form suggest worlds like Jupiter and Saturn should orbit the Sun in circular paths that share the same plane. But the orbits of Jupiter, Saturn, and Uranus are out-of-round by up to 9 percent. And the orbital planes of Saturn, Uranus, and Neptune tilt as much as 2° to that of Jupiter’s.

Second, something stirred up Jupiter’s so-called Trojan asteroids. Thousands of these rocks are trapped in gravitationally stable zones (Lagrange points) 60° ahead of and behind Jupiter. But instead of orbiting in Jupiter’s plane, the Trojan orbits display a range of tilts.

Then there’s the Kuiper Belt’s “missing mass.” For big objects like Eris and Pluto to form, theorists think some 10 Earth-masses of icy material must have been present between 30 and 50 astronomical units (AU) from the Sun. (One AU equals the mean Earth-Sun distance.) The trouble is, astronomers estimate the Kuiper Belt now holds less than 1/10 of an Earth-mass. So, where did all this material go?

Another mystery: Based on the distribution of craters on the Moon and analysis of lunar rock samples, many planetary scientists believe the inner planets experienced a late round of comet or asteroid impacts. This Late Heavy Bombardment, some 3.9 to 3.8 billion years ago, was brief and intense. But where did the impactors come from? And why did the event happen so late in the solar system’s formation?

Roaming giants
Recent studies by Rodney Gomes (National Observatory, Rio de Janeiro, Brazil), Kleomenis Tsiganis and Alessandro Morbidelli (Observatory of the Côte d’Azure, Nice, France), and Hal Levison (Southwest Research Institute, Boulder, Colorado) suggest one answer for all of these puzzles. The scientists published their results as three separate papers in the May 26, 2005, issue of Nature. In essence, they argue Jupiter and Saturn shook up the solar system long after the planets formed.
Planetary computer simulation
In this computer simulation, the giant planets start off closer together than they do today. Neptune (blue) starts off between Saturn (gold) and Uranus (purple). A disk of comets (green) extending to 35 AU fringes the young solar system; tick marks indicate 5 AU. The model undergoes a dramatic change after some 870 million years (Myr).
R. Gomes, et. al., Nature, May 26, 2005.
In their model of the young solar system, all of the giant planets orbit closer together than they do today, Neptune lies between Saturn and Uranus, and a truncated disk of icy comets surrounds them. Computer simulations show the two giants diverging as they migrate, with Jupiter heading toward the Sun, and Saturn heading outward. This continues until Jupiter completes two circuits of the Sun in the time it takes Saturn to make one. At this point, called the 1:2 mean-motion resonance, a dramatic change takes place.

The resonance creates a situation in which the solar system’s largest planets give each other repeated gravitational nudges, whose effects quickly build. Jupiter and Saturn force one another into ever more eccentric orbits. Saturn, with less mass, experiences the greatest orbital change.

Planetary billiards
Saturn’s increasingly out-of-round orbit brings it close to less-massive Neptune and Uranus. Their orbits become unstable and expand. In some runs, Neptune leapfrogs Uranus or is ejected outright.

In either case, Neptune disrupts the debris disk and scatters the comets, some of which become trapped in resonances to form the present-day Kuiper Belt. Many objects ejected from the giant-planet region form the Oort comet cloud. But enough of the mass rains toward the inner solar system to explain the Late Heavy Bombardment’s intensity and sudden onset.

Meanwhile, the act of scattering Neptune and Uranus settles Saturn into an orbit with its present high eccentricity, and scattering the comets stops the outward migrations of Neptune and Uranus.

Some scientists suggest more asteroids than comets pummeled the inner planets. That’s not a problem, because the asteroid belt also gets shook up when Jupiter and Saturn cross their mean-motion resonance. As Jupiter’s orbit changed, gravitational resonances swept through the asteroid belt, and these led to changes in the rocks’ orbital eccentricity and tilt. Once the giant planets settled down, any asteroids that happened to lie near Jupiter’s Lagrange points became trapped there. The peculiar orbital inclinations astronomers now see in Jupiter’s Trojan population record a time of great upheaval in the early solar system, say the scientists.

But can Saturn, Uranus, and Neptune retain their satellites during this gravitational rough-and-tumble? The team looked at this aspect of the problem and found that, in most cases, they can. In about 75 percent of the simulations, Saturn and Neptune kept their moons. Uranus did only slightly worse.

Hal Levison likes to present this scenario as a fairy tale — “a nice, compact story” that ties together a lot of solar-system loose ends. Many details, such as the original orbits of the giant planets or the makeup of the icy disk, still need explanation. But Levison and his colleagues feel the picture is essentially correct.

In this fairy tale, it’s the giants who win the day.