Shaking Up a Nursery of Giant Planets

What are Uranus and Neptune doing so far from the sun? The question has puzzled theorists for decades. According to a new model, sibling rivalry might be to blame for their banishment. Runty Uranus and Neptune may have grown up in tight quarters much closer to the sun, only to have the big bruisers Jupiter and Saturn fling them into the outer reaches of the solar system.

The nine planets formed in a disk of dust and gas, where chunks of primordial matter collided to form bigger and bigger chunks and eventually planets. For the next several million years, the resulting ice-rock cores of the outer planets grabbed gas from the nebula until the gas was all taken up or blown away by the sun. But out on the nebula's fringes, matter was spread too thin for anything like planets to form.

Apparently, Uranus and Neptune formed somewhere else, presumably closer to the sun where the nebula was far denser. But how did they move outward billions of kilometers without disrupting their nicely circular orbits, which are in the same plane as the rest of the solar system? In this week's issue of Nature, planetary dynamicists Edward Thommes and Martin Duncan of Queen's University in Kingston, Ontario, and Harold Levison of the Boulder, Colorado, branch of the Southwest Research Institute demonstrate a two-step method that works, at least in their model. They assume that not just two but four or five ice-rock cores formed where Jupiter and Saturn now reside.

Conventional thinking also has it that once a core reached a critical mass--about 15 times that of Earth--its growth would speed up as its increasing mass gave it greater and greater gravitational pull on the gas. By chance, in Thommes and Duncan's scenarios, Jupiter hit runaway growth first, letting it grab 71% of the total mass of the outer planets; Saturn came in second with 21%, but late bloomers Uranus and Neptune got only about 3% and 4% of the mass, respectively, leaving them at the mercy of nearby Jupiter and Saturn. In the first 100,000 years of many of the simulations, Jupiter and Saturn gravitationally fling their nursery mates into steeply tilted, highly elongated orbits that can carry them outward 30 to 40 times the distance from Earth to the sun.

That's a dangerous situation for an undersized giant because the big guys could, and in some of the simulations do, eject a planet from the solar system entirely. But the debris remaining in the disk can step in to defend the bullied planets. In a process called dynamical friction, innumerable gravitational interactions with bits of disk debris push Uranus and Neptune around as they pass through and over the disk. The friction eases the wildly orbiting planets once again into circular orbits in the plane of the other planets but beyond the disruptive influence of Jupiter and Saturn. Further planet-disk interactions can move the relocated planets even farther out. Of the 24 simulations Thommes and his colleagues have run, about half produced an outer solar system resembling ours.

Some researchers question the realism of the model and note that no one is sure what the earliest solar system was actually like. Renu Malhotra of the Lunar and Planetary Institute in Houston believes cores would have interacted with gas and ice-rock bodies in the giant planet nursery where they formed. Such interactions, which don't appear in the model, would "tend to damp this violent physics" of throwing Uranus and Neptune out. "Whether the modeling reflects what happens in nature is not demonstrated," she says. Yet, "it's a fascinating result," says planetary dynamicist Brett Gladman of the Observatory of Nice in France. "I think it's marvelous it works."