Planet Formation on the Fast Track

Growing up in a hurry

It’s textbook astronomy. Planets form little by little, as material slowly congeals within the disk of gas, dust, and ice known to swaddle young stars. First, gravity gathers together bits of dust, which merge to form boulder-size bodies, which themselves coalesce into bigger and bigger objects. In about a million years, these form rocky planets, like Earth and Mars. Over the next few million years, gas from the disk settles around some of these solid bodies, and they grow far bigger, becoming giants like gaseous Saturn and Jupiter.

NEW IDEA. Simulation of two different protoplanetary disks (top and bottom at left). In just a few hundred years, each disk fragments into clumps (right) large enough to form Jupiterlike planets. Lighter color denotes higher density. Mayer, Quinn, et al./Science
STANDARD VIEW. Planets form within a swirling disk of material that surrounds young stars. According to the most widely accepted model, gravity gathers dust together, creating larger and larger solid bodies. L. Cook
STAR BABIES. After about a million years, the heaviest of these solid embryos (top) could weigh more than 10 times that of Earth. The embryo continues to grow (middle) by piling on gas from the disk, eventually clearing a gap around itself (bottom). Adapted from a sketch by A. Showman, Lissauer, and S. Lubow

But several astronomers now say that this model for making planets may not be entirely correct. They’ve devised an alternative theory in which planets as massive as Jupiter–whether orbiting our sun or a distant star–would form completely within just a few hundred years, rather than the millions mandated by today’s most popular planet-formation model.

Both models start with the same reservoir of planet-making materials. This spinning cloud of gas, dust, and ice, like tossed pizza dough, rapidly flattens into a disk.

Over time, gravity causes material in this so-called protoplanetary disk to clump into planet-size objects. The two models are poles apart, however, when it comes to the speed of this clustering and the size of the initial clumps.

According to the standard model, known as the core-accretion model, making Jupiter required the initial formation of a solid core 5 to 10 times Earth’s mass. That buildup would have taken about a million years. This large core then had enough gravity to attract a massive amount of gas from the protoplanetary disk to create a planet of Jovian proportions. In the accretion model, these so-called gas giants may take as much as 10 million years to form.

That’s several million years too long, contends Lucio Mayer of the University of Zurich in Switzerland. Direct telescope observations suggest that protoplanetary disks don’t last more than about 7 million years, and studies of the environment in which stars form suggest that many disks may evaporate in much less time.

War of the world-makers

The typical star in the Milky Way is born into a tough neighborhood, Mayer says. Most stars hatch in dense molecular clouds, which amount to crowded stellar nurseries. The youngsters are extremely hot, and the ultraviolet light they blast into space can evaporate a protoplanetary disk in less than 100,000 years. In the accretion model, that’s not enough time for a Jupiterlike planet to form.

Even if there are no other hot stars around, recent simulations show that when molecular clouds fragment into individual stars, the gravitational tug of war between neighbors can lop off the gaseous, outer parts of protoplanetary disks in 100,000 years or less. “If a gas giant planet can’t form quickly, it probably can’t form at all,” concludes Thomas Quinn of the University of Washington in Seattle.

If the core-accretion model is correct, gas giants ought to be rare, Quinn argues. Yet since 1995, astronomers have found more than 100 extrasolar planets, and most of them are at least as massive as Jupiter.

Quinn, Mayer, and their colleagues recently revisited the standard model of planet formation, investigating whether giant planets could form quickly. Astronomer Gerard Kuiper made such a proposal in the early 1950s, and Alan P. Boss of the Carnegie Institution of Washington (D.C.) did more extensive work beginning in the late 1980s.

Boss had been using computer simulations to study the transport of angular momentum, or rotational motion, and mass in the sun’s protoplanetary disk. He was surprised to find that gravity could cause the swirling disk, after just a few orbits about its parent star, to suddenly fragment into clumps as big as a modest-size planet. These clumps would be so massive that they’d continue pulling in more and more material.

This model is known as the gravitational-instability model.

“I did not set out to upset the apple cart,” Boss recalls. “Rather, I stumbled upon excellent reasons for thinking that there might be a better way to make giant planets.”

Also, he notes, “the extrasolar-planet discoveries have opened people’s minds in general to new ideas.” Moreover, he says, recent calculations by other scientists have suggested that many of the solid bodies that might serve as the rocky core for Jupiter-size planets in the traditional theory would spiral into the parent star before a Jupiter could form.

Further analysis by planetary scientists has shown that other effects could also cause a protoplanetary disk to become unstable and break into large fragments. For instance, within the disk, electrically charged material might pile up, triggering the disk to fragment. Or a powerful gravitational disturbance–either the tug of a star passing nearby or of a companion star to the parent–could produce instability in the disk.

But the gravitational-instability model has problems of its own. Foremost, it’s mathematically complicated and computationally intensive. So, no one has followed the simulations long enough and with enough precision to unequivocally demonstrate that the model allows for the formation of massive planets.

“Because gravitational instability is, by definition, a highly nonlinear process, only a sophisticated computer model can properly investigate it,” says Mayer. In the Nov. 29, 2002 Science, he and his colleagues describe the results of an extensive simulation based on the instability model. The team spent 2 years refining calculations to track what would happen to a protoplanetary disk over 1,000 years, more than any other simulation had considered.

Mayer and his colleagues had previously devoted a decade to state-of-the-art simulations of the formation and evolution of galaxies. For that reason, “we had already developed a fast code that can run in parallel on machines with hundreds of processors,” says Mayer. “It was almost natural to think of applying our tools and experience in numerical simulations to such a fundamental problem like planet formation.”

“I’m thrilled with what they’ve done,” says Boss. The findings, he notes, put the instability model on a surer footing and reveal that protoplanetary disks fragment into massive clumps in just a few hundred years.

Data dearth

Not every planetary scientist is convinced that the gravitational-instability model can supercede the core-accretion model. One of these skeptics is Jack Lissauer of NASA’s Ames Research Center in Mountain View, Calif. He says that although the study by Quinn, Mayer, and their colleagues is the most complete test of the instability model so far, the researchers still haven’t proven that a protoplanetary disk inevitably undergoes the type of instability required to form large planets rapidly.

Lissauer maintains that in the team’s model, the disk will first undergo a different kind of instability: Material would initially clump but then quickly spread out uniformly, hindering or preventing the large-scale clumping required to make big planets.

Mayer argues that his group has shown that a protoplanetary disk can evolve through a sequence of milder instabilities, coming to a point where the disk fragments into planet forming clumps. “I have no doubt that we are seeing that our clumps will become a real giant planet,” he says.

Lissauer has thrown another challenge to the champions of the gravitational-instability model. He notes that his team and several others have recently demonstrated that, with refinements, the core-accretion model indeed can yield a Jupiter-mass planet within the lifetime of known protoplanetary disks.

Several proposed studies may settle the controversy. In a few months, NASA is scheduled to launch the Space Infrared Telescope Facility. In conjunction with SOFIA, an infrared telescope mounted in an airplane, researchers plan to more accurately measure the lifetime of protoplanetary disks–especially the gaseous components so critical for making Jupiterlike planets, says Mayer. Improved infrared observations will help scientists because the dust in protoplanetary disks glows brightest at infrared wavelengths.

Mayer says another test of the competing models will depend on whether astronomers can find a giant, Jupiterlike planet at a distance from its parent star comparable to that of Neptune’s distance from the sun. At such a distance, the standard model would require 100 million years to make a giant planet. That’s far longer than any protoplanetary disk could survive.

For Boss, an important test would be a study of Jupiter’s deep interior to learn how much of the planet’s core is rocky. In the gravitational-instability model, the disk doesn’t form clumps heavier than six times Earth’s mass. So, if a mission to Jupiter were to find a core no heavier than that, that result would support the instability model.

“Ten years ago, we all ‘knew’ that the giant planets formed by core accretion,” says Boss. Ten years from now, he says, textbooks may tell a different story–that giant planets grew up in a hurry.

Modeling planet formation

Origins of underlying theory go back a couple centuries

The notion that our solar system’s planets emerged from a swirling disk of gas and dust dates back more than 2 centuries. Prussian philosopher Immanuel Kant had the idea first. In a 1755 treatise, he noted that several properties indicated that all the planets arose from some common material source that had originally enveloped the sun. All the known planets seemed to orbit in nearly the same plane and direction about the sun, rotated in that same direction, and have moons that orbit about their planets in the same direction. French mathematician and astronomer Pierre-Simon Laplace presented a more quantitative argument in 1796. The origin of the solar system, he asserted, was “a large nebula [that] rotated and because of the gravitation of mass to its center, a sun formed itself in the middle and condensed. The outer parts of the nebula broke into rings, and the rings rolled themselves into globes–the planets.”

In listing their reasons for assuming that planets were born from a swirling disk of matter, Laplace and Kant weren’t entirely on target. In fact, Venus and Uranus, which was discovered in 1781, rotate opposite to the direction of the other planets. Moreover, some moons are now known to orbit in the opposite direction from the others.

With the recent discovery of more than 100 massive extrasolar planets, there’s a new focus on planet origins. The scenario proposed by Kant and Laplace provides the basis for the two major models.


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