In the footsteps of Descartes: does stormy weather spawn planets?

Nearly four centuries ago, the French mathematician and philosopher Rene Descartes envisioned a cosmos in which whirlpools of matter called vortices generated the structure of the universe by pulling matter together. By the time Isaac Newton published his theory of gravitation, about 50 years later, Descartes' primitive notion had begun to fall by the wayside.

Now, an astrophysicist suggests that vortices may indeed hold the key to creation -- at least when it comes to the birth of the giant planets.

For the past 15 years, astronomers have gathered more and more evidence that many newborn stars are encircled by flattened disks of gas and dust. Though the disks survive less than 100 million years -- the blink of an eye in astronomical time -- they hold a revolution in the making. Under the right conditions, ice and dust particles in the disk collide and clump together, spawning a plethora of new worlds.

According to standard theory, the raw material for the solar system's innermost residents, including Mercury, Venus, Earth, and Mars, originated as microscopic dust grains in a disk surrounding the sun. Toward the center of the solar disk, dust grains gathered into kilometer-size boulders known as planetesimals. Colliding and sometimes sticking together, these planetesimals grew bigger and more massive. As their gravitational tug increased, they grew larger still, ultimately forming the solid cores of the inner planets.

In more distant, colder regions of the disk, ice existed along with dust. Together, dust and ice accumulated to form the solid core of the giant planets Jupiter and Saturn (Jupiter's core alone is 10 to 20 times as massive as Earth). These behemoths also possess huge amounts of hydrogen and helium gases, which form their thick atmospheres. Presumably, their massive cores gravitationally grabbed these gases from the solar disk. But therein lies a problem.

Studies suggest that sunlike stars can hold onto hydrogen and helium in their disks for only about 10 million years, because these light gases can easily escape gravity's pull (SN: 2/18/95, p.111). The cores of Jupiter and Saturn must therefore have snared the gases within that short time. However, some models indicate that the cores would have had difficulty in gravitationally trapping gas so quickly.

Alternatively, the dust and gas that make up Jupiter could have coalesced all at once, in one big lump. But most models can't then explain how the mixture of materials could have quickly segregated into a dense, rocky core and a hydrogen-rich exterior.

Such obstacles have prompted a number of planetary scientists to conclude that planets akin to Jupiter and Saturn --if they exist at all outside the solar system -- are probably rare.

But Fred C. Adams of the University of Michigan in Ann Arbor has a different idea.

He speculates that cyclones, or vortices, generated in the preplanetary disk could help form bodies like Jupiter or Saturn rapidly. The vortices would resemble the whirlpool of an earthly hurricane or the raging Jovian storm known as the Great Red Spot.

Adams described the unpublished work, developed in collaboration with Richard Watkins of the University of Michigan, during an astronomy colloquium in February at the University of Maryland in College Park.

Adams and Watkins began their study by showing that vortices can occur naturally within a rotating, circumstellar disk. They note that any system with differential rotation -- rotation that shears apart adjacent circulating layers in the disk -- can generate such whirlpools.

The typical vortex might not last long. But if certain types of vortices persist, dust and gas would get pushed to the center by the so-called Coriolis force, an effect of the whirling motion.

However, the storm also generates an outward pressure; this force expels the gas but has little influence on the movement of the solid dust particles. Thus, argue Adams and Watkins, some types of vortices can rapidly separate the raw materials for a Jupiter or a Saturn into a rocky core and a gas-rich exterior.

"If you have just gravity acting, there's no way to segregate heavy elements -- the rocks -- from the gas," says Adams. "You need a further mechanism...and we suggest vortices can do just that."

Adams notes that even those vortices that fail to produce a planet may play a leading role in the evolution of the star at the disk's center. When vortices take shape, the friction created as neighboring layers of the rotating disk rub against each other robs the disk of angular momentum. This slows the disk's rotation, hastening its gravitational capture by the young, sunlike star at the center. The vortex motion may thus explain why disks around young stars don't survive for very long, Adams says.

Douglas N.C. Lin of the University of California, Santa Cruz, says he finds the study intriguing but notes several caveats. "It's interesting work, but it's not yet fully developed," he cautions.

Lin observes that the kind of vortex Adams and Watkins propose has orderly internal motion, like a well-behaved merry-go-round. In fact, he says, the typical storm system, like the Great Red Spot, contains all kinds of tiny whirlpools and eddies. In such a complex, real-life system, dust may not migrate to the center of the vortex, a key aspect of the planet-forming theory.

He adds that the standard picture of the formation of the solar system explains neatly why the inner planets are mostly rock and many of the outer planets have massive atmospheres. The outer planets presumably formed in the more distant, chillier parts of the solar disk, where gas pressure is lower. The lower pressure allows the gravitational pull of a rocky core to capture more gas at the outskirts of the solar disk than it could closer to the center.

In contrast, the vortices envisioned by Adams and Watkins would occur at random, with equal probability everywhere in the disk, Lin says. Thus, their model can't explain why the gas giants formed only in the outer part of the solar system.

Adams responds that although vortices can indeed be generated anywhere in the solar system, they may form more readily and grow larger in the outer reaches. He notes that planet formation may require a combination of vortices and the standard method of accumulating gas and dust into planetesimals.

In an independent study, Adams and Willy Benz of the University of Arizona in Tucson consider the earliest stage of star formation, when the amount of material in the disk is comparable to the mass of the fledgling star. During this era, they note in their unpublished work, density variations in the disk can create knots of gas that orbit the immature star.

An orbiting knot typically contains about 1 percent of the mass in the disk. If this tiny companion grabs an appreciable share of the matter that has yet to rain down on the embryonic star, it might evolve into a giant planet -- or even become a star in its own right. Adams and Benz note that many stars seem to be born in pairs. Their model, they say, offers an explanation of how a single star and its disk could fragment into two.