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CALIFORNIA WILD

Skywatcher

The Planet Maker
Douglas Lin Images the Birth of Other Worlds and the Survival of Solar Systems

Robert Adler

Until 1995, Astronomers pondering the birth and growth of planets had only our solar system to study. But in a mere four years, observers have detected more than 20 massive planets orbiting distant stars. In April, Paul Butler and Geoffrey Marcy, the leading discoverers of extrasolar planets, announced that astronomers had found and confirmed a full-fledged solar system: three giant planets circling Upsilon Andromedae, a Sunlike star just 44 light-years from Earth.

For the first time, theorists have a variety of planetary systems to scrutinize. Our cozy solar system now looks increasingly like a member of the aristocracy-distinctly in the minority, staunchly conservative, an island of propriety amid a fast and decadent crowd. The rest of the collection is full of surprises: Jupiter-mass planets whizzing around in Mercury-sized orbits, others careering about like loose cannonballs, and a tiny minority orbiting within their star's habitable zone. Theorists are eager to test their models against these beasts, to see if and how well their models match what observers are finding. They want answers to several vital questions: How many stars sport planets? How often do solar systems survive their wild adolescence and settle down into stable adulthood? How quickly do planetary systems form? Are Earth-like planets a rare species, or as common as Canada geese?

One of the leading planetary theorists is Douglas Lin of the University of California at Santa Cruz. Lin's primary tools are not telescopes, but mathematics and his imagination-a high-powered combination that has led him to some surprising ideas of how planets accrete and migrate, often years ahead of others, but equally often confirmed by later observations. A case in point: In 1982, Lin predicted that planets would spawn readily, but that most of them would spiral inexorably toward their stars and die. It took until October 1995, when Swiss astronomers found the first extrasolar planet skimming just above the surface of its star, 51 Pegasi, for observations to start to catch up with him.

Lin is a study in contrasts. Extremely productive, yet seemingly unhurried, intellectually expansive, yet exquisitely clear, he appears somehow casual and formal at the same time. Ask an ordinary question, and you get a thoughtful, often colorful answer. Ask a more interesting question, like why Pluto hasn't been elbowed out of the solar system, and you see Lin's eyes glow and sense ideas dancing in his mind.

It's starting to look as though as many as one in five Sunlike stars has planetary companions. Astronomers now think that planets form readily, grow quickly, and for the most part, die young. It falls to theorists like Lin to explain their presence, their persistence, and their peculiarities. Debra Fischer, part of the group that detected the Upsilon Andromedae system, says that this discovery implies that our galaxy is teeming with planetary systems. But Lin points out, "If planets are a dime a dozen, there shouldn't be any problems with planet formation. Yet, to be honest, theoretically we have many problems forming them."

The first challenge, Lin says, is finding enough matter to make a planet. Planets form from the so-called protoplanetary disks of hydrogen, helium, and other gases sprinkled with ices and cosmic dust that surround most young stars. A disk starts out as a rotating cloud; gravitational attraction among its particles causes the disk to flatten and spread like a spinning lump of pizza dough that grows to the size of our solar system or several times larger. Although disks appear to have plenty of hydrogen and helium, it's not obvious that they have enough of the heavier elements like carbon, oxygen, and silicon to build large planets. To account for our solar system, astronomers have to assume that the nine planets gobbled up every bit of the available heavy material.

Lin interprets a protoplanetary disk as a kind of dynamic fluid circulating around a developing star. Almost all of the matter that forms a star first spirals through its disk. "It's like your bank account," Lin says. "At a given time there is only a certain amount of money available. But this doesn't represent how much goes through it." It's this inward flow of material, he believes, that recharges the disk and provides enough of the heavy elements to form planetary cores.

Even if those fluid protoplanetary disks supply plenty of material, embryonic planets face other obstacles. For one, the pea- to ping-pong-ball-sized bits of ice and dust must clump together to comprise larger objects, but their minute gravity is far too weak to accomplish this. To make matters worse, at the frigid temperatures typical of the regions where giant planets form, colder than minus 175 ºC, chunks of ice are bouncier than Superballs. Even dust grains refuse to stick together.

Bothered by this roadblock, Lin and U.C. Santa Cruz physicist Frank Bridges devised a barrel-shaped gadget in which they could collide iceballs at carefully controlled speeds and temperatures. They found that the spheres of ice almost never stuck. When one pair did, the scientists noticed that it had picked up a thin coating of frost. Further experimentation with other frost-forming materials found in protoplanetary disks led to their key finding, published in 1995, that frosty coatings, especially of organic molecules like methanol, act like Velcro, multiplying ice's stickiness a hundredfold. So organic molecules, which are abundant in interstellar dust clouds and protoplanetary disks, may play a crucial role in turning tiny pellets into objects massive enough to grow through gravitational attraction.

Once this organic glue bonds dust and ice into planetesimals, solid objects up to a kilometer in size, Lin believes that gravity can then assemble the rocky inner planets and the dense cores of giant planets before the material in the protoplanetary disk disappears into the young star or out into space. Lin's models suggest that when a rocky core reaches a critical size, about ten times the mass of Earth, gravity allows it to gobble up the surrounding gas voraciously. "It's like a hungry wolf," he says. "It just eats up everything in its path."

Lin sees planet making as a slow, two-step process in which the rocky cores form first, but a very different model developed by Alan Boss, at the Carnegie Institution of Washington, shows gaseous planets forming much faster. Boss finds that gravity can compress random knots of gas into proto-giant planets in just a few thousand years. Both theories have to squeeze a lot of planet formation into the brief lives of protoplanetary disks, which observers now think may last just two or three million years.

What stops the planetary wolf from gobbling up all of the available gas? Lin and post-doctoral student Geoff Bryden designed a computer model to study what happens as a hungry planet sweeps through a disk. The growing planet quickly clears out a huge gap by adding energy to the slower-moving material outside its orbit, while draining energy from the faster-moving dust and gas inside its orbit. The Hubble Space Telescope has recently captured images of a disk with just such a gap, around a star in the constellation Libra. So the ravenous wolf matures into a territorial sheepdog that keeps everything out, and, as a result, can grow no bigger. "I think that's fascinating," says Lin, "because it provides a natural account of why Jupiter has the mass it does."

But while giant planets conveniently stop growing in Lin's models, they refuse to stay in place. The same hydrodynamic processes that spawned them and spurred their growth also set their orbital paths on the proverbial slippery slope. Inwardly moving dust and gas piles up outside the planet's orbit, but thins out inside it. As the planet tugs on the slower-moving material stacked up outside its orbit, it gives away more energy than it can regain from the thinned-out material tugging it from inside. "This is deficit spending," Lin says, "so the planet would lose angular momentum. As a result, it will spiral in toward the proto-star."

In the early 1980s, Lin predicted that giant planets would migrate toward their stars. Thirteen years later he was shown to be both right and wrong. When astronomers discovered the first extrasolar planet, circling 51 Pegasi some 50 light-years from Earth, it was not a surprise that their find was a giant planet. Massive planets are the only ones their indirect method could detect. What surprised almost everyone, though, was its orbital period. Rather than circling far from the sun, as our giant planets do, in stately cycles measured in decades, this strange creature zoomed around its star in four days. It looked like one of Lin's migrating planets had been caught in the act. But instead of falling into its sun, it teetered on the brink. "It didn't surprise me that they would migrate," Lin says. "It did surprise me that they would stop."

Lin and two colleagues raced to explain how a planet could narrowly avert a fiery death. They found two mechanisms that could shunt a planet into a parking orbit close to its sun. One possibility is that the star's magnetic field carves out an inner hole in the disk. Cut loose from the gravitational tug-of-war with the disk, the planet could no longer shed angular momentum and would have to stop its migration. The second mechanism depends on how fast young stars spin. If a star is rotating rapidly, tidal effects can transfer enough angular momentum from the star to the planet to slam on the brakes.

But stars gradually slow down, which leads to one of Lin's most surprising predictions, a kind of planetary death march. Like an airplane in a holding pattern above an overtaxed airport, a giant planet orbits and waits. In due time, however, its star is no longer spinning fast enough to keep it in orbit, so the planet plunges into the star. But if a rechargeable protoplanetary disk can spawn one suicidal planet, why not more? "This type of process," Lin says, "could actually cause many Jupiters to have migrated toward the Sun during its infancy, and been accreted, before the rest of the [current] solar system was formed."

The fate of a planet falling into a star seems clear, but Lin looks for details to a planet's demise, hoping that the crime scene won't always be scrubbed clean of clues. He and his colleagues have recently created an exquisitely detailed computer model of a planet's encounter with its parent star. Applying basic physical laws, the simulation starts with a Jupiter-sized planet orbiting in the outer reaches of the sun's gaseous envelope. In its first orbit of 165 minutes, the planet is stripped of about eleven percent of its mass. It never completes a second orbit. Creating a titanic shock wave as it plows through increasingly dense layers of the sun's atmosphere, the planet heats up, flattens, and starts to vaporize. Its rocky core lasts the longest, but it also melts within 100 seconds.

The planet vaporizes but does not vanish. If Lin's model is right, a planet like Jupiter would leave 90 percent of its remains as high levels of heavy elements in its sun's outer envelope, the part we can see. In fact, observers have measured the spectra of the parent stars of the first 16 extrasolar planets discovered; four of the stars have more iron than usual, and two, Lin says, are "super-metal-rich." These clues lend support to Lin's theory, but once again he has to wait for observations-perhaps from proposed orbiting interferometers and ground-based radio telescopes-to catch up.

Although the majority of the giant planets discovered to date fit the 51 Pegasi profile of a planet orbiting its star closer than Mercury orbits the Sun, a handful buck this trend for an even more radical one. While they take familiar lengths of time to orbit their stars, from half a year to four-and-a-half years, they career in highly eccentric orbits-dramatically unlike the circular orbits that have kept the planets in our solar system out of each other's way for billions of years.

To explain these wayward wanderers, Lin and other theoreticians, like Fred Rasio at MIT, follow a line of reasoning advanced by French mathematician Pierre Laplace 200 years ago. When several giant planets form at the same time, small perturbations caused by the gravitational tug of one on another gradually build up, distorting both orbits. Once the orbits of two giant planets cross, one is likely to be slung from its solar system, while the other is tugged into an elliptical orbit.

As unlikely as it seems for Earth-like planets to survive in a system disrupted by either marching or marauding Jupiters, they may still be common. Planet hunter Marcy places his bet on the 95 percent of stars surveyed that do not possess Jupiters in elongated orbits. Lin is more optimistic, noting that the small, rocky planets in our solar system formed in vastly different environments within the Sun's protoplanetary disk. What's more, he believes that after the last giant planet's death plunge, enough time and material remain to form a few Earths, which would account for our solar system. Lin even finds encouragement in Upsilon Andromedae's small family, where short- and long-orbit planets peacefully co-exist.

So Lin makes another prediction. "We don't know any Earth-like objects around other stars yet, because our sensitivity is not quite there," he says. "But eventually, people will discover these. And I believe they are very common. I believe around most stars you would find them." Within a decade or so, powerful new instruments could confirm or refute Lin's claim. If he's right this time, an even more farsighted thinker will be vindicated. In 1600, the Renaissance scholar Giordano Bruno was burned at the stake in Rome for advocating much the same idea. Bruno had sealed his fate some two decades earlier when he wrote:

"Innumerable suns exist; innumerable earths revolve around these suns in a manner similar to the way the seven planets revolve around our sun. Living beings inhabit these worlds."

Robert Adler currently writes for New Scientist in London. His article on archeoastronomy appeared in the Summer 1999 issue of California Wild.

Fall 1999

Vol. 52:4