Science and Medicine : Are We Alone? : Technology Is Advancing the Search for Other Planetary Systems

Scientists have succeeded in recent years in unraveling some of the great mysteries of the universe, but some where out there, trillions of miles away, the grandest trophy of all awaits discovery.

That trophy is a simple answer to the most haunting question of all: Are we alone?

The quest for an answer has proved nearly as frustrating as it is important, because the distance between stars is so vast that it has been impossible to prove whether other stars even have planets and thus could support life. But today, scientists see themselves on the brink of discovering whether planetary systems are rare or common throughout the universe.

“The technology is now at hand to search for planetary systems,” said David Black, director of the Lunar and Planetary Institute in Houston. “We are witnessing the birth of a new discipline.”

That technology will include a wide range of orbiting telescopes that will have to function on a level of perfection almost beyond imagination. It will take years, and possibly decades, to design, build and put the telescopes in orbit, but scientists believe they are ready to begin answering a question that has bedeviled humans since the dawn of history.

The search is hampered by the fact that there is only one known planetary system throughout the entire universe.

“We only have one we can study, and that is our own,” Peter Goldreich of Caltech told a planetary seminar at UC Berkeley marking the 100th birthday of the Astronomical Society of the Pacific.

But because the solar system formed about 4.5 billion years ago, there is little evidence remaining that has not been severely altered by the passage of time.

“All the clues we have are very old,” Goldreich said.

What happened all those many years ago to cause nine planets to form around a young, undistinguished star?

While scientists disagree on the details, there is a consensus on how it all began.

Experts believe that in its prenatal stage the sun was a giant cloud of gas and dust, called a “solar nebula,” drifting through the galaxy. Like clouds in the Earth’s atmosphere, some areas were more dense than others, and they gradually accumulated enough material to create weak gravitational fields.

As the denser pockets grew in mass they swirled around each other and ultimately merged, causing the core of the cloud to grow ever more compact.

In time, the inner core of the cloud collapsed from the increasingly strong gravitational field, causing an implosion that set off a nuclear chain reaction. It was a spectacular--but not unusual--event, repeated billions of times throughout the Milky Way galaxy.

Another star had been born, a star destined to become a modest, average performer, like trillions and trillions of others.

After its birth, the sun remained cloaked in a cloud of gas and dust for awhile. But the new star generated powerful forces, including an immense magnetic field and fierce solar winds that soon began sweeping the cloud away from the sun’s poles and into a flat disk that swirled around its equator. The dust and gas then coalesced into planets.

In recent years, space-borne observatories and advances in the sensitivity of ground-based telescopes have found evidence that disks routinely form around young stars, but they do not last very long.

Richard Terrile of the Jet Propulsion Laboratory and Brad Smith of the University of Arizona captured a dramatic photograph of a star known as Beta Pictoris, showing a band of infrared radiation around its central core, indicating the presence of a disk.

Astronomers Steven Beckwith of Cornell University and Anneila Sargent of Caltech took that one step further and discovered several young stars with disks. But their evidence suggests that a disk lasts only through a star’s infancy.

Other findings support that.

The husband-and-wife team of Steve and Karen Strom of the University of Massachusetts found that 60% of the stars they looked at showed signs of a disk.

“But as they age, stars lose the infrared signature,” Steve Strom said.

The disk disappears by the time a star is only about 3 million to 10 million years old.

That leaves astronomers with the predicament of having evidence that disks are a common part of star formation, but with real difficulties in showing how a system like the sun’s should be able to form before the star blows the gas and dust away.

“The question is, can you form planets before the solar nebula goes away?” asked astronomer Peter Bodenheimer of Lick Observatory.

Does that suggest that our solar system is possibly unique, an aberration in the star formation process?

Probably not, most experts believe.

“We expect to find that planetary systems occur rather frequently in nature,” Black, of the Lunar and Planetary Institute, said.

That optimism is supported by a team of Canadian astronomers who announced two years ago that they had detected evidence of planets orbiting around at least two nearby stars. The stars are so bright, and the planets so dim, that it is not possible to see them, but astronomer Bruce Campbell, who led the Canadian team, insists they are there.

Planets would not simply travel in perfect circles around a stationary star. Instead, the entire system would orbit around a point at the center of the system’s mass.

That would cause the star to “wobble” slightly, Campbell said. Sometimes the star would appear to be moving slightly closer to a distant observer, and at other times it would appear to be receding.

But since the star is so much more massive than the planets, the point around which everything rotates would still be within the star, just slightly off center. That, in turn, makes detection a formidable task.

Scientists are now proposing some extremely sophisticated telescopes in hopes of finally resolving the issue. All of the telescopes would have to be placed in orbit because the measurements are so exacting that the Earth’s atmosphere would distort the results.

Physicist Eugene Levy of the University of Arizona believes it should be possible to detect planets through a process known as “astrometry.” That involves tracking stars so precisely that any deviation--such as that caused by orbiting planets--could be demonstrated.

A star with planets should travel in small circles. That would cause a slight change in the angle when viewed from the Earth.

But what a tiny change it would be.

Levy tried to dramatize that point in a presentation during the Berkeley symposium. Standing behind a lectern, Levy tapped the side of a light on top of the speaker’s stand.

He told his listeners to look at him while sighting along the side of the light. Then, he said, move in along the light the width of one atom. That’s right, one atom.

The change in the angle is comparable to what the orbiting observatory would have to measure.

“We’re talking small here,” he said.

Furthermore, it takes years for large planets to orbit a star, so the observatory would have to work flawlessly for 10 to 20 years, unattended, in space.

It would be so sensitive to movement that even if astronauts could get up to it they could not touch it, so “it would not be possible to go up there and diddle with it from time to time,” he added. William Borucki of NASA’s Ames Research Center in Mountain View has proposed a system that could detect planets more quickly, but it is at least as demanding as Levy’s.

Borucki said that if a planet orbiting another star passes between the observer and the star, it would block out part of the star’s light during its transit. An extremely sensitive orbiting observatory should be able to detect that drop in luminosity if the planet is in an orbit close to the star.

Borucki’s plan would have the advantage of detecting Earth-like planets that might be in the proper orbit to sustain life, but the precision demanded by such an observatory is mind-boggling.

“Planets are small and stars are large, so your signal is not going to be very strong,” Borucki said. “But it’s within the realm of possibility.”

Yet even he admits, “It would take a real act of courage to say you’ve found a planet.”

JPL’s Terrile believes the only way people are going to be satisfied that there are planets out there is if they can see them. That is not possible with existing technology because planets are so dim compared to stars that it would be comparable to seeing a lighted match in front of a glaring searchlight.

“A planet the size of Jupiter would be about one billion times fainter than its star,” Terrile said.

Nonetheless, Terrile is working on a NASA proposal to develop a telescope that would block out the light of the star and still be sensitive enough to see the planet. The telescope would need a five-foot mirror free of microscopic imperfections. And it would have to remain so even after being launched into orbit.

No one has ever built such a perfect mirror, but Terrile is working with the people who fabricated the optics for the Hubble Space Telescope and he believes it may be possible.

Many astronomers are eager to see devices such as these made available, because they are convinced that planetary systems abound throughout the universe. All of that gas and dust surrounding newborn stars ends up someplace, and they see no reason why the sun’s planetary system should be unique.

But a few suggest that in many cases stellar disks form something else entirely--the dreaded “brown dwarfs.”

It is possible that the gas and dust left over after star formation simply form another star, many theorists have argued, but there isn’t enough of it for the star to get massive enough to ignite. If so, there could be trillions of “brown dwarfs” in the galaxy, small stars that are too dim to be seen.

It would greatly diminish the number of planetary systems--and thus the chances of intelligent life elsewhere--if many stellar disks accrete into brown dwarfs instead of planets.

Many scientists, however, see no reason to think the Earth is unusual.

“We haven’t found any reason to think that a planetary system is unique or a miraculous thing,” said Alan Boss of the Carnegie Institute.

If that turns out to be wrong, it would suddenly become a very lonely universe.