A renaissance in the search for planets
HONOLULU — For millennia, man-kind has pondered and searched for worlds outside our solar system - for planets like Earth that could support life.
But since the advent of modern astronomy centuries ago, detection of distant planets has proved to be as difficult as finding grains of sugar on a beach. Stars, billions of times more brilliant than the worlds that circle them, make planets all but impossible to find. And decades of intense observation yielded only false alarms, earning planet-hunting a reputation as a backwater of astronomy.
During the past three years, however, this perception has radically changed. Through advances in technology, an improved understanding of planetary behavior, and increased access to better telescopes, astronomers have found 17 planets since 1995. These discoveries have revolutionized planetary science, forcing scientists to revise long-held theories about the universe and making planet-searching one of the hottest fields of astronomy.
"The major change has been access to large telescopes like the Keck [a telescope in Hawaii with a mirror 30 feet across]," says William Cochran, an astronomer at the University of Texas at Austin. "With big scopes, you get a lot more light. And the faster you can get light, the faster you can detect these planets."
In many ways, the recent discovery of what could be a nascent solar system 220 light-years from Earth is a symbol of this planet-hunting renaissance. Images of the would-be solar system were first captured by the Keck telescope. Later, using the Hubble Space Telescope, University of Hawaii astronomer Bradford Smith discovered that there might be a planet within the new solar system. He found the planet by searching the heavens in a different way - by looking at disks of dust around stars.
He sifted infrared images of the star 220 light years away, dubbed HR 4796A. Inside its disk sat a ring that looked like hula hoop. "When we pulled the image of this star ring up on the computer screen, it looked like Saturn," says Dr. Smith. "It was like, 'Wow!' We had not really expected that."
The same image that floored Smith brought a room full of normally sedate astronomers to their feet for a standing ovation at the American Astronomical Society meeting earlier this month.
Indeed, the find is a breakthrough. According to previous theories, no planetary candidate should be there. The star is only 8 million to 10 million years old, supposedly far too young to have developed a large, far-flung planet like the one indicated in the Hubble images.
But that's really no great surprise. Beyond the quixotic object found by Smith in the gloaming of deep space, astronomers are finding other planets that do not conform to traditional models.
For one thing, many are massive - as large as Jupiter but with short orbits closer to their host stars than Mercury is to the sun. Theoretically, Jupiter-like planets were supposed to be found orbiting farther away from their host stars, where they would not be sucked in by the stellar bodies' strong gravitational pull.
Many of the newly discovered planets were also found to have elliptical orbits - unlike those of planets in our solar system, which are largely circular.
Theorists are now scrambling to rebuild planetary theory based on these observations. They have already come up with some explanations. "Hot Jupiters" might be migrating toward their host stars, destroying other planets that lay in their paths, scientists say. Meanwhile, the elliptical orbits of extrasolar planets might be caused by the gravitational pull of either a nearby star or planet.
IN addition to learning more about planets themselves, scientists have learned more about how to look for them. Before the binge in planetary discovery, money to finance planet searches was hard to come by. "For a long time, whenever anyone asked for money to find planets, everybody laughed and threw their proposal away," says William Borucki, a researcher at the NASA/Ames Research Center in California.
Then in October 1995, two astronomers in Switzerland noticed that the light spectrum of the star 51 Pegasi was wobbling. Upon closer examination, the scientists concluded that the phenomenon was caused by a large planet orbiting very close to the star. After that, the wobble - previously only a theory - became a telltale sign for planet hunters and helped resuscitate the field.
In fact, new research shows that 5 percent of the stars surveyed show evidence of planets.
Still, astronomers are far from a consensus that an Earth-like body exists. "It is still being argued today by many people that our own solar system is an absolute anomaly," says Smith. "On the other side of it, there are many people who believe that when you form stars you form planets at the same time."
In the next decade, a new generation of highly sensitive telescopes will turn their mirrors heavenward. Most will dedicate sky time to planet searches. Scientists are also perfecting new techniques that use less-expensive equipment to locate planets. And direct imaging of distant stars with infrared instruments promises to produce more snapshots of stars and planets, too.
But all these new high-powered tools still can only give scientists evidence that planets exist - they cannot give a direct picture of an Earth-like planet. "The problem is Earth-like planets are small and whatever effect they have is going to be much more difficult to detect," says Smith.