- HONOLULU - For thousands of years, humans have thought about and searched
for worlds outside our solar system - for planets like ours that can support
life.
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- 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.
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- 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.
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- "The major change has been access
to large telescopes like the Keck 1/8a telescope in Hawaii with a mirror
30 feet across 3/8," 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."
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- 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.
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- 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," Smith says. "It
was like, 'Wow!' We had not really expected that."
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- In fact, new research shows that 5 percent
of the stars surveyed show evidence of planets.
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- 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."
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- 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.
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- 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," Smith says.
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