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- Nashville, Tenn., and Berkeley, Calif. - Astronomers
have witnessed for the first time a distant planet passing in front of
its star, providing direct and independent confirmation of the existence
of extrasolar planets that to date have been inferred only from the wobble
of their star.
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- "This is the first independent confirmation of a
planet discovered through changes in a star's radial velocity and demonstrates
that our indirect evidence for planets really is due to planets,"
said Geoffrey Marcy, a professor of astronomy at the University of California,
Berkeley.
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- Marcy and his colleagues, Paul Butler of the Department
of Terrestrial Magnetism at the Carnegie Institution of Washington in Washington,
D.C., and Steve Vogt of UC Santa Cruz and Lick Observatory, first detected
a wobble in the star called HD 209458 on Nov. 5. Ascribing the wobble to
a nearby planet, they were able to estimate its orbit and approximate mass.
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- As with all new planets they detect, the team immediately
brought it to the attention of collaborator Greg Henry, an astronomer at
the Tennessee State University Center of Excellence in Information Systems
in Nashville. He conducts research with several automatic telescopes at
Fairborn Observatory, a non-profit research foundation located in the Patagonia
Mountains of southern Arizona.
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- Henry turned one of his automated telescope on the star
at the time Marcy and Butler predicted the planet would cross the face
of the star if the planet's orbital plane were lucky enough to carry it
between Earth and the star. Until now, none of the 18 other extrasolar
planets Marcy and Butler have discovered has had its orbital plane oriented
edge-on to Earth so that the planet could be seen to transit the star,
nor have any of the other planets discovered by other researchers.
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- However, on Nov. 7, Henry observed a 1.7 percent dip
in the star's brightness. Because the planet orbits its star once every
3.523 days, he plans to repeat his observations on Sunday, Nov. 14.
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- "This planetary transit occurred at exactly the
time predicted from Marcy's observations, confirming absolutely the presence
of a companion," Henry said. "The amount of dimming of the star's
light during the transit also gives us the first-ever measure of the size
and density of an extrasolar planet. We've essentially seen the shadow
of the planet and used it to measure the planet's size."
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- The star HD 209458 is 47 parsecs (153 light years or
1.4 million billion kilometers or 859,000 billion miles) away in the constellation
of Pegasus, and is about the same age, color and size as our own Sun. It
is very near the star, 51 Pegasi, around which the first extrasolar planet
was discovered in 1995.
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- With the orbital plane of the planet known, the astronomers
for the first time could determine precisely the mass of the planet and,
from the size of the planet measured during transit, its density.
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- Interestingly, while the planet's mass is only 63 percent
of Jupiter's mass, its radius is 60 percent bigger than that of Jupiter.
This fits with theories that predict a bloated planet when, as here, the
planet is very close to the star.
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- The density, about 0.2 grams per cubic centimeter, means
it is a gas giant like Jupiter. However, such gas giants could not have
formed at the distance this planet is from its star.
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- "This supports the theory that extrasolar planets
very near their star did not form where they are, but formed farther out
and migrated inward," Henry said.
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- Various groups around the world have been searching for
planets by looking for dimming of stars, or as Marcy says, "staring
at the sky and seeing if any star blinks." To date, none of these
searches has turned up a new planet.
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- "With this one, everything hangs together,"
Marcy said. "This is what we've been waiting for."
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- The research was supported by the National Aeronautics
and Space Administration, the National Science Foundation, Sun Microsystems
and the Richard Lounsbery Foundation.
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- _____
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- Geoff Marcy can be reached at gmarcy@etoile.berkeley.edu
or (510) 642-1952; Greg Henry is at henry@schwab.tsuniv.edu or (615) 963-7017;
Paul Butler is at paul@dtm.ciw.edu or (202) 686-4370 x4401; and Steve Vogt
is at vogt@ucolick.org
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