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Posted by baalke on September 14, 2006, 3:14 pm
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http://www.cfa.harvard.edu/press/pr0624.html
Harvard-Smithsonian Center for Astrophysics
Press Release
Release No.: 06-24
For Release: EMBARGOED UNTIL 10:00 a.m. EDT, September 14, 2006
Note to editors: High-resolution images to accompany this release are
online at http://www.cfa.harvard.edu/press/pr0624image.html.
Strange New Planet Baffles Astronomers
Washington, DC - Using a network of small automated telescopes known as
HAT, Smithsonian astronomers have discovered a planet unlike any other
known world. This new planet, designated HAT-P-1, orbits one member of
a
pair of distant stars 450 light-years away in the constellation
Lacerta.
"We could be looking at an entirely new class of planets," said Gaspar
Bakos, a Hubble fellow at the Harvard-Smithsonian Center for
Astrophysics (CfA). Bakos designed and built the HAT network and is
lead
author of a paper submitted to the Astrophysical Journal describing the
discovery.
With a radius about 1.38 times Jupiter's, HAT-P-1 is the largest known
planet. In spite of its huge size, its mass is only half that of
Jupiter.
"This planet is about one-quarter the density of water," Bakos said.
"In
other words, it's lighter than a giant ball of cork! Just like Saturn,
it would float in a bathtub if you could find a tub big enough to hold
it, but it would float almost three times higher."
HAT-P-1 revolves around its host star every 4.5 days in an orbit
one-twentieth of the distance from Earth to the Sun. Once each orbit,
it
passes in front of its parent star, causing the star to appear fainter
by about 1.5 percent for more than two hours, after which the star
returns to its previous brightness.
HAT-P-1's parent star is one member of a double-star system called ADS
16402 and is visible in binoculars. The two stars are separated by
about
1500 times the Earth-Sun distance. The stars are similar to the Sun but
slightly younger - about 3.6 billion years old compared to the Sun's
age
of 4.5 billion years.
Although stranger than any other extrasolar planet found so far,
HAT-P-1
is not alone in its low-density status. The first planet ever found to
transit its star, HD 209458b, also is puffed up about 20 percent larger
than predicted by theory. HAT-P-1 is 24 percent larger than expected.
"Out of eleven known transiting planets, now not one but two are
substantially bigger and lower in density than theory predicts," said
co-author Robert Noyes (CfA). "We can't dismiss HD209458b as a fluke.
This new discovery suggests something could be missing in our theories
of how planets form."
Theorists had already considered a number of possibilities to explain
the large size of HD 209458b, but so far without success. The only way
to puff up these giant planets beyond the size calculated from
planetary
structure equations would be to supply additional heat to their
interiors. Simple heating of the surface due to the host star's
proximity would not work. (If it could, all close-in transiting giant
planets should be expanded, not just two of them.)
One way to inject energy into the planet's center is by tipping it on
its side, similar to Uranus in the solar system. A planet in that state
orbiting close to its star would be subjected to tidal heating of the
interior. But according to Smithsonian astronomer Matthew Holman (who
was not a member of the discovery team), "the circumstances required to
tip over a planet are so unusual that this would seem unlikely to
explain both known examples of inflated worlds."
According to co-author Dimitar Sasselov (CfA), "Another explanation for
HD 209458b's large size was tidal heating due to an eccentric orbit,
but
recent observations have pretty much ruled that out."
The scientists will continue observing HAT-P-1 to see if such an
explanation could hold in this case, but "until we can find an
explanation for both of these swollen planets, they remain a great
mystery," Sasselov said.
The HAT network consists of six telescopes, four at the Smithsonian
Astrophysical Observatory's Whipple Observatory in Arizona and two at
its Submillimeter Array facility in Hawaii. These telescopes conduct
robotic observations every clear night, each covering an area of the
sky
300 times the size of the full moon with every exposure.
HAT searches for planets by watching for stars that dim slightly when
an
orbiting planet crosses directly in front of the star as viewed from
Earth - a sort of mini-eclipse. Transits offer astronomers a unique
opportunity to measure a planet's physical size from the amount of the
dimming. Combined with the mass, which is determined by measuring the
amount of the star's wobble as the planet orbits it, researchers then
calculated a planet's density. Measurements of the wobble of HAT-P-1's
parent star were led by co-author Debra Fischer of San Francisco State
University.
Major funding for HATnet was provided by NASA. More information about
HAT is available online at http://www.cfa.harvard.edu/~gbakos/HAT/.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016
cpulliam@cfa.harvard.edu
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