CfA Release No.: 06-24
September 14, 2006

Note to editors: A high-resolution image to accompany this release will be
posted 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://cfa-www.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 Aguilar
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
617-495-7463
cpulliam@cfa.harvard.edu