Harvard-Smithsonian Center for Astrophysics
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/.
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