Astronomers Find Super-massive Planet (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Sun, 6 May 2007 22:13:55 GMT
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Harvard-Smithsonian Center for Astrophysics
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Christine Pulliam
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For Release: Wednesday, May 02, 2007
Release No.: 2007-11
Astronomers Find Super-massive Planet
Cambridge, MA -- Today, astronomers at the Harvard-Smithsonian Center for
Astrophysics (CfA) announced that they have found the most massive known
transiting extrasolar planet. The gas giant planet, called HAT-P-2b,
contains more than eight times the mass of Jupiter, the biggest planet in
our solar system. Its powerful gravity squashes it into a ball only
slightly larger than Jupiter.
HAT-P-2b shows other unusual characteristics. It has an extremely oval
orbit that brings it as close as 3.1 million miles from its star before
swinging three times farther out, to a distance of 9.6 million miles. If
Earth's orbit were as elliptical, we would loop from almost reaching
Mercury out to almost reaching Mars. Because of its orbit, HAT-P-2b gets
enormously heated up when it passes close to the star, then cools off as
it loops out again. Although it has a very short orbital period of only
5.63 days, this is the longest period planet known that transits, or
crosses in front of, its host star.
"This planet is so unusual that at first we thought it was a false alarm
-- something that appeared to be a planet but wasn't," said CfA astronomer
Gaspar Bakos. "But we eliminated every other possibility, so we knew we
had a really weird planet."
Bakos is lead author of a paper submitted to the Astrophysical Journal
describing the discovery. That paper is available online at
http://arxiv.org/abs/0705.0126
HAT-P-2b orbits an F-type star, which is almost twice as big and somewhat
hotter than the Sun, located about 440 light-years away in the
constellation Hercules. Once every 5 days and 15 hours, it crosses
directly in front of the star as viewed from Earth-a sort of mini-eclipse.
Such a transit offers astronomers a unique opportunity to measure a
planet's physical size from the amount of dimming.
Brightness measurements during the transit show that HAT-P-2b is about
1.18 times the size of Jupiter. By measuring how the star wobbles as the
planet's gravity tugs it, astronomers deduced that the planet contains
about 8.2 times Jupiter's mass. A person who weighs 150 pounds on Earth
would tip the scale at 2100 pounds, and experience 14 times Earth's
gravity, by standing on the visible surface (cloud tops) of HAT-P-2b.
CfA astronomer and co-author Robert Noyes said, "All the other known
transiting planets are like 'hot Jupiters.' HAT-P-2b is hot, but it's not
a Jupiter. It's much denser than a Jupiter-like planet; in fact, it is as
dense as Earth even though it's mostly made of hydrogen."
"This object is close to the boundary between a star and a planet," said
Harvard co-author Dimitar Sasselov. "With 50 percent more mass, it could
have begun nuclear fusion for a short time."
An intriguing feature of HAT-P-2b is its highly eccentric (e=0.5) orbit.
Gravitational forces between star and planet tend to circularize the orbit
of a close-in planet. There is no other planet known with such an
eccentric, close-in orbit. In addition, all other known transiting planets
have circular orbits.
The most likely explanation is the presence of a second, outer world whose
gravity pulls on HAT-P-2b and perturbs its orbit. Although existing data
cannot confirm a second planet, they cannot rule it out either.
HAT-P-2b orbits the star HD 147506. With visual magnitude 8.7, HD 147506
is the fourth brightest star known to harbor a transiting planet, making
the star (but not the planet) visible in a small, 3-inch telescope.
HAT-P-2b was discovered using a network of small, automated telescopes
known as HATNet, which was designed and built by Bakos. 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. As part of an international campaign, the Wise
HAT telescope, located in the Negev desert (Israel) also took part in the
discovery. The HAT 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. About 26,000 individual observations were made
to detect the periodic dips of intensity due to the transit.
Major funding for HATnet was provided by NASA. More information about HAT
is available online at
http://www.hatnet.hu
.
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