Most massive transiting extrasolar planet discovered

Astronomers at the Harvard-Smithsonian Centre for Astrophysics (CfA) claim to have discovered the most massive known transiting extrasolar planet.

Washington, May 3 : Astronomers at the Harvard-Smithsonian Centre for Astrophysics (CfA) claim to have discovered the most massive known transiting extrasolar planet.

The gas giant planet, HAT-P-2b, contains more than eight times the mass of Jupiter, the biggest planet in our solar system, but, its powerful gravity makes it only slightly larger than Jupiter. A person weighing 150 pounds on Earth would weigh as much as 2100 pounds on HAT-P-2b and experience 14 times the gravity on Earth.

HAT-P-2b shows other unusual characteristics as well.

An intriguing feature is its highly eccentric (e=0.5) orbit. 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. In Earth's context that would mean looping in an elliptical orbit stretching from Mercury to Mars.

Due to its unusual orbit, the planet also gets enormously heated up when it passes close to the star, and then cools off as it loops out again.

Normally, gravitational forces between a star and planets tend to circularize the orbit of a close-in planet. There is also no other planet known with such an eccentric, close-in orbit. All other known transiting planets also have circular orbits.

Researchers believe 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 do not rule it out either.

Although the planet 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, scientists further say. "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. But we eliminated every other possibility, so we knew we had a really weird planet," said CfA astronomer G�sp�r Bakos.

"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," said CfA astronomer and co-author Robert Noyes.

"This object is close to the boundary between a star and a planet. With 50 percent more mass, it could have begun nuclear fusion for a short time," added Harvard co-author Dimitar Sasselov.

HAT-P-2b orbits the star HD 147506, 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. With visual magnitude 8.7, HD 147506 is the fourth brightest star known to harbour a transiting planet.

Once every five days and 15 hours, the planet crosses directly in front of the star as viewed from Earth - a sort of mini-eclipse.

Astronomers say such a transit offers a unique opportunity to measure a planet's physical size from the amount of dimming.

Researchers discovered HAT-P-2b 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 findings appear in the Astrophysical Journal.

ANI