"Weird" New Planet Weighs as Much as 2,500 Earths

Kate Ravilious
for National Geographic News
May 3, 2007

With temperatures ranging from 1000 to 2000°C, gravity 15 times stronger than Earth's, and a year that lasts just 5.6 of our days, HAT-P-2b is not a planet you'd want to visit for vacation.

The unusual gas giant—located 440 light-years away in the constellation Hercules—turns out to be the most massive planet found outside our solar system so far.

Astronomers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, spotted the superdense planet using the HATNet global network of automated telescopes, which scans a large fraction of the Northern Hemisphere sky every night to search for planets.

HAT-P-2b, the second planet the project has discovered, stunned scientists with far-out features unprecedented for an alien world. (Related: "New Planet 'Bonanza' Discovered at Center of Milky Way [October 4, 2006].)

Weird World

"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 Center for Astrophysics astronomer Gáspár Bakos in a statement. Bakos is lead author of a paper submitted to the Astrophysical Journal that describes the discovery.

"But we eliminated every other possibility, so we knew we had a really weird planet," he said.

The research team was able to determine some of the planet's properties by watching as it passed across the face of its star—known as HD 147506—causing a slight dimming of the star's light. (Related: "First Habitable Earthlike Planet Found, Experts Say" [April 24, 2007].)

"We estimate that it has a high surface gravity, around 15 times greater than the Earth," Bakos said. That means a person who weighs 150 pounds (68 kilograms) on Earth would weigh around 2,100 pounds (950 kilograms) on HAT-P-2b.

The scientists also calculated that the planet is incredibly dense: Though HAT-P-2b is only slightly larger than Jupiter, it weighs around eight times as much—equivalent to the mass of 2,500 Earths.

Partner Planet?

Perhaps most unusual of all is the newly discovered world's boomerang orbit. HAT-P-2b closes to within 3.1 million miles (5 million kilometers) of its star before swinging out to around 9.6 million miles (15 million kilometers) away, all in just 5.6 days.

"If Earth had such an elliptical orbit, it would be really strange, like going from Mercury to Mars," Bakos said.

One theory to explain the extremely oval orbit is that another planet may be hiding out nearby.

"It is possible that [HAT-P-2b] is interacting with another planet further out that we haven't seen," Bakos said.

Calculations had suggested that such bizarre, superdense planets could exist, the researchers say. But the bizarre new find finally provides concrete confirmation.

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