Largest Known Planet Discovered, Astronomers Announce

Brian Handwerk
for National Geographic News

September 14, 2006

Astronomers say they have discovered what appears to be an entirely new kind of planet, an extra-large gas giant unlike any known world in our solar system or beyond.

"This questions our understanding of how giant planets are formed and evolve," said Robert Noyes, a senior physicist at the Harvard-Smithsonian Center for Astrophysics (CfA).

The planet, dubbed HAT-P-1, is located some 450 light-years from Earth in the constellation Lacerta.

It is the largest planet ever discovered and boasts a radius nearly 1.4 times larger than Jupiter's.

HAT-P-1 is also the least dense of all known planets.

"This planet is about one quarter the density of water," said Gaspar Bakos, a Hubble fellow at CfA.

"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."

Unlike more familiar gas giants, including those in our solar system, HAT-P-1 does not appear to have a solid core.

(CfA scientists also announced today a new method for detecting life on other planets.)

Planet Poses "Unsolved Problem"

The planet's large size and low density can't be explained by current theories of giant planet formation.

Scientists suggest that additional heat in its interior could account for its "puffed-up" size, but as yet they can't explain how such heat could be generated.

"We've discovered a very bizarre new object, so the people who do the theoretical structure models are left scratching their heads as to what could possibly be going on," Noyes told reporters during a press conference this morning at the Smithsonian Institution in Washington, D.C.

"At the moment it's an unsolved problem."

HAT-P-1 orbits one of a pair of twin stars that is very much like our sun but nearly a billion years younger.

(See a virtual solar system.)

The star is bright enough to be seen with a pair of binoculars, but HAT-P-1 is not visible.

The planet was detected by measuring the dip in starlight, lasting about two hours, that occurs each time HAT-P-1 passes between its star and Earth.

Many extrasolar planets have been discovered in recent years. But HAT-P-1 is only the 12th that transits between its star and Earth, enabling scientists to directly measure its size and mass and calculate its density.

HAT-P-1 is the second extrasolar planet found to be perplexingly large, confirming that the first planet is not a fluke.

The new planet was spied by the HAT telescope network, which consists of four instruments at the Smithsonian Astrophysical Observatory's Whipple Observatory in Arizona and two others at its Submillimeter Array facility in Hawaii.

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