Artist's impression of the newly discovered world HAT-P-1. The new planet has a size inflated well above what theory would predict.

Credit: David Aguilar

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WASHINGTON DC: Astronomers have discovered a new planet with a size inflated well above what theory would predict, according to a new U.S. study. Dubbed HAT-P-1, the planet is the largest yet discovered, with a radius 1.38 times Jupiter's, but with only half Jupiter's mass.

"We could be looking at an entirely new class of planets," said Gaspar Bakos, a Hubble Fellow at the Harvard-Smithsonian Centre for Astrophysics in Boston. Bakos designed and built the network of small automated telescopes (called HAT) used to detect the planet, and is lead author of a paper submitted to the Astrophysical Journal.

"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 orbits its parent star - which is 450 lightyears away - once every 4.5 days in an orbit one-twentieth of the distance from the 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 per cent for more than two hours, after which the star returns to its previous brightness.

The parent star is one member of a double-star system called ADS 16402 and is visible through 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 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, is also about 20 per cent larger than would be predicted by theory. HAT-P-1 is 24 per cent 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, also of the Harvard-Smithsonian Centre for Astrophysics. "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 inflate these giant planets beyond the size calculated from planetary structure equations would be to supply additional heat to their interiors.

One way to inject energy into the planet's center is by tipping it on its side, similar to Uranus in our 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, "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, "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." 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 mass, which is determined by measuring the star's wobble as the planet orbits, researchers can calculate a planet's density.