Distant giant is lighter than a giant ball of cork

Astronomers are just like ordinary people, even if they do spend an inordinate amount of time peering into the darkness, and like us, they occasionally become excited.

Recently, they became quite animated when they discovered a planet unlike any other. Designated HAT-P-1, it orbits one of a pair of distant stars 450 light-years away, in the constellation Lacerta. With a radius about 1,38 times that of Jupiter, it is the largest planet discovered so far. However, its mass is only half that of the gas giant.

Using a network of small automated telescopes known as HAT (Hungarian Automated Telescope), the astronomers discovered that the planet revolves around its host star every 4,5 days in an orbit one-twentieth of the distance from Earth to the Sun. Once each orbit, it passes in front of its parent star, causing it to appear fainter by about 1,5 per cent for more than two hours, after which the star returns to its previous brightness.

HAT-P-1’s 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 1 500 times the Earth-Sun distance. They are similar to the Sun, but slightly younger – about 3,6 billion years old compared to the Sun’s age of 4,5 billion years.

“We could be looking at an entirely new class of planets,” said Gaspar Bakos, a Hubble fellow at the Harvard-Smithsonian Centre for Astrophysics. Bakos designed and built the HAT network and is lead author of a paper submitted to the Astrophysical Journal describing the discovery.

“This planet is about one-quarter the density of water,” Bakos said. “In other words, it’s lighter than a giant ball of cork. 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.”

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 puffed up to a size about 20 per cent larger than predicted by theory.

“Out of 11 known transiting planets, we now have two that are substantially bigger and lower in density than theory predicts,” said co-author Robert Noyes. “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 puff up these giant planets beyond the size calculated from planetary structure equations would be to supply additional heat to their interiors. Simple heating of the surface due to the host star’s proximity would not work. (If it could, all close-in transiting giant planets should be expanded, and not just the two.)

One way to inject energy into the planet’s centre is by tipping it on its side, like Uranus. 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 (not a member of the discovery team), “the circumstances required to tip over a planet are so unusual that this would seem unlikely to explain both known examples of inflated worlds”.

Said 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.” The 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”.

* The HAT network consists of six telescopes, four at the Smithsonian Astrophysical Observatory’s Whipple Observatory in Arizona and two at its Submillimetre 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. Together with the mass, which is determined by measuring the star’s wobble as the planet orbits it, researchers are able to calculate a planet’s density.