The newly discovered world HAT-P-1 is puffed up much larger than theory predicts.

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.”