The distant world is the largest yet discovered and is more than one and a third the diameter of our own biggest planet Jupiter. But it is only half as dense and is even lighter than cork.
The puffed-up planet was discovered using a network of small automatic telescopes called HAT. It is in orbit around one of a pair of stars 450 light-years from Earth in the constellation of Lacerta, the Lizard.
Discoverer Gaspar Bakos, of the Harvard-Smithsonian Centre for Astrophysics (CfA) in Massachusetts, said yesterday: “We could be looking at an entirely new class of planets.
“This planet is about one-quarter the density of water. 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.”
The new planet, labelled HAT-P-1, revolves around its parent star every 4.5 days in an orbit only a twentieth the distance from Earth to the Sun. Once each orbit, it passes in front of its own sun, causing its brightness to fade by about 1.5 percent for more than two hours each time. It was that brightness dip that gave the planet’s presence away to astronomers.
The star, which is 3.6 billion years old, compared to the Sun’s 4.5 billion years, is visible in binoculars but the fade is too little to be seen with the eye alone.
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, also is puffed up about 20 percent larger than predicted by theory. HAT-P-1 is 24 percent larger than expected.
Robert Noyes, co-author of a paper to be submitted to the Astrophysical Journal, said: “Out of eleven known transiting planets, now not one but two are substantially bigger and lower in density than theory predicts. We can’t dismiss HD209458b as a fluke. This new discovery suggests something could be missing in our theories of how planets form.”
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.
The image shows a CfA artist’s impression of the new puffed-up planet.