Astronomers have found a strange new world that has them pondering again the essential properties of a planet.
An artist's impression of the strange new world, HAT-P-1b
This new object, designated HAT-P-1b, orbits one member of a pair of stars 450 light-years away in the constellation Lacerta.
Although its radius is about 1.38 times that of our own Jupiter, it has a mass that is only half that of Jupiter.
This makes it much bigger and lower in density than planets are usually, raising questions about how it formed.
The mathematical equations describing planetary structure do not fit.
"This planet is about one-quarter the density of water," said Gaspar Bakos, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics (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."
HAT-P-1b is one of the 200 or so planets that have been detected outside our Solar System. It has the largest radius yet measured.
Like many of these extrasolar bodies, it orbits close to its parent star, revolving around it in just once every 4.5 Earth days.
HAT-P-1b is too far away to be imaged directly, but scientists know it is there because of the way light from its parent star dims as the planet passes in front.
Scientists know of one other extrasolar planet, HD 209458b, which is also puffed up about 20% bigger than predicted by theory. HAT-P-1b is 24% larger than expected.
Several ideas had been proposed solve the puzzle but none seemed to quite fit, said the CfA research team, which has detailed its discovery in a paper submitted to the Astrophysical Journal.
"Until we can find an explanation for both of these swollen planets, they remain a great mystery," said co-researcher Dimitar Sasselov.
The astronomers used a network of telescopes in the states of Arizona and Hawaii to discover the planet.
The double-star system, of which HAT-P-1b's parent forms a part, is called ADS 16402 and is visible through binoculars.