From

http://news.bbc.co.uk/1/hi/sci/tech/5346998.stmThis new object, designated HAT-P-1, orbits one member of a pair of stars 450 light-years away in the constellation Lacerta.

Although HAT-P-1's 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.

It would float in a bathtub if you could find a tub big enough to hold it

Gaspar Bakos

"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-1 is the largest of the 200 or so planets that have been detected outside our Solar System.

Like many of these extrasolar bodies, it orbits close to its parent star, revolving around it in just once every 4.5 Earth days.

Scientists know of one other extrasolar planet, HD 209458b, which is also puffed up about 20% bigger than predicted by theory. HAT-P-1 is 24% larger than expected.

Gaia xxx