One of first transiting exoplanets discovered by Kepler announced in a batch of 5 Jan 2010 (Kepler 4-9). It, like the others, is a hot Jupiter. It is the largest of the batch in diameter, its mass is 50% of Jupiter's, but its diameter is 50% greater than Jupiter, making its density that of styrofoam. This is the least dense planet found to date.
Kepler 7 System Web Pages
Kepler 7 System In the News
First batch of five planets discovered by Kepler (2010)