Posted: Fri, Sep 15, 2006, 8:24 AM ET (1224 GMT)
Astronomers announced Thursday that they have found an extrasolar planet that is larger than Jupiter but only half its mass, making it the less dense planet ever found. The exoplanet, HAT-P-1, closely orbits one star of a double-star system 450 light-years from Earth, completing one orbit every 4.5 days. The planet transits across the disk of its star as sen from Earth, allowing astronomers to measure its size and mass. The planet turns out to have a diameter nearly 40% larger than Jupiter's, but only has half its mass, giving the planet a density about one-quarter than of water. Astronomers said they're not sure what caused the planet to be so large, having ruled out tidal heating caused by an eccentric orbit or a highly-inclined axis.

Related Links:

•  New Scientist article
•  SPACE.com article
•  New York Times article
•  National Geographic News article
•  Harvard-Smithsonian Center for Astrophysics press release