Sept 14, 2006 (ScienceNow) – Astronomers searching for extrasolar planets have discovered a real corker: a gigantic world more than twice the size of Jupiter but only half as massive, which means it’s literally about as dense as a cork. Announced today at a press conference in Washington, D.C., the puffball is the largest planet yet discovered and the second largest and lightest exoplanet ever found. The finds leave theorists with some explaining to do. According to current models of planet structure and evolution, such large, lightweight orbs shouldn’t exist.

More than 200 planets orbiting distant stars have been discovered. Typically, researchers detect the telltale wobble in a star’s position caused by the gravitational tug of the orbiting planet. But 11 exoplanets have been spotted another way, when they cross in front of their stars, which happens only if observers on Earth see a planet’s orbit edge-on. From the amount of light blocked, researchers can tell how big a planet is…

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