A Puffed-Up Extrasolar Planet

Maggie McKee writes, "New Scientist Space reports astronomers have found a planet less dense than a wine cork and 38% larger than Jupiter. It circles a star about 450 light years from Earth. A similarly bloated planet has been found before (HD 209458b), so these puffed-up planets may be quite common. But no one knows how they got so swollen. One possibility is 'that some poorly understood mechanism has separated hydrogen and helium in each planet.'"