Puffed-up planets are heated like toast

A PLANET-sized version of an electric toaster could explain why some exoplanets get so large. A related phenomenon could be responsible for keeping in check the gusting winds that form the stripes of Jupiter.

More than 150 planets have been found orbiting closer to their host stars than Mercury is to the sun. Many of these star-hugging gas giants - known as "hot Jupiters" because they can have surface temperatures of 2000 °C or more - have a similar mass to Jupiter but can have up to six times the volume.

Something must be heating the interior of these planets to make them puff up in this way - but what? Radiation from the host star can't be the source, as most of it is reradiated into space from gas at the surface.

Gravitational heating effects might work for planets with elongated orbits. The ever-changing gravitational tug of the host star on the orbiting planet would create friction by flexing its interior, possibly generating enough heat to cause the expansion we see. But this mechanism can't explain how some planets with a circular orbit - such as TrES-4, which is less massive than Jupiter but 1.8 times as wide - get to be so large.