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 鈥� known as 鈥渉ot Jupiters鈥� because they can have surface temperatures of 2000 掳C or more 鈥� 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 鈥� but what? Radiation from the host star can鈥檛 be the source, as most of it is reradiated into space from gas at the surface.
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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鈥檛 explain how some planets with a circular orbit 鈥� such as TrES-4, which is less massive than Jupiter but 1.8 times as wide 鈥� get to be so large.
and David Stevenson of the California Institute of Technology in Pasadena now suggest that the missing energy could originate in a wind of charged particles circling the planet. The temperature in the atmospheres of hot Jupiters is high enough to knock large numbers of electrons off atoms like sodium and potassium. These electrons could then be whipped around by the planet鈥檚 winds and interact with its magnetic field, generating a current that can extend deep into the planet (see diagram), heating up its interior like the element of an electric toaster ().
鈥淓lectrons whipped around by winds interact with the planet鈥檚 magnetic field, generating a current鈥�
鈥淭he little power that you鈥檙e depositing there may be enough to inflate the planet,鈥� says of Princeton University, who models the properties of exoplanets but was not associated with the study.
Burrows adds that more detailed modelling is needed to determine whether currents generated this way reach far enough into the interiors of hot Jupiters to puff them up. 鈥淚t might only work at some times in some planets.鈥�
If the theory proves correct, it could 鈥渒ill two birds with one stone鈥�, says Burrows, because a similar mechanism could play a part in maintaining the speed of wind bands that whip around Jupiter and Saturn. These winds may be driven by the temperature variations between regions receiving different amounts of sunlight or by the churning generated by the planet鈥檚 own heat. But some other process is needed to keep the winds moving at constant speeds.
Even though these gas giants are too cool to liberate electrons in the same way as on hot Jupiters, the heat deeper within the planet may strip electrons from hydrogen and other elements. The interaction of these electrons with a planet鈥檚 magnetic field, as Batygin has proposed for exoplanets, may create a counter-force that helps restrain the winds.
NASA鈥檚 James Webb Space Telescope, due to launch in 2014, could help refine the model by narrowing down the speed of winds on puffed-up exoplanets.
