by GEORGE MUSSER
AUSTIN, Texas—Astrophysicists have a funny attitude toward magnetic fields. You might say they feel both repelled and attracted. Gravitation is assumed to rule the cosmos, so models typically neglect magnetism, which for most researchers is just as well, because the theory of magnetism has a forbidding reputation. The basic equations are simple enough, solving them less so. Electromagnetism is a standard weeder course in graduate school, and magnetohydrodynamics ranks up there with quantum field theory as the hardest subject known to mortal minds. That said, when astrophysicists don’t understand something, they often invoke the m-word. “When all else fails, introduce a magnetic field,” exoplanet theorist Dimitar Sasselov of Harvard University told an audience at the American Astronomical Society meeting this week.
Judging from his and others’ talks, all else has been failing a lot lately. One of the many mysteries about the Jupiter-like planets being found around other stars is why their density is so low—some are as fluffy as styrofoam or balsa wood. Orbiting so close to their stars, these planets are baked by stellar radiation, but even that’s not enough to puff them up, at least not directly. Sasselov described a new model by Konstantin Batygin of Caltech and his colleagues in which the planet acts like a giant induction stove. Magnetic fields set up electric currents in the ionized gases of the planet, further heating and bloating it (see above diagram).
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