Magnetoastrocoolness: How Cosmic Magnetic Fields Shape Planetary Systems
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).
Magnetic fields also muck up planet and star formation, as Susana Lizano of the Universidad Nacional Autonoma de México in Morelia explained. The interstellar clouds of gas and dust out of which planets and stars coalesce are threaded with magnetic fields—weak ones, a thousandth as strong as Earth's. (Astronomers gauge the field strength by looking at how light from the dust is polarized.) As these clouds collapse, basic theory predicts the field should intensify a billionfold. But if that happened, the field would become powerful enough to stop the collapse. Even leaving this problem aside, as the cloud settles into a swirling disk, the field should bring the swirling motion in the inner part of the disk to a halt. Somehow the fields must dissipate, perhaps through a variant of induction-stove effect.
Even when the magnetic field weakens, it sculpts the nascent planetary system. It causes gas to revolve around the star more slowly than freely orbiting objects do. Embryonic planets thus experience a drag force and and spiral inward. The field also stabilizes the disk, keeping it from fragmenting—further evidence that planets form by step-by-step agglomeration rather than gravitational breakup of the disk. All in all, Lizano built a persuasive case that astrophysicists ignore magnetism at their peril.
Finally, a reminder of the unexpected usefulness of magnetic fields came in a talk by Bill Atwood of the University of California at Santa Cruz, co-leader of the Fermi Gamma-Ray Space Telescope. Last fall Fermi confirmed earlier findings that positrons are peppering our planet; these cosmic particles, the antimatter twin of electrons, might originate in the self-destruction of dark-matter particles. The peculiar thing about this discovery is that Fermi isn't even an antimatter detector. Unlike other instruments such as the Alpha Magnetic Spectrometer, it lacks an onboard magnet, which, by bending the paths of electrons one way and positrons the other, can tell the two apart. But the Fermi team exploited the fact that the instrument is embedded in a magnetic field—Earth's.