New theory predicts strength of heavenly bodies' magnetic fields
German scientists have developed a theory that can predict the magnetic field of planets and stars alike. Their computer simulations reveal that the strength of a heavenly body's magnetic field is determined by the amount of energy (in the form of heat or light, for example) that it emits. The theory is backed up by observational data and could help astronomers predict which planets and stars should have detectable magnetic fields. Many stars and planets have magnetic fields. These are generated when liquid or gaseous material in the hot interior of a star or planet rises to the surface, cools, and sinks back down. As this material is able to conduct electricity, its movement creates the magnetic fields, and the fast rotation of planets and stars gives the currents a shape that promotes a 'dynamo principle'. The Sun's magnetic field contributes to the creation of solar flares, which fling charged particles into space. Meanwhile, the Earth's magnetic field protects us from this bombardment. The strength of the magnetic fields generated by different stars and planets varies widely; Jupiter's magnetic field is ten times as strong as Earth's, and the magnetic fields of some stars even dwarf Jupiter's magnetic field by a factor of 1,000 or more! However, until now the causes behind these differences were a mystery. One theory proposed that the strength of the magnetic field was determined by the speed at which the planet rotates. However, while this held true in some situations it did not apply to fast-rotating bodies, such as Earth and Jupiter, or small stars. In this latest study, scientists at the Max Planck Institute for Solar System Research in Germany used computer simulations to develop a new theory, which states that the strength of the magnetic field depends on the amount of energy emitted into space by the star or planet in question. The team tested their theory on observational data from Earth and Jupiter, as well as on different kinds of rapidly rotating stars. Despite the widely varying nature of these objects, the theory held true for all of them. Crucially, the law also applies to stars whose density changes with depth. 'Our results imply that the dynamo process in planets and stars is not as different as we thought,' commented Professor Ulrich Christensen of the Max Planck Institute for Solar System Research. The theory could now be used to predict the magnetic field strength of planets whose magnetic fields have not yet been detected. For example, some stars have planets that are much larger than Jupiter, and these could have magnetic fields to match. At the moment, there are no antennae on Earth sensitive enough to detect the intense radio waves that these massive planets probably emit. However, the LOFAR ('Low frequency array for radio astronomy') array, which will eventually consist of a network of antennae dotted across Europe, will be able to pick up these signals.
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