A magnetic field evolution scenario for brown dwarfs and giant planets

Very little is known about magnetic fields of extrasolar planets and brown dwarfs. We use the energy flux scaling law presented by Christensen et al. to calculate the evolution of average magnetic fields in extrasolar planets and brown dwarfs under the assumption of fast rotation, which is probably the case for most of them. We find that massive brown dwarfs of about 70 M(Jup) can have fields of a few kilo-Gauss during the first few hundred Million years. These fields can grow by a factor of two before they weaken after deuterium burning has stopped. Brown dwarfs with weak deuterium burning and extrasolar giant planets start with magnetic fields between similar to 100G and similar to 1 kG at the age of a few Myr, depending on their mass. Their magnetic field weakens steadily until after 10 Gyr it has shrunk by about a factor of 10. We use observed X-ray luminosities to estimate the age of the known extrasolar giant planets that are more massive than 0.3 M(Jup) and closer than 20 pc. Taking into account the age estimate, and assuming sun-like wind-properties and radio emission processes similar to those at Jupiter, we calculate their radio flux and its frequency. The highest radio flux we predict comes out as 700 mJy at a frequency around 150 MHz for tau Boo b, but the flux is below 60 mJy for the rest. Most planets are expected to emit radiation between a few Mhz and up to 100 MHz, well above the ionospheric cutoff frequency.