2004ApJ...609L..87S

We study the internal structure and the convective motions of giant extrasolar planets in order to calculate their dynamo-generated surface magnetic field and dipolar magnetic moment. Using evolutionary models, we investigate the existence of magnetism in planets with masses ranging from 0.3M_J to 10M_Jand with rotation periods ranging from synchronism in ``hot Jupiters'' (with periods of 3-4 days) to the breakdown centrifugal limit (with periods of 2-5 hr). We find that the high Rayleigh and low Ekman numbers in the interior metallic-hydrogen region guarantee convective motions even for low-mass and evolved (aged) planets. The convective velocities estimated from mixing-length theory and from ``magnetostrophic'' balances yield high Reynolds and magnetic Reynolds numbers (∼10³-10⁶), permitting the self-excited dynamo mechanism. Strong magnetism occurs in young massive and rapidly rotating extrasolar planets (with surface magnetic fields of ∼30-60 G), but older or orbitally synchronized planets should have fields of ∼1 G. We discuss the implications of these results for the detection of magnetic fields in extrasolar planets.