A similarity approach to the atmospheric dynamics of giant extrasolar planets and brown dwarfs

Abstract

We present an assessment of the most plausible dynamical regimes operating in the atmospheres of giant extrasolar planets (EGP) and cold ("methane") brown dwarfs from the available data on a selected group of objects. The most important parameters controlling the atmospheric circulation are the rotation angular velocity and the energy balance between the internal heat source and the star's insolation. The first parameter can be reasonably constrained for some of these objects by theoretical arguments. The second is constrained by the observations. Assuming a hydrogen composition, we discuss possible scenarios for the first order atmospheric motions in terms of characteristic geophysical fluid dynamic numbers and representative time constants. The analysis is applied to the family of extrasolar giant planets classified recently by Sudarsky et al. (2000) according to their effective temperature and Bond albedo. For completeness we extend this study to cold ("methane") brown dwarfs. Three main dynamical regimes emerge from this analysis: (A) Close EGP ("hot jupiters") with spin-orbit locked (slowly rotating) planets, have their atmospheres mainly under the star's radiative control. Super-rotating atmospheric motions between the heated and cooled hemispheres can be expected. (B) Atmospheres with their dynamics controlled by both the internal and external energy sources, with Coriolis forces producing zonal motions (Jupiter like objects). (C) Cold brown dwarfs, with motions controlled by the internal heat source (thermally driven turbulent convection) producing intense vertical velocities that dominate the motion field.