Atmospheric circulation and tides of “51 Pegasus b-like” planets

Free access article

Issue		A&A Volume 385, Number 1, April I 2002


Page(s)		166 - 180
Section		Formation and evolution of planetary systems
DOI		http://dx.doi.org/10.1051/0004-6361:20020101

A&A 385, 166-180 (2002)
DOI: 10.1051/0004-6361:20020101

Atmospheric circulation and tides of "51 Pegasus b-like" planets

A. P. Showman¹ and T. Guillot²

¹ University of Arizona, Department of Planetary Sciences and Lunar and Planetary Laboratory, Tucson, AZ 85721, USA
e-mail: showman@lpl.arizona.edu
² Observatoire de la Côte d'Azur, Laboratoire Cassini, CNRS UMR 6529, 06304 Nice Cedex 4, France

(Received 31 August 2001 / Accepted 16 January 2002 )

Abstract
We examine the properties of the atmospheres of extrasolar giant planets at orbital distances smaller than 0.1 AU from their stars. We show that these "51 Peg b-like" planets are rapidly synchronized by tidal interactions, but that small departures from synchronous rotation can occur because of fluid-dynamical torques within these planets. Previous radiative-transfer and evolution models of such planets assume a homogeneous atmosphere. Nevertheless, we show using simple arguments that, at the photosphere, the day-night temperature difference and characteristic wind speeds may reach ~500 K, and ~2 km s ^-1, respectively. Substantial departures from chemical equilibrium are expected. The cloud coverage depends sensitively on the dynamics; clouds could exist predominantly either on the dayside or nightside, depending on the circulation regime. Radiative-transfer models that assume homogeneous conditions are therefore inadequate in describing the atmospheric properties of 51 Peg b-like planets. We present preliminary three-dimensional, nonlinear simulations of the atmospheric circulation of HD 209458b that indicate plausible patterns for the circulation and generally agree with our simpler estimates. Furthermore, we show that kinetic energy production in the atmosphere can lead to the deposition of substantial energy in the interior, with crucial consequences for the evolution of these planets. Future measurements of reflected and thermally-emitted radiation from these planets will help test our ideas.

Key words: planets and satellites: general -- stars: planetary systems

Offprint request: T. Guillot, guillot@obs-nice.fr

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