Atmospheric Dynamics of Hot Giant Planets and Brown Dwarfs

Abstract

Groundbased and spacecraft telescopic observations, combined with an intensive modeling effort, have greatly enhanced our understanding of hot giant planets and brown dwarfs over the past ten years. Although these objects are all fluid, hydrogen worlds with stratified atmospheres overlying convective interiors, they exhibit an impressive diversity of atmospheric behavior. Hot Jupiters are strongly irradiated, and a wealth of observations constrain the day-night temperature differences, circulation, and cloudiness. The intense stellar irradiation, presumed tidal locking and modest rotation leads to a novel regime of strong day-night radiative forcing. Circulation models predict large day-night temperature differences, global-scale eddies, patchy clouds, and, in most cases, a fast eastward jet at the equator—equatorial superrotation. The warm Jupiters lie farther from their stars and are not generally tidally locked, so they may exhibit a wide range of rotation rates, obliquities, and orbital eccentricities, which, along with the weaker irradiation, leads to circulation patterns and observable signatures predicted to differ substantially from hot Jupiters. Brown dwarfs are typically isolated, rapidly rotating worlds; they radiate enormous energy fluxes into space and convect vigorously in their interiors. Their atmospheres exhibit patchiness in clouds and temperature on regional to global scales—the result of modulation by large-scale atmospheric circulation. Despite the lack of irradiation, such circulations can be driven by interaction of the interior convection with the overlying atmosphere, as well as self-organization of patchiness due to cloud-dynamical-radiative feedbacks. Finally, irradiated brown dwarfs help to bridge the gap between these classes of objects, experiencing intense external irradiation as well as vigorous interior convection. Collectively, these diverse objects span over six orders of magnitude in intrinsic heat flux and incident stellar flux, and two orders of magnitude in rotation rate—thereby placing strong constraints on how the circulation of giant planets (broadly defined) depend on these parameters. A hierarchy of modeling approaches have yielded major new insights into the dynamics governing these phenomena.

Indications for very high metallicity and absence of methane in the eccentric exo-Saturn WASP-117b

“…Because these atmospheres tend to have deep, stably stratified radiative zones, the quenching point of most species typically does not lie within the (deeper) convective zone, and the traditional mixing-length theory breaks down. Instead, vertical mixing in irradiated exoplanets is described as being caused by medium-to large-scale atmospheric circulation, which when globally averaged may or may not act as a diffusive process (see, e.g., recent discussions in Zhang 2020;Showman et al 2020). It is possible to study these atmospheric motions in 3D using general circulation models, and from these, derive order-of-magnitude parameterizations for the 1D eddy diffusion coefficient K zz .…”

Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: global and equatorial dynamics

“…These observations motivate the investigation of global atmospheric dynamics of BDs and directly imaged EGPs, their fundamental properties and effects on cloud formation and chemistry. There have been several studies investigating atmospheric dynamics appropriate for these objects (Freytag et al 2010;Zhang & Showman 2014;Tan & Showman 2017;Showman et al 2019;, and see recent reviews by Showman et al 2020 andZhang 2020). Cloud radiative feedback has been proposed as a robust and novel mechanism to drive spontaneous atmospheric variability and dynamics in these atmospheres .…”

Hot Jupiters: Origins, Structure, Atmospheres

“…GCMs find that the atmospheric circulations of hot Jupiters are characterized by large dayside-to-nightside temperature contrasts. The large dayside-to-nightside temperature gradient forces fast winds which manifest as eastward (propagating in the direction of rotation) jets with maximum speeds in equatorial regions (Hammond & Pierrehumbert, 2018;Showman & Polvani, 2011;Tsai et al, 2014). These fast winds lead to a shift in the temperature maximum (or "hot spot") eastward (downwind) of the substellar point.…”