Atmospheric Heat Redistribution on Hot Jupiters

Abstract

Infrared light curves of transiting hot Jupiters present a trend in which the atmospheres of the hottest planets are less efficient at redistributing the stellar energy absorbed on their daysides-and thus have a larger day-night temperature contrast-than colder planets. To this day, no predictive atmospheric model has been published that identifies which dynamical mechanisms determine the atmospheric heat redistribution efficiency on tidally locked exoplanets. Here we present a shallow water model of the atmospheric dynamics on synchronously rotating planets that explains why heat redistribution efficiency drops as stellar insolation rises. Our model shows that planets with weak friction and weak irradiation exhibit a banded zonal flow with minimal day-night temperature differences, while models with strong irradiation and/or strong friction exhibit a day-night flow pattern with order-unity fractional day-night temperature differences. To interpret the model, we develop a scaling theory which shows that the timescale for gravity waves to propagate horizontally over planetary scales, τ wave , plays a dominant role in controlling the transition from small to large temperature contrasts. This implies that heat redistribution is governed by a wave-like process, similar to the one responsible for the weak temperature gradients in the Earth's tropics. When atmospheric drag can be neglected, the transition from small to large day-night temperature contrasts occurs when τ wave ∼ √ τ rad /Ω, where τ rad is the radiative relaxation time and Ω is the planetary rotation frequency. Alternatively, this transition criterion can be expressed as τ rad ∼ τ vert , where τ vert is the timescale for a fluid parcel to move vertically over the difference in day-night thickness. These results subsume the more widely used timescale comparison for estimating heat redistribution efficiency between τ rad and the horizontal day-night advection timescale, τ adv . Only because τ adv ∼ τ vert for hot Jupiters does the commonly assumed timescale comparison between τ rad and τ adv yield approximately correct predictions for the heat redistribution efficiency.

Pseudo 2D chemical model of hot-Jupiter atmospheres: application to HD 209458b and HD 189733b

“…Indeed, important temperature contrasts between different planetary sides of hot Jupiters, noticeably between day and night sides, have been predicted (Showman & Guillot 2002), observed for a dozen hot Jupiters (see Knutson et al 2007 for the first one), qualitatively understood (Cowan & Agol 2011;Perez-Becker & Showman 2013), and confirmed by three-dimensional general circulation models (e.g. Perna et al 2012).…”

Accuracy tests of radiation schemes used in hot Jupiter global circulation models

“…Brightness maps Majeau et al 2012) and wind velocities (Snellen et al 2010) are now accessible, and constraints on the composition are becoming available, but large uncertainties remain. Observations indicate a hotspot shifted eastward of the substellar point Majeau et al 2012) and temperature contrasts smaller than what is expected for these planets without winds (Knutson et al , 2009, indicating transport of heat from the day-side to the night-side (Watkins & Cho 2010;Perez-Becker & Showman 2013). HD 209458b appears to have a temperature inversion in its upper atmosphere Burrows et al 2007) while HD 189733b does not Barman 2008;Knutson et al 2009), indicating that, despite similar orbital properties, hot Jupiters may have very different circulation patterns that still need to be understood.…”

An irradiated brown-dwarf companion to an accreting white dwarf

“…Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of M 2 = 0.055 ± 0.008M and an average spectral type of L1 ± 1, supporting both theoretical predictions and model-dependent observational constraints. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the day and night sides on the sub-stellar donor, ∆T 57 K, and the maximum difference between the hottest and coolest parts of its surface, of ∆T max 200 K. The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of A B < 0.54 at the 2-σ confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the donor's atmosphere 4,5 .…”

“…We find a limit for the redistribution efficiency of < 0.54 at 2-σ confidence. Efficient reprocessing, coupled with poor heat redistribution, has also been found in hot Jupiters in this atmospheric temperature regime 5 and suggested for low-mass stars in accreting white dwarfs 4 . If irradiation is the main cause of the larger-than-predicted donor radii 19 , efficient heat redistribution is required.…”

“…The system also provides an independent new benchmark for theoretical models that predict the albedo and heat redistribution efficiency in irradiated planetary atmospheres 5 (email: j.v.hernandez@soton.ac.uk).…”