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Register a new userAtmospheric Circulation of Eccentric Hot Neptune GJ436b
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GJ436b is a unique member of the transiting extrasolar planet population being one of the smallest and least irradiated and possessing an eccentric orbit. Because of its size, mass and density, GJ436b could plausibly have an atmospheric metallicity similar to Neptune (20-60 times solar abundances), which makes it an ideal target to study the effects of atmospheric metallicity on dynamics and radiative transfer in an extrasolar planetary atmosphere. We present three-dimensional atmospheric circulation models that include realistic non-gray radiative transfer for 1, 3, 10, 30, and 50 times solar atmospheric metallicity cases of GJ436b. Low metallicity models (1 and 3 times solar) show little day/night temperature variation and strong high-latitude jets. In contrast, higher metallicity models (30 and 50 times solar) exhibit day/night temperature variations and a strong equatorial jet. Spectra and light curves produced from these simulations show strong orbital phase dependencies in the 50 times solar case and negligible variations with orbital phase in the 1 times solar case. Comparisons between the predicted planet/star flux ratio from these models and current secondary eclipse measurements support a high metallicity atmosphere (30-50 times solar abundances) with disequilibrium carbon chemistry at play for GJ436b. Regardless of the actual atmospheric composition of GJ436b, our models serve to illuminate how metallicity influences the atmospheric circulation for a broad range of warm extrasolar planets.
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The hot-Jupiter HAT-P-2b has become a prime target for Spitzer Space Telescope observations aimed at understanding the atmospheric response of exoplanets on highly eccentric orbits. Here we present a suite of three-dimensional atmospheric circulation models for HAT-P-2b that investigate the effects of assumed atmospheric composition and rotation rate on global scale winds and thermal patterns. We compare and contrast atmospheric models for HAT-P-2b, which assume one and five times solar metallicity, both with and without TiO/VO as atmospheric constituents. Additionally we compare models that assume a rotation period of half, one, and two times the nominal pseudo-synchronous rotation period. We find that changes in assumed atmospheric metallicity and rotation rate do not significantly affect model predictions of the planetary flux as a function of orbital phase. However, models in which TiO/VO are present in the atmosphere develop a transient temperature inversion between the transit and secondary eclipse events that results in significant variations in the timing and magnitude of the peak of the planetary flux compared with models in which TiO/VO are omitted from the opacity tables. We find that no one single atmospheric model can reproduce the recently observed full orbit phase curves at 3.6, 4.5 and 8.0 microns, which is likely due to a chemical process not captured by our current atmospheric models for HAT-P-2b. Further modeling and observational efforts focused on understanding the chemistry of HAT-P-2bs atmosphere are needed and could provide key insights into the interplay between radiative, dynamical, and chemical processes in a wide range of exoplanet atmospheres.
We present an analysis of seven primary transit observations of the hot Neptune GJ436b at 3.6, 4.5 and $8~mu$m obtained with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. After correcting for systematic effects, we fitted the light curves using the Markov Chain Monte Carlo technique. Combining these new data with the EPOXI, HST and ground-based $V, I, H$ and $K_s$ published observations, the range $0.5-10~mu$m can be covered. Due to the low level of activity of GJ436, the effect of starspots on the combination of transits at different epochs is negligible at the accuracy of the dataset. Representative climate models were calculated by using a three-dimensional, pseudo-spectral general circulation model with idealised thermal forcing. Simulated transit spectra of GJ436b were generated using line-by-line radiative transfer models including the opacities of the molecular species expected to be present in such a planetary atmosphere. A new, ab-initio calculated, linelist for hot ammonia has been used for the first time. The photometric data observed at multiple wavelengths can be interpreted with methane being the dominant absorption after molecular hydrogen, possibly with minor contributions from ammonia, water and other molecules. No clear evidence of carbon monoxide and dioxide is found from transit photometry. We discuss this result in the light of a recent paper where photochemical disequilibrium is hypothesised to interpret secondary transit photometric data. We show that the emission photometric data are not incompatible with the presence of abundant methane, but further spectroscopic data are desirable to confirm this scenario.
We study the feasibility of observationally constraining the rotation rate of hot Jupiters, planets that are typically assumed to have been tidally locked into synchronous rotation. We use a three-dimensional General Circulation Model to solve for the atmospheric structure of two hot Jupiters (HD 189733b and HD 209458b), assuming rotation periods that are 0.5, 1, or 2 times their orbital periods (2.2 and 3.3 days, respectively), including the effect of variable stellar heating. We compare two observable properties: 1) the spatial variation of flux emitted by the planet, measurable in orbital phase curves, and 2) the net Doppler shift in transmission spectra of the atmosphere, which is tantalizingly close to being measurable in high-resolution transit spectra. Although we find little difference between the observable properties of the synchronous and non-synchronous models of HD 189733b, we see significant differences when we compare the models of HD 209458b. In particular, the slowly rotating model of HD 209458b has an atmospheric circulation pattern characterized by westward flow and an orbital phase curve that peaks after secondary eclipse (in contrast to all of our other models), while the quickly rotating model has a net Doppler shift that is more strongly blue-shifted than the other models. Our results demonstrate that the combined use of these two techniques may be a fruitful way to constrain the rotation rate of some planets, and motivate future work on this topic.
We present results from an atmospheric circulation study of nine hot Jupiters that comprise a large transmission spectral survey using the Hubble and Spitzer Space Telescopes. These observations exhibit a range of spectral behavior over optical and infrared wavelengths which suggest diverse cloud and haze properties in their atmospheres. By utilizing the specific system parameters for each planet, we naturally probe a wide phase space in planet radius, gravity, orbital period, and equilibrium temperature. First, we show that our model grid recovers trends shown in traditional parametric studies of hot Jupiters, particularly equatorial superrotation and increased day-night temperature contrast with increasing equilibrium temperature. We show how spatial temperature variations, particularly between the dayside and nightside and west and east terminators, can vary by hundreds of K, which could imply large variations in Na, K, CO and CH4 abundances in those regions. These chemical variations can be large enough to be observed in transmission with high-resolution spectrographs, such as ESPRESSO on VLT, METIS on the E-ELT, or with MIRI and NIRSpec aboard JWST. We also compare theoretical emission spectra generated from our models to available Spitzer eclipse depths for each planet, and find that the outputs from our solar-metallicity, cloud-free models generally provide a good match to many of the datasets, even without additional model tuning. Although these models are cloud-free, we can use their results to understand the chemistry and dynamics that drive cloud formation in their atmospheres.
Atmospheric circulation on tidally-locked exoplanets is driven by the absorption and reradiation of heat from the host star. They are natural heat engines, converting heat into mechanical energy. A steady state is possible only if there is a mechanism to dissipate mechanical energy, or if the redistribution of heat is so effective that the Carnot efficiency is driven to zero. Simulations based on primitive, equivalent-barotropic, or shallow-water equations without explicit provision for dissipation of kinetic energy and for recovery of that energy as heat, violate energy conservation. More seriously perhaps, neglect of physical sources of drag may overestimate wind speeds and rates of advection of heat from the day to the night side.
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