[Paper Review] A New Atmospheric Model for HD 189733 b

Jonathan Langton, Gregory Laughlin|ArXiv.org|Aug 22, 2008

Stellar, planetary, and galactic studies16 references3 citations

TL;DR

This paper presents a new two-dimensional, hydrostatically-balanced hydrodynamic model for HD 189733 b’s atmosphere, simulating supersonic, turbulent winds that induce chaotic temperature variations. The model successfully reproduces the observed 8-μm light curve, including the unexpected 30° eastward offset of the hot spot and westward offset of the cold spot, by attributing these features to large-scale atmospheric turbulence rather than steady-state flow.

ABSTRACT

We have developed a new two-dimensional hydrostatically-balanced isobaric hydrodynamic model for use in simulation of exoplanetary atmospheres. We apply this model to the infrared photosphere of the hot Jupiter HD 189733 b, for which an excellent 8-micron light curve has been obtained. For reasonable parameter choices, the results of our model are consistent with these observations. In our simulations, strongly turbulent supersonic flow develops, with wind speeds of approximately 5 km/s. This flow geometry causes chaotic variation of the temperature distribution, leading to observable variations in the light curve from one orbit to the next.

Motivation & Objective

• To develop a more physically consistent two-dimensional atmospheric model for hot Jupiters that maintains hydrostatic balance, overcoming limitations of prior shallow-water and compressible models.
• To explain the observed 8-μm phase curve of HD 189733 b, particularly the anomalous 30° eastward offset of the hottest region and westward offset of the coldest region.
• To test whether large-scale atmospheric turbulence can account for orbital variations in flux extrema, offering a dynamic explanation for the observed light curve morphology.
• To provide a testable hypothesis: if turbulence is real, flux minima and maxima should shift by several hours between orbits, which can be verified with future observations.

Proposed method

• The model employs a two-dimensional, isobaric, hydrostatically-balanced hydrodynamic framework, ensuring vertical hydrostatic equilibrium while resolving horizontal dynamics at high resolution.
• It uses a fully compressible hydrodynamics approach with a modified equation set that enforces hydrostatic balance, avoiding the unphysical features of earlier compressible models.
• The model incorporates radiative forcing with adjustable parameters to simulate stellar irradiation, though it does not include full multi-wavelength radiative transfer.
• Simulations are initialized with a strong temperature gradient and driven by intense irradiation, leading to supersonic, turbulent wind patterns.
• The model outputs time-dependent temperature and wind fields, which are then used to compute synthetic light curves at 8 μm by integrating flux over the visible disk.
• The synthetic light curves are compared directly to the observed 8-μm flux curve from Knutson et al. (2007), assessing fit quality and orbital variability.

Experimental results

Research questions

• RQ1 Can a two-dimensional, hydrostatically-balanced hydrodynamic model reproduce the observed 8-μm phase curve of HD 189733 b, including the non-steady-state flux variations?
• RQ2 Why does the hottest region on HD 189733 b lie 30° east of the substellar point, and the coldest region 30° west of the antistellar point, contrary to expectations of symmetric, zonal flow?
• RQ3 Does large-scale atmospheric turbulence, rather than steady-state wind patterns, explain the observed orbital variability in flux extrema?
• RQ4 Can the model’s prediction of time-variable flux minima and maxima—shifted by several hours between orbits—be tested with future observations?
• RQ5 To what extent do the observed flux curve features at 8 μm and 24 μm differ due to atmospheric dynamics at different pressure levels or due to turbulent variability?

Key findings

• The model produces supersonic, turbulent wind speeds of approximately 5 km s⁻¹, which drive chaotic, non-steady temperature distributions across the planet’s dayside and nightside.
• The simulated light curve exhibits orbital variability in flux extrema, with the timing of minima and maxima shifting by several hours between orbits, consistent with observational uncertainty of ±1 hour.
• The model successfully reproduces the observed 30° eastward offset of the flux maximum and the 30° westward offset of the flux minimum relative to the substellar and antistellar points.
• The model is not excluded by current observations, as the fit to the 8-μm light curve is qualitatively reasonable despite limited radiative forcing parameterization.
• The model provides the first explanation for the anomalous cold spot offset in HD 189733 b’s 8-μm phase curve, attributing it to turbulent advection rather than symmetric zonal flow.
• The model suggests that future observations should monitor flux timing across multiple orbits to test for turbulence-induced shifts, offering a direct observational test of the hypothesis.