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Extrasolar Planet HD209458b Upper-Atmosphere Model and Ultraviolet Observation Techniques, 15-9484

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Principal Investigators
Kurt D. Retherford
G. Randy Gladstone

Inclusive Dates:  07/01/04 – 11/01/04

Background - The discovery of planets around other stars has changed our view of the universe and revolutionized our understanding of planetary formation. The detection of planets orbiting very close to their host star, "hot Jupiters," has also broadened our concept of what other planets might look like in terms of size, composition, temperature, and physical processes (e.g., star-planet interactions). The planet we currently know most about is the transiting planet HD209458b. The technique of transmission spectroscopy allows us to understand the composition and structure of its upper atmosphere, as performed with the Space Telescope Imaging Spectrograph. The goal of our project was to investigate the upper-atmospheric composition, temperature, and density structure of newly detected extrasolar planets and pertinent physical processes such as radiative transfer by developing modeling tools and ultraviolet observational techniques.

Approach - Our approach was to develop an upper atmosphere model for HD209458b to use in constraining its physical properties with current observations of hydrogen Lyman-alpha emissions and help plan future observations. The simple one-dimensional model bridges the region between thermosphere and exosphere models currently reported by Yelle, Icarus, 2004 and LeCavelier-des-Etangs et al., A&A, 2004, respectively. Hydrogen emissions become optically thick around three planet radii, where the atmospheric properties are best described with a transitional formalization for rarefied gas flows rather than Navier-Stokes or Liouville equations, respectively. We therefore choose the Direct Simulation Monte Carlo (DSMC) method to model the upper atmosphere region probed by the observations.

Accomplishments - We installed the package of DSMC software made publicly available by G. A. Bird, which is the standard model in this field. The lower boundary conditions for our model are to be set by the Yelle, Icarus, 2004 model results (which based on our review of the literature is currently the best available model). Full development of the exoplanet exosphere model is currently the subject of future proposals.

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