Retrieval of atmospheric properties of extrasolar planets

Abstract

We present a new method to retrieve molecular abundances and temperature profiles from exoplanet atmosphere photometry and spectroscopy. Our method allows us to run millions of 1-D atmosphere models in order to cover the large range of allowed parameter space. In order to run such a large number of models, we have developed a parametric pressure-temperature (P-T) profile coupled with line-by-line radiative transfer, hydrostatic equilibrium, and energy balance, along with prescriptions for non-equilibrium molecular composition and energy redistribution. The major difference from traditional 1-D radiative transfer models is the parametric P-T profile, which essentially means adopting energy balance only at the top of the atmosphere and not in each layer. We see the parametric P-T model as a parallel approach to the traditional exoplanet atmosphere models that rely on several free parameters to encompass unknown absorbers and energy redistribution. The parametric P-T profile captures the basic physical features of temperature structures in planetary atmospheres (including temperature inversions), and reproduces a wide range of published P-T profiles, including those of solar system planets. We apply our temperature and abundance retrieval method to two exoplanets which have the best data available, HD 189733b and HD 209458b. For each planet, we compute - 107 atmospheric spectra on a grid in the parameter space, and report contours of the error surface, given the data. For the day-side of HD 189733b, we place constraints on the atmospheric properties based on three different data sets available. Our best-fit models to one of the data sets allow for very efficient daynight energy redistribution in HD 189733b. The different constraints on molecular abundances confirm the presence of H20, CH4 , CO and CO 2 in HD 189733b. Our results also rule out the presence of a thermal inversion in this planet. The model constraints due to the different data sets indicate that the planetary atmosphere is variable, both, in its energy redistribution state and in the chemical abundances. The variability is evident in the data; some key observations with different instruments at the same wavelength differ at the - 2- level. If, on the other hand, the differences in data represent underestimated errors, and if all the data sets have to be reconciled simultaneously, then we are unable to make specific constraints on the molecular abundances or on the temperature profile, beyond identification of molecules and the presence or absence of a thermal inversion. For HD 209458b, we confirm and constrain a thermal inversion in the day-side atmosphere, and the data allows for very efficient day-night redistribution of energy. We report detection of CO, CH4 and CO 2 on the dayside of HD 209458b, along with placing an upper-limit on the amount of H2 0. We also report atmospheric models for three transiting exoplanets with limited data: TrES-2, HAT-P-7b, GJ 436b. For TrES-2 and HAT-P-7b, where only four observations each are available, we find that the data can be fit with models with and without thermal inversions, if we make no assumptions of chemical equilibrium. Finally, in this work, we report the first steps towards developing a parameter estimation procedure for exoplanetary atmospheres. We demonstrate with simulated data that our model can be used with a formal Bayesian parameter estimation algorithm, like MCMC, to place constraints on the atmospheric properties of hot Jupiters.