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Current observations of the atmospheres of close-in exoplanets arepredominantly obtained with two techniques: low-resolution spectroscopy withspace telescopes and high-resolution spectroscopy from the ground. Although theobservables delivered by the two methods are in principle highly complementary,no attempt has ever been made to combine them, perhaps due to the differentmodeling approaches that are typically used in their interpretation. Here wepresent the first combined analysis of previously-published dayside spectra ofthe exoplanet HD 209458b obtained at low resolution with HST/WFC3 andSpitzer/IRAC, and at high resolution with VLT/CRIRES. By utilizing a novelretrieval algorithm capable of computing the joint probability distribution oflow- and high-resolution spectra, we obtain tight constraints on the chemicalcomposition of the planet's atmosphere. In contrast to the WFC3 data, we do notconfidently detect H2O at high spectral resolution. The retrieved waterabundance from the combined analysis deviates by 1.9 sigma from theexpectations for a solar-composition atmosphere in chemical equilibrium.Measured relative molecular abundances of CO and H2O strongly favor anoxygen-rich atmosphere (C/O<1 at 3.5 sigma) for the planet when compared toequilibrium calculations including O rainout. From the abundances of the sevenmolecular species included in this study we constrain the planet metallicity to0.1-1.0x the stellar value (1 sigma). This study opens the way to coordinatedexoplanet surveys between the flagship ground- and space-based facilities,which ultimately will be crucial for characterizing potentially-habitableplanets.

A framework to combine low- and high-resolution spectroscopy for the atmospheres of transiting exoplanets