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Absorption lines from water, methane and carbon monoxide are detected in theatmosphere of exoplanet HR8799b. A medium-resolution spectrum presented hereshows well-resolved and easily identified spectral features from all threemolecules across the K band. The majority of the lines are produced by CO andH2O, but several lines clearly belong to CH4. Comparisons between these dataand atmosphere models covering a range of temperatures and gravities yield logmole fractions of H2O between -3.09 and -3.91, CO between -3.30 and -3.72 andCH4 between -5.06 and -5.85. More precise mole fractions are obtained for eachtemperature and gravity studied. A reanalysis of H-band data, previouslyobtained at similar spectral resolution, results in a nearly identical waterabundance as determined from the K-band spectrum. The methane abundance isshown to be sensitive to vertical mixing and indicates an eddy diffusioncoefficient in the range of 10^6 to 10^8 cm^2 s^-1, comparable to mixing in thedeep troposphere of Jupiter. The model comparisons also indicate a C/O between~ 0.58 and 0.7, encompassing previous estimates for a second planet in the samesystem, HR8799c. Super-stellar C/O could indicate planet formation bycore-accretion, however, the range of possible C/O for these planets (and thestar) is currently too large to comment strongly on planet formation. Moreprecise values of the bulk properties (e.g., effective temperature and surfacegravity) are needed for improved abundance estimates.

Simultaneous Detection of Water, Methane and Carbon Monoxide in the Atmosphere of Exoplanet HR8799b