2015ApJ...804...61B

Absorption lines from water, methane, and carbon monoxide are detected in the atmosphere of exoplanet HR 8799 b. A medium-resolution spectrum presented here shows well-resolved and easily identified spectral features from all three molecules across the K band. The majority of the lines are produced by CO and H₂O, but several lines clearly belong to CH₄. Comparisons between these data and atmosphere models covering a range of temperatures and gravities yield log mole fractions of H₂ O between -3.09 and -3.91, CO between -3.30 and -3.72, and CH₄between -5.06 and -5.85. More precise mole fractions are obtained for each temperature and gravity studied. A reanalysis of H-band data, previously obtained at a similar spectral resolution, results in a nearly identical water abundance as determined from the K-band spectrum. The methane abundance is shown to be sensitive to vertical mixing and indicates an eddy diffusion coefficient in the range of 10⁶-10⁸ cm2/s, comparable to mixing in the deep troposphere of Jupiter. The model comparisons also indicate a carbon-to-oxygen ratio (C/O) between ∼0.58 and 0.7, encompassing previous estimates for a second planet in the same system, HR 8799 c. Super-stellar C/O could indicate planet formation by core-accretion; however, the range of possible C/O for these planets (and the star) is currently too large to comment strongly on planet formation. More precise values of the bulk properties (e.g., effective temperature and surface gravity) are needed for improved abundance estimates.