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Using the integral field spectrograph OSIRIS, on the Keck II telescope, broadnear-infrared H and K-band spectra of the young exoplanet HR8799b have beenobtained. In addition, six new narrow-band photometric measurements have beentaken across the H and K bands. These data are combined with previouslypublished photometry for an analysis of the planet's atmospheric properties.Thick photospheric dust cloud opacity is invoked to explain the planet's rednear-IR colors and relatively smooth near-IR spectrum. Strong water absorptionis detected, indicating a Hydrogen-rich atmosphere. Only weak CH4 absorption isdetected at K band, indicating efficient vertical mixing and a disequilibriumCO/CH4 ratio at photospheric depths. The H-band spectrum has a distincttriangular shape consistent with low surface gravity. New giant planetatmosphere models are compared to these data with best fitting bulk parameters,Teff = 1100K +- 100 and log(g) = 3.5 +-0.5 (for solar composition). Given theobserved luminosity, log(Lobs/Lsun) ~ -5.1, these values correspond to a radiusof 0.75 Rjup (+0.17, -0.12) and mass ~ 0.72 Mjup (+2.6,-0.6) -- strikinglyinconsistent with interior/evolution models. Enhanced metallicity (up to ~ 10times that of the Sun) along with thick clouds and non-equilibrium chemistryare likely required to reproduce the complete ensemble of spectroscopic andphotometric data and the low effective temperatures (< 1000K) required by theevolution models.

Clouds and Chemistry in the Atmosphere of Extrasolar Planet HR8799b