  Using the integral field spectrograph OSIRIS, on the Keck II telescope, broad
near-infrared H and K-band spectra of the young exoplanet HR8799b have been
obtained. In addition, six new narrow-band photometric measurements have been
taken across the H and K bands. These data are combined with previously
published photometry for an analysis of the planet's atmospheric properties.
Thick photospheric dust cloud opacity is invoked to explain the planet's red
near-IR colors and relatively smooth near-IR spectrum. Strong water absorption
is detected, indicating a Hydrogen-rich atmosphere. Only weak CH4 absorption is
detected at K band, indicating efficient vertical mixing and a disequilibrium
CO/CH4 ratio at photospheric depths. The H-band spectrum has a distinct
triangular shape consistent with low surface gravity. New giant planet
atmosphere 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 the
observed luminosity, log(Lobs/Lsun) ~ -5.1, these values correspond to a radius
of 0.75 Rjup (+0.17, -0.12) and mass ~ 0.72 Mjup (+2.6,-0.6) -- strikingly
inconsistent with interior/evolution models. Enhanced metallicity (up to ~ 10
times that of the Sun) along with thick clouds and non-equilibrium chemistry
are likely required to reproduce the complete ensemble of spectroscopic and
photometric data and the low effective temperatures (< 1000K) required by the
evolution models.
