Using the integral field spectrograph OSIRIS , on the Keck \ @ slowromancap 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 CH _ { 4 } absorption is detected at K band , indicating efficient vertical mixing and a disequilibrium CO/CH _ { 4 } 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 , { T } _ { eff } = 1100K \pm 100 and \log ( g ) = 3.5 \pm 0.5 ( for solar composition ) .
Given the observed luminosity ( \log L _ { obs } / L _ { \odot } \sim - 5.1 ) , these values correspond to a radius of 0.75 { R } _ { Jup } ^ { +0.17 } _ { -0.12 } and mass \sim 0.72 { M } _ { Jup } ^ { +2.6 } _ { -0.6 } – strikingly inconsistent with interior/evolution models .
Enhanced metallicity ( up to \sim 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 .