CLOUDS AND CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HR8799b

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 narrowband 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 = 1100 K ±100 and log ( g ) = 3.5 ± 0.5 (for solar composition). Given the observed luminosity (log L obs / L ☉ ∼ − 5.1), these values correspond to a radius of 0.75 R Jup +0.17 − 0.12 and a mass of ∼0.72 M Jup +2.6 − 0.6 —strikingly inconsistent with interior/evolution models. Enhanced metallicity (up to ∼10 × 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 (<1000 K) required by the evolution models.