Clouds and Chemistry in the Atmosphere of Extrasolar Planet HR8799b

TL;DR

This study analyzes the atmosphere of exoplanet HR8799b using spectroscopic and photometric data, revealing thick clouds, water absorption, disequilibrium chemistry, and discrepancies with evolution models, suggesting complex atmospheric composition.

Contribution

First detailed atmospheric analysis of HR8799b combining new spectra and photometry, highlighting the need for enhanced metallicity and clouds in models.

Findings

Thick cloud opacity explains red near-IR colors.

Weak methane absorption indicates disequilibrium chemistry.

Derived parameters are inconsistent with standard evolution models.

Abstract

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.