Simultaneous Detection of Water, Methane and Carbon Monoxide in the Atmosphere of Exoplanet HR8799b

TL;DR

This study detects water, methane, and carbon monoxide in exoplanet HR8799b's atmosphere using spectral data, providing insights into its composition, atmospheric mixing, and potential formation history.

Contribution

First simultaneous detection and quantification of water, methane, and CO in HR8799b's atmosphere with detailed atmospheric modeling.

Findings

Detected spectral lines from H2O, CH4, and CO in the exoplanet's atmosphere.

Estimated molecular abundances and atmospheric mixing parameters.

Indicated a super-stellar C/O ratio, suggesting possible planet formation pathways.

Abstract

Absorption lines from water, methane and carbon monoxide are detected in the atmosphere of exoplanet HR8799b. A medium-resolution spectrum presented here shows well-resolved and easily identified spectral features from all three molecules across the K band. The majority of the lines are produced by CO and H2O, but several lines clearly belong to CH4. Comparisons between these data and atmosphere models covering a range of temperatures and gravities yield log mole fractions of H2O between -3.09 and -3.91, CO between -3.30 and -3.72 and CH4 between -5.06 and -5.85. More precise mole fractions are obtained for each temperature and gravity studied. A reanalysis of H-band data, previously obtained at similar spectral resolution, results in a nearly identical water abundance as determined from the K-band spectrum. The methane abundance is shown to be sensitive to vertical mixing and indicates an eddy diffusion coefficient in the range of 10^6 to 10^8 cm^2 s^-1, comparable to mixing in the deep troposphere of Jupiter. The model comparisons also indicate a C/O between ~ 0.58 and 0.7, encompassing previous estimates for a second planet in the same system, HR8799c. Super-stellar C/O could indicate planet formation by core-accretion, however, the range of possible C/O for these planets (and the star) is currently too large to comment strongly on planet formation. More precise values of the bulk properties (e.g., effective temperature and surface gravity) are needed for improved abundance estimates.