Characterizing Cool Gas Giant Exoplanets Detected by Radial Velocity

A Quick Study of the Characterization of Radial Velocity Giant Planets in Reflected Light by Forward and Inverse Modeling

Authors:

Marley et al

Abstract:

We explored two aspects of the problem of characterizing cool extrasolar giant planets in scattered optical light with a space based coronagraph. First, for a number of the known radial velocity (RV) giants we computed traditional forward models of their atmospheric structure and clouds, given various input assumptions, and computed model albedo spectra. Such models have been computed before, but mostly for generic planets. Our new models demonstrate that there is likely interesting spectral diversity among those planets that are most favorable for direct detection. Second, we applied a powerful Markov Chain Monte Carlo (MCMC) retrieval technique to synthetic noisy data of cool giants to better understand how well various atmospheric parameters--particularly molecular abundances and cloud properties--could be constrained. This is the first time such techniques have been applied to this problem. The process is time consuming, so only a dozen or so cases could be completed in the limited time available. Nevertheless the results clearly show that even at S/N ~ 5, scientifically interesting and valuable conclusions can be drawn about the properties of giant planet atmospheres from noisy spectra. We find that atmospheric abundances are best constrained when the planet gravity is bounded. Thus direct imaging observations of known radial velocity planets are extremely valuable as limits on a target planet's gravity can be obtained from astrometric imaging and reflectivity and mass-radius relationship arguments (for mass and radius, respectively). Further retrieval studies are clearly warranted and would be valuable to help guide instrument design decisions.