Cloud and Haze in the Atmospheres of Wide-Separation Exoplanets

Imaging and characterizing wide-separation exoplanets with spaceborne coronagraph will write a new chapter of exoplanet science. Most of the exoplanets to be observed by coronagraph will be located further away from their parent stars than is Earth from the Sun. These "cold" exoplanets have atmospheric environments conducive for the formation of water and/or ammonia clouds by condensation. Above the condensation clouds, photochemical processes driven by UV irradiation can lead to formation of haze particles. Understanding the cloud and haze in the atmosphere of wide-separation exoplanets is essential, because they determine the planets' spectral signal and how well we can measure the planets' atmospheric abundances. Using atmospheric chemistry and radiative transfer models, I find that the mixing ratio of methane and the pressure level of the uppermost cloud deck on these planets can be uniquely determined from their reflection spectra, if a strong band and a weak band of methane are measured at moderate spectral resolutions. This determination can however be biased by a haze layer above the cloud. To constrain the uncertainty, atmospheric photochemistry models are used to estimate the amount of haze particles.