The close-in extrasolar giant planets (CEGPs) reside in irradiated
environments much more intense than that of the giant planets in our solar
system. The high UV irradiance strongly influences their photochemistry and the
general current view believed that this high UV flux will greatly enhance
photochemical production of hydrocarbon aerosols. In this letter, we
investigate hydrocarbon aerosol formation in the atmospheres of CEGPs. We find
that the abundances of hydrocarbons in the atmospheres of CEGPs are
significantly less than that of Jupiter except for models in which the CH4ā
abundance is unreasonably high (as high as CO) for the hot (effective
temperatures ā³1000 K) atmospheres. Moreover, the hydrocarbons will be
condensed out to form aerosols only when the temperature-pressure profiles of
the species intersect with the saturation profiles--a case almost certainly not
realized in the hot CEGPs atmospheres. Hence our models show that photochemical
hydrocarbon aerosols are insignificant in the atmospheres of CEGPs. In
contrast, Jupiter and Saturn have a much higher abundance of hydrocarbon
aerosols in their atmospheres which are responsible for strong absorption
shortward of 600 nm. Thus the insignificance of photochemical hydrocarbon
aerosols in the atmospheres of CEGPs rules out one class of models with low
albedos and featureless spectra shortward of 600 nm.Comment: ApJL accepte