The atmospheres of extrasolar planets

The characteristics of irradiated solar system planetary atmospheres have been studied for decades. Modern planetary science benefits from an exhaustive body of ground- and space-based data. The study of extrasolar planetary atmospheres, by contrast, is in its infancy and currently rests on a few score of datapoints, mostly of the transiting planets. As the study of exoplanetary atmospheres blossoms, it is well worth remembering lessons learned from the solar system. In this contribution, based on my Les Houches lecture on exoplanet atmospheres, I briefly review a few of the key concepts relevant to understanding planetary atmospheres and point to examples both in our own solar system and among the exoplanets. This short survey aims not to review the field but rather stresses the importance of understanding the structure, dynamics, cloud processes and photochemistry of planetary atmospheres and points the reader towards useful reference works. Here I particularly explain the importance of cloud and photochemical processes in controlling the spectra and albedos of extrasolar planets and urge that care be taken in the interpretation of exoplanet colors.