A combination of observations and theory is beginning to reveal the nature of the atmospheres of extrasolar giant planets.

Infrared observations of secondary transits hold great promise as diagnostics of the atmospheres, providing information about composition, about dayside temperatures, and, by inference, about heat flow and dynamics. The Spitzer Space Telescope provides five bandpasses in which secondary transits of the nearer transiting planets can be detected. Although five-wavelength spectrophotometry of two or three planets is scarcely all that one might desire, it is vastly more information about the planetary atmospheres than has been available hitherto.

High-resolution near-IR spectroscopy of common molecules such as H2O and CO may yet prove successful, but efforts to date have failed to provide detections of the primary transit spectra of even the most accessible transiting planets. Theory suggests that these spectra contain a lot of information. Adequate observations may however await the launch of space telescopes designed for this purpose; none of these are now on the horizon (although JWST holds some promise for at least some kinds of observations). The upper limits on molecular band strengths that have been set using ground-based observations suggest that sources of continuous opacity (clouds, perhaps) are likely important in the atmospheres of hot Jupiters at pressures of a few mb or less.

Visible-light (but space-borne) observations of primary transits have revealed absorption in the Na D lines; the small amplitude of the absorption is consistent with the idea of high clouds, but other explanations are possible as well. The mass-radius relation for extrasolar planets is measured using accurate transit light curves, and it provides constraints on the bulk composition of the planets. Most of the transiting planets agree tolerably well with theoretical expectations for irradiated hydrogen-helium configurations, but the planet orbiting HD 149026 is too small (likely because of a large and dense core), while that orbiting HD 209458 is too large (for reasons that remain mysterious).

Finally, the observations of the transit of Venus described above still await a complete analysis, but a preliminary look shows that the transit spectrum of Venus's atmosphere can easily be seen in the near-IR lines of CO2. These data (and those that will no doubt be obtained at the next Venus transit in 2012), provide both an inspiration and a testing ground for methods that we would like to use on planets of other stars.

We are grateful to the staff of the KIS vacuum tower telescope and to Manuel Collados Vera and the staff of the Instituto de Astrofisica de Canarias for their generous assistance with the observations of the Venus transit. NSO/Kitt Peak FTS data used here were produced by NSF/NOAO.

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