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We are developing a technique to measure the atmospheric composition of extrasolar planets through transit spectroscopy. Current observational capabilities have not yet reached enough sensitivity to detect the Earth-like planets that life as we know it requires to evolve. We anticipate, however, that this technique will detect constituents of the atmospheres of Earth-like planets once future space-based observatories become sensitive enough. We are currently using our methods on the extrasolar close-in giant planet HD 209458b. Bulk and orbital parameters are well constrained for this planet and there are measurements of atmospheric sodium, hydrogen, carbon and oxygen. However, nothing is known about the abundances of molecules relevant to life. We are studying the modulation of the stellar spectrum as the planet transits in front of the star. Different wavelengths become extinct at different levels in the exoplanet, causing the occulting area, and therefore the modulation, to be wavelength-dependent. This dependency allows us to identify atmospheric constituents. Signal-to-noise estimates show that data we have obtained from the Very Large Telescope (VLT), the Infrared Telescope Facility (IRTF), Palomar, and Keck are sensitive enough to measure or place useful limits on the atmospheric abundances of water and maybe even of carbon monoxide and methane. In order to detect the very weak expected modulation (∼ 4 parts in 10000), we are building a detailed radiative-transfer model to cross correlate with the data. This model will accept atmospheric temperature, density, composition, and cloud distributions profiles for the planet. We will then be able to measure the abundance of molecules relevant to life in the atmosphere of an extrasolar planet. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]