Spectroscopic indicators of life on other planets

Astronomers have now identified over 300 extrasolar planets orbiting nearby stars. Most of these planets have been found by using ground-based instruments to measure Doppler shifts in the spectrum of the parent star. For stars similar to our Sun, this method is unable to find planets as small as Earth. Within the next two (three?) decades, however, NASA hopes to launch space-based telescopes that will be able to search directly for extrasolar planets. NASA's planned \textit{Terrestrial Planet Finder (TPF)} missions, will look for Earth-like planets around nearby stars and, if they exist, provide spectroscopic information on their atmospheres. \textit{TPF-C} will be a coronagraph that operates in the visible/near-IR. A variant of this idea, called \textit{TPF-O}, would replace the internal coronagraph with a free-flying occulting disk. \textit{TPF-I} is envisioned as a free-flying interferometer operating in the thermal-IR. On a planet like modern Earth, \textit{TPF-C} or \textit{TPF--O} should be able to see absorption bands of O$_{2}$, H$_{2}$O, and possibly O$_{3}$. \textit{TPF-I} would be able to see CO$_{2}$, H$_{2}$O, and O$_{3}$. Both O$_{2}$ and O$_{3}$ are considered to be good indicators of life for planets orbiting within the liquid water habitable zone of their parent star. Even better evidence for life would be the simultaneous observation of O$_{2}$ (or O$_{3})$ and a reduced gas such as CH$_{4}$ or N$_{2}$O. That may not be possible with a first-generation \textit{TPF} instrument but should ultimately be possible in the more distant future.