Could we detect O2 in the atmosphere of a transiting extra-solar earth-like planet?

Although the extra-solar planets discovered so far are of the giant, gaseous type, the increased sensitivity of future surveys will result in the discovery of lower mass planets. The detection of O-2 in the atmosphere of a rocky extra-solar planet would be a potential indicator of life. In this paper we address the specific issue of whether we would be able to detect the O-2 A-band absorption feature in the atmosphere of a planet similar to the Earth, if it were in orbit around a nearby star. Our method is empirical, in that we use observations of the Earth's O-2 A-band, with a simple geometric modification for a transiting extra-solar planet, allowing for limb-darkening of the host star. We simulate the spectrum of the host star with the superposed O-2 A-band absorption of the transiting planet, assuming a spectral resolution of similar to8 km s(-1)(typical of current echelle spectrographs), for a range of spectral signal-to-noise ratios. The main result is that in principle we may be able to detect the O-2 A-band of the transiting planet for host stars with radii R less than or equal to 0.3 R.. However, using existing instrumentation and 8 in telescopes, this requires target NI-stars with m (V) approximate to 10 or brighter for integration times of similar to 10 hours or less. The number of such stars over the sky is small. Larger aperture telescopes and/or improved instrumentation efficiency would enable surveys of M-stars down to in (V) approximate to 13 and greatly improve the chances of discovering life elsewhere.