  Exoplanet observations promise one day to unveil the presence of
extraterrestrial life. Atmospheric compounds in strong chemical disequilibrium
would point to large-scale biological activity just as oxygen and methane do in
the Earth's atmosphere. The cancellation of both the Terrestrial Planet Finder
and Darwin missions means that it is unlikely that a dedicated space telescope
to search for biomarker gases in exoplanet atmospheres will be launched within
the next 25 years. Here we show that ground-based telescopes provide a strong
alternative for finding biomarkers in exoplanet atmospheres through transit
observations. Recent results on hot Jupiters show the enormous potential of
high-dispersion spectroscopy to separate the extraterrestrial and telluric
signals making use of the Doppler shift of the planet. The transmission signal
of oxygen from an Earth-twin orbiting a small red dwarf star is only a factor 3
smaller than that of carbon monoxide recently detected in the hot Jupiter tau
Bootis b, albeit such a star will be orders of magnitude fainter. We show that
if Earth-like planets are common, the planned extremely large telescopes can
detect oxygen within a few dozen transits. Ultimately, large arrays of
dedicated flux collector telescopes equipped with high-dispersion spectrographs
can provide the large collecting area needed to perform a statistical study of
life-bearing planets in the solar neighborhood.
