  Visible and near-infrared spectra of transiting hot Jupiter planets have
recently been observed, revealing some of the atmospheric constituents of their
atmospheres. In the near future, it is probable that primary and secondary
eclipse observations of Earth-like rocky planets will also be achieved. The
characterization of the Earth's transmission spectrum has shown that both major
and trace atmospheric constituents may present strong absorption features,
including important bio-markers such as water, oxygen and methane. Our
simulations using a recently published empirical Earth's transmission spectrum,
and the stellar spectra for a variety of stellar types, indicate that the new
generation of extremely large telescopes, such as the proposed 42-meter
European Extremely Large Telescope(E-ELT), could be capable of retrieving the
transmission spectrum of an Earth-like planet around very cool stars and brown
dwarfs (Teff < 3100 K). For a twin of Earth around a star with Teff around 3100
K (M4), for example, the spectral features of water vapor, methane, carbon
dioxide, and oxygen in the wavelength range between 0.9 and 2.4 micron can
simultaneously be detected within a hundred hours of observing time, or even
less for a late-M star. Such detection would constitute a proof for the
existence of life in that planet. The detection time can be reduced to a few
hours for a super-Earth type of planet with twice the Earth's radius.
