  Exoplanetary science is among the fastest evolving fields of today's
astronomical research. Ground-based planet-hunting surveys alongside dedicated
space missions (Kepler, CoRoT) are delivering an ever-increasing number of
exoplanets, now numbering at ~690, with ESA's GAIA mission planned to bring
this number into the thousands. The next logical step is the characterisation
of these worlds: what is their nature? Why are they as they are? The use of the
HST and Spitzer Space Telescope to probe the atmospheres of transiting hot,
gaseous exoplanets has demonstrated that it is possible with current technology
to address this ambitious goal. The measurements have also shown the difficulty
of understanding the physics and chemistry of these environments when having to
rely on a limited number of observations performed on a handful of objects. To
progress substantially in this field, a dedicated facility for exoplanet
characterization with an optimised instrument design (detector performance,
photometric stability, etc.), able to observe through time and over a broad
spectral range a statistically significant number of planets, will be
essential. We analyse the performances of a 1.2/1.4m space telescope for
exoplanet transit spectroscopy from the visible to the mid IR, and present the
SNR ratio as function of integration time and stellar magnitude/spectral type
for the acquisition of spectra of planetary atmospheres in a variety of
scenarios: hot, warm, and temperate planets, orbiting stars ranging in spectral
type from hot F to cool M dwarfs. We include key examples of known planets
(e.g. HD 189733b, Cancri 55 e) and simulations of plausible terrestrial and
gaseous planets, with a variety of thermodynamical conditions. We conclude that
even most challenging targets, such as super-Earths in the habitable-zone of
late-type stars, are within reach of a M-class, space-based spectroscopy
mission.
