Since the discoveries of giant planets outside our Solar System, over 800 extra-solar planets have been detected and several thousands candidates are awaiting confirmation. They have revolutionized planetary science, by placing our once unique solar system into context. The subset of extrasolar planets that transit their parent star have had most impact on our understanding of their planetary structure and atmospheric physics: they are the only ones for which one can simultaneously measure mass and radius, and therefore infer internal composition. The few that transit a host star bright enough for detailed spectroscopic follow-up provide, in addition, observational information on the composition and physics of extrasolar planetary atmospheres.Much interest is now focused on finding and characterizing terrestrial mass planets, ideally in the habitable zone of their host stars. The present ERC project offers a novel method to dramatically improve the precision of both the detection and the characterization of exoplanets. The method makes use of multi-object spectrographs to add spectroscopic resolution on traditional differential photometry. This enables the fine correction of the atmospheric variations that would otherwise hinder ground-based observations.We propose to setup small-size telescopes equipped with a multi-object near-IR spectrograph and observe 800 M dwarfs. This will be the most sensitive survey for Earth-size planets transiting bright nearby stars. It shall yield dozens exo-Earths amenable to atmospheric characterization, including several habitable exo-Earths.To perform their atmospheric characterization, we also propose to apply the technique of differential spectro-photometry with multi-object spectrographs available on large telescopes. Our observations will represent a step forward in transmission spectroscopy and prepare for the identification of bio-markers in exo-Earth atmospheres with the future ELTs.