Up to today, more than 5000 exoplanets have been detected and confirmed. Among them, about 60 are habitable Earth-like planets. It is expected that the ongoing and future space-borne planet survey missions including TESS, PLATO will detect thousands of small to medium-sized planets via the transit technique, including over a hundred habitable terrestrial rocky planets. To conduct a detailed atmospheric study of these terrestrial planets, particularly Earth-like planets in the habitable zones around solar-like stars, the exoplanet community has proposed various follow-up missions. In this talk, I will briefly summarize the main technique used for the characterization of exoplanet atmospheres, and the major results achieved so far. In addition, I will outline the currently available missions/instruments that can be used for the study of exoplanets and point out the lack of missions for the characterization of Earth 2.0 and the search for extraterrestrial biosignature for the upcoming 20 years. Therefore, we propose a 6-m class UV-to-optical space telescope, a mega mission aiming to study the atmospheres of exoplanets systematically, and to look for evidence of extraterrestrial life around our solar system. I will briefly introduce the preliminary design, main scientific objectives, and current status of this mission.
