This program is dedicated to the simulation and characterization of Extrasolar Terrestrial Planet (ETP) atmospheres. Thanks to new generation codes, the team E3ARTHS aims to provide a top expertise in a key domain of astrobiology: the origin, evolution and identification of habitable worlds, and the quest for biomarkers on Earth-like planets. The team will also revisit early Earth models for a better understanding of the context of the origins of life, in the light of recent works on Earth formation, impact history and Solar evolution. The observable signatures of an ETP and its ability to sustain life are determined by atmospheric properties: chemistry, radiative transfer, climate. Although these processes are usually treated separately, they evolve in a tightly coupled scheme under the influence of astrophysical, geophysical and, if present, biological mechanisms. Eventually, realistic planetary environments will thus have to be modeled with self-consistent 3D tools, involving a multidisciplinary and international approach. Although ambitious by today's standards, such enterprise is a necessary counterpart of the planned ETP searches, and is required to study the discovered planets. Observatories like Darwin/TPF and ELTs will provide direct information on ETPs within 10-15 years. Ongoing transit searches (CoRoT, and Kepler), and radial-velocity surveys, are on the verge of detecting ETPs. In this context, E3ARTHS can become one of the cores in European theoretical research on ETPs, in close interaction with observation programs. Since his PhD, F. Selsis has developed his own research on ETPs, which already had important implications for the design of instruments for TEP search and characterization. His plan is now to take this research at the next level by creating a dedicated team that will integrate new tools such as 3D climate, photochemical and radiative transfer codes, produce virtual observations of ETPs, and study their potential for life.