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Description
After that a new technique combining high-contrast imaging and high-dispersion spectroscopy successfully detected the atmosphere of a giant planet, it soon became contemplated as one of the most promising avenues to study the atmosphere of Earth-size worlds. With the forthcoming ELTs, it shall gain the angular resolution and sensitivity to even detect O2 in the atmosphere of planets orbiting red dwarfs. This is a strong motivation to make the census of planets around cool stars which habitable zones can be resolved by ELTs, i.e. for M dwarfs within ~5-parsecs. In that context, our HARPS survey is already a major contributor to that sample of nearby planets. Here we report on our radial-velocity observations of Ross 128 (Proxima Virginis, GJ447, HIP 57548), a M4 dwarf just 3.4-parsec away from our Sun. We detect it is host of an exo-Earth with a projected mass m*sini=1.35M_{sun}_ and an orbital period of 9.9-days. Ross 128 b receives ~1.38 as much flux as Earth from the Sun and has an equilibrium temperature between 269K (resp. 213K) for an Earth-like (resp. Venus-like) albedo. According to recent studies, it is located at the inner edge of the so called habitable zone. An 80-day long light curve performed by K2 during campaign C01 excludes Ross 128 b is a transiting planet. Together with ASAS photometry and other activity indices, it argues for a long rotational period and a weak activity which, in the context of habitability, gives a high merit to the detection. Today, Ross 128 b is the second closest known exo-Earth after Proxima Centauri b (1.3 parsec) and the closest known temperate planet around a quiet star. At maximum elongation, the planet-star angular separation of 15 milli-arcsec will be resolved by the ELT (>3{lambda}/D) in all optical bands of O_2_.
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