The HARPS search for southern extra-solar planets XI. Super-Earths (5 & 8 M_Earth) in a 3-planet system

This Letter reports on the detection of two super-Earth planets in the Gl581 system, already known to harbour a hot Neptune. One of the planets has a mass of 5 M_Earth and resides at the ``warm'' edge of the habitable zone of the star. It is thus the known exoplanet which most resembles our own Earth. The other planet has a 7.7 M_Earth mass and orbits at 0.25 AU from the star, close to the ``cold'' edge of the habitable zone. These two new light planets around an M3 dwarf further confirm the formerly tentative statistical trend for i) many more very low-mass planets being found around M dwarfs than around solar-type stars and ii) low-mass planets outnumbering Jovian planets around M dwarfs.