The HARPS search for southern extra-solar planets - XVIII. An Earth-mass planet in the GJ 581 planetary system
M. Mayor, X. Bonfils, T. Forveille, X. Delfosse, S. Udry, J.-L. Bertaux, H. Beust, F. Bouchy, C. Lovis, F. Pepe, C. Perrier, D. Queloz and N. C. Santos
