The HARPS search for southern extra-solar planets XIII. A planetary system with 3 super-Earths (4.2, 6.9, and 9.2 M-circle plus)

We report the detection of a planetary system with three Super-Earths orbiting HD40307. HD40307 is a K2V metal-deficient star at a distance of only 13 parsec, which is part of the HARPS GTO high-precision planet-search programme. The three planets on circular orbits have very low minimum masses of 4.2, 6.9, and 9.2 Earth masses and periods of 4.3, 9.6, and 20.5 days, respectively. The planet with the shortest period is the lowest mass planet detected to-date orbiting a main sequence star. The detection of the correspondingly low amplitudes of the induced radial-velocity variations is secured completely by the 135 high-quality HARPS observations illustrated by the radial-velocity residuals around the 3-Keplerian solution of only 0.85 ms(-1). Activity and bisector indicators exclude any significant perturbations of stellar intrinsic origin, which supports a planetary interpretation. In contrast to most planet-host stars, HD40307 has a significantly sub-solar metallicity ([Fe/H] = -0.31), which suggests that very light planets might have a different dependence on host star metallicity than gas giant planets. In addition to the 3 planets close to the central star, a small drift in the radial-velocity residuals implies the presence of another companion in the system, the nature of which is still unknown.