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This paper reports on the detection of a planetary system with threeSuper-Earths orbiting HD40307. HD40307 is a K2V metal-deficient star at adistance of only 13 parsec, part of the HARPS GTO high-precision planet-searchprogramme. The three planets on circular orbits have very low minimum masses ofrespectively 4.2, 6.9 and 9.2 Earth masses and periods of 4.3, 9.6 and 20.5days. The planet with the shortest period is the lightest planet detectedto-date orbiting a main sequence star. The detection of the correspondingly lowamplitudes of the induced radial-velocity variations is completely secured bythe 135 very high-quality HARPS observations illustrated by the radial-velocityresiduals around the 3-Keplerian solution of only 0.85 m/s. Activity andbisector indicators exclude any significant perturbations of stellar intrinsicorigin, which supports the planetary interpretation. Contrary to mostplanet-host stars, HD40307 has a marked sub-solar metallicity ([Fe/H]=-0.31),further supporting the already raised possibility that the occurrence of verylight planets might show a different dependence on host star's metallicitycompared to the population of gas giant planets. In addition to the 3 planetsclose to the central star, a small drift of the radial-velocity residualsreveals the presence of another companion in the system the nature of which isstill unknown.

The HARPS search for southern extra-solar planets. XIII. A planetary system with 3 Super-Earths (4.2, 6.9, & 9.2 Earth masses)