This paper reports on the detection of a planetary system with three Super-Earths orbiting HD 40307 .
HD 40307 is a K2 V metal-deficient star at a distance of only 13 parsec , part of the HARPS GTO high-precision planet-search programme .
The three planets on circular orbits have very low minimum masses of respectively 4.2 , 6.9 and 9.2 Earth masses and periods of 4.3 , 9.6 and 20.5 days .
The planet with the shortest period is the lightest planet detected to-date orbiting a main sequence star .
The detection of the correspondingly low amplitudes of the induced radial-velocity variations is completely secured by the 135 very 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 the planetary interpretation .
Contrary to most planet-host stars , HD 40307 has a marked sub-solar metallicity ( [ Fe/H ] = - 0.31 ) , further supporting the already raised possibility that the occurrence of very light planets might show a different dependence on host star ’ s metallicity compared to the population of gas giant planets .
In addition to the 3 planets close to the central star , a small drift of the radial-velocity residuals reveals the presence of another companion in the system the nature of which is still unknown .