The HARPS search for southern extra-solar planets ^*,**

XIII. A planetary system with 3 super-Earths (4.2, 6.9, and 9.2 M)

¹ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: michel.mayor@obs.unige.ch
² Physikalisches Institut, Universität Bern, Silderstrasse 5, 3012 Bern, Switzerland
³ Service d'Aéronomie du CNRS/IPSL, Université de Versailles Saint-Quentin, BP 3, 91371 Verrières-le-Buisson, France
⁴ Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France

Received: 23 June 2008
Accepted: 1 August 2008

Abstract

We report 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, 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, HD 40307 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.