The HARPS search for southern extra-solar planets - XII. A giant planet orbiting the metal-poor star HD171028

In this paper we present the detection of a 1.8 M-Jup planet in a 538-day period trajectory orbiting the metal-poor star HD171028 ([Fe/H] = -0.49). This planet is the first to be discovered in the context of a HARPS program searching for planets around metal-poor stars. Interestingly, HD171028 is one of the least metal-poor stars in the sample. This discovery is placed in the context of the models of planet formation and evolution.