The HARPS search for southern extra-solar planets XXXIX. HD175607, the most metal-poor G dwarf with an orbiting sub-Neptune

Context. The presence of a small-mass planet (M-p \textless 0.1 M-Jup) seems, to date, not to depend on metallicity, however, theoretical simulations have shown that stars with subsolar metallicities may be favoured for harbouring smaller planets. A large, dedicated survey of metal-poor stars with the HARPS spectrograph has thus been carried out to search for Neptunes and super-Earths. Aims. In this paper, we present the analysis of HD175607, an old G6 star with metallicity [Fe/H] = -0.62. We gathered 119 radial velocity measurements in 110 nights over a time span of more than nine years. Methods. The radial velocities were analysed using Lomb-Scargle periodograms, a genetic algorithm, a Markov chain Monte Carlo analysis, and a Gaussian processes analysis. The spectra were also used to derive stellar properties. Several activity indicators were analysed to study the effect of stellar activity on the radial velocities. Results. We find evidence for the presence of a small Neptune-mass planet (M-p sin i = 8.98 +/- 1.10 M-circle times) orbiting this star with an orbital period P = 29.01 +/- 0.02 days in a slightly eccentric orbit (e = 0.11 +/- 0.08). The period of this Neptune is close to the estimated rotational period of the star. However, from a detailed analysis of the radial velocities together with the stellar activity, we conclude that the best explanation of the signal is indeed the presence of a planetary companion rather than stellar related. An additional longer period signal (P similar to 1400 d) is present in the data, for which more measurements are needed to constrain its nature and its properties. Conclusions. HD175607 is the most metal-poor FGK dwarf with a detected low-mass planet amongst the currently known planet hosts. This discovery may thus have important consequences for planet formation and evolution theories.