A planetary system around HD 155358: The lowest metallicity planet host star

We report the detection of two planetary mass companions to the solar-type star HD 155358. The two planets have orbital periods of 195.0 and 530.3 days, with eccentricities of 0.11 and 0.18. The minimum masses for these planets are 0.89 and 0.50M(J), respectively. The orbits are close enough to each other, and the planets are sufficiently massive, that the planets are gravitationally interacting with each other, with their eccentricities and arguments of periastron varying with periods of 2300-2700 yr. While large uncertainties remain in the orbital eccentricities, our orbital integration calculations indicate that our derived orbits would be dynamically stable for at least 108 yr. With a metallicity [Fe/H] of -0.68, HD 155358 is tied with the K1 III giant planet host star HD 47536 for the lowest metallicity of any planet host star yet found. Thus, a star with only 21% of the heavy-element content of our Sun was still able to form a system of at least two Jovian-mass planets and have their orbits evolve to semimajor axes of 0.6-1.2 AU.