We detected 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 0.50 Jupiter masses 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 years. While large uncertainties remain in the orbital eccentricities, our orbital integration calculations indicate that our derived orbits would be dynamically stable for at least 10^8 years. With a metallicity [Fe/H] of -0.68, HD 155358 is tied with the K1III 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 semi-major axes of 0.6 - 1.2 AU.

Time series of the HET radial velocity data (green) for HD 155358. The best-fit 2-planet Keplerian orbit is shown as solid blue line.
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Phase plot of the HET radial velocity data (green) and the 2 radial velocity orbits for planet 1 (upper panel) and planet 2 (lower panel). The data are repeated for a second cycle (gray points).
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Parameters of the 2 planets and their orbits:
P1 = 195.0 ± 1.1 [days]
T1 periastron = 2453950.0 ± 10.4 [BJD]
K1 = 34.6 ± 3.0 [m/s]
e1 = 0.112 ± 0.037
omega1 = 162 ± 20 [deg]
M1 sin i = 0.89 ± 0.12 [Jupiter]
a1 = 0.628 ± 0.020 [AU]

P2 = 530.3 ± 27.2 [days]
T2 periastron = 2454420.3 ± 79.3 [BJD]
K2 = 14.1 ± 1.6 [m/s]
e2 = 0.176 ± 0.174
omega2 = 279 ± 38 [deg]
M2 sin i = 0.504 ± 0.075 [Jupiter]
a2 = 1.224 ± 0.081 [AU]

Results from the dynamical study of the HD 155358 planetary system showing how the 2 planets gravitationally interact. The middle panel shows how they are trading eccentricities on a time scale of ~2500 years.
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Parameters of the host star:
Spectral type = G0
M = 0.87 ± 0.07 M [Sun]
[Fe/H] = -0.68 ± 0.07
Teff = 5760 ± 101 [K]
logg = 4.09 ± 0.1 [cgs]
age = ~10 [Gyr]

References:
Cochran W. D., Endl M., Wittenmyer R. A., Bean, J. 2007, ApJ in press astro-ph/0705.3228

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