The HARPS search for southern extra-solar planets

Precise radial-velocity measurements with the HARPS spectrograph reveal the presence of two planets orbiting the solar-type star HD 45364. The companion masses are M sin i = 0.187 MJup and 0.658 MJup, with semi-major axes of $a = 0.681$ AU and 0.897 AU, and eccentricities of $e = 0.168$ and 0.097, respectively. A dynamical analysis of the system further shows a 3:2 mean motion resonance between the two planets, which prevents close encounters and ensures the stability of the system over 5 Gyr. This is the first time that such a resonant configuration has been observed for extra-solar planets, although there is an analogue in our Solar System formed by Neptune and Pluto. This singular planetary system may provide important constraints on planetary formation and migration scenarios