A Second Giant Planet In 3:2 Mean-Motion Resonance In The HD 204313 System

We present eight years of high-precision radial velocity (RV) data for HD 204313 from the 2.7 m Harlan J. Smith Telescope at McDonald Observatory. The star is known to have a giant planet (M sin i = 3.5 M-J) on a similar to 1900 day orbit, and a Neptune-mass planet at 0.2 AU. Using our own data in combination with the published CORALIE RVs of Segransan et al., we discover an outer Jovian (M sin i = 1.6 M-J) planet with P similar to 2800 days. Our orbital fit suggests that the planets are in a 3: 2 mean motion resonance, which would potentially affect their stability. We perform a detailed stability analysis and verify that the planets must be in resonance.Australian government through ARC DP0774000UNSWNational Aeronautics and Space Administration NNX07AL70G, NNX09AB30GHungarian OTKA K76816, K83790, MB08C 81013Hungarian Academy of ScienceAstronom