The Anglo-Australian Planet Search XXV : A Candidate Massive Saturn Analog Orbiting HD 30177

We report the discovery of a second long-period giant planet orbiting HD 30177, a star previously known to host a massive Jupiter analog (HD 30177b: a=3.8$\pm$0.1 au, m sin $i=9.7\pm$0.5 Mjup). HD 30177c can be regarded as a massive Saturn analog in this system, with a=9.9$\pm$1.0 au and m sin $i=7.6\pm$3.1 Mjup. The formal best fit solution slightly favours a closer-in planet at $a\sim$7 au, but detailed n-body dynamical simulations show that configuration to be unstable. A shallow local minimum of longer-period, lower-eccentricity solutions was found to be dynamically stable, and hence we adopt the longer period in this work. The proposed $\sim$32 year orbit remains incomplete; further monitoring of this and other stars is necessary to reveal the population of distant gas giant planets with orbital separations $a\sim$10 au, analogous to that of Saturn.