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G. Laughlin (UCO/Lick Observatory), J. Chambers (NASA Ames Research Center), D. Fischer (UC Berkeley)
Thirteen years of Doppler velocity measurements have revealed the presence of two planets orbiting the star 47 Ursa Majoris on low eccentricity orbits. A 2-Keplerian fit to the radial velocity data suggests that the inner planet has a period Pb = 1089.0 ±2.9 d, and a nominal mass m\sin i = 2.54 MJup, while the outer planet has a period Pc = 2594 ±90 d, and a mass m \sin i = 0.76 MJup. These mass and period ratios suggest a possible kinship to the Jupiter-Saturn pair in our own solar system. We explore the current dynamical state of this system with numerical integrations, and compare the results with analytic secular theory. We find that the planets in the system are likely participating in a secular resonance in which the arguments of pericenter librate around zero. The system may also currently be in a 7:3 mean-motion resonance. Using a self-consistent fitting procedure in conjunction with numerical integrations, we show that stability considerations restrict the mutual inclination between the two planets to 40 degrees or less, and that this result is relatively insensitive to the total mass of the two planets. We present hydrodynamical simulations which measure the torques exerted on the planets by a hypothesized external protoplanetary disk. We show that planetary migration in response to torques from the disk may have led to capture of the system into a 7:3 mean-motion resonance, although it is unclear how the eccentricities of the planets would have been damped after capture occured. We show that Earth-mass planets can survive for long periods in some regions of the habitable zone of the nominal co-planar system. A set of planetary accretion calculations, however, shows that it is unlikely that large terrestrial planets can form in the 47UMa habitable zone. This work was funded by the NASA Origins Program, and by a NASA Ames Director's Discretionary Fund Award.