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Planet–planet scattering in the upsilon Andromedae system


Doppler spectroscopy has detected 152 planets around nearby stars1. A major puzzle is why many of their orbits are highly eccentric; all planets in our Solar System are on nearly circular orbits, as is expected if they formed by accretion processes in a protostellar disk. Several mechanisms have been proposed to generate large eccentricities after planet formation, but so far there has been little observational evidence to support any particular model. Here we report that the current orbital configuration of the three giant planets around upsilon Andromedae2,3 (υ And) probably results from a close dynamical interaction with another planet4, now lost from the system. The planets started on nearly circular orbits, but chaotic evolution caused the outer planet (υ And d) to be perturbed suddenly into a higher-eccentricity orbit. The coupled evolution of the system then causes slow periodic variations in the eccentricity of the middle planet (υ And c). Indeed, we show that υ And c periodically returns to a very nearly circular state every 6,700 years.

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Figure 1: Secular evolution of the planetary system around υ And.
Figure 2: Probability distribution for the minimum eccentricity of υ And c.
Figure 3: Observational constraints on the secular evolution parameters.
Figure 4: Dynamical evolution of a hypothetical planetary system similar to υ And.


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We are very grateful to D. Fischer for providing us with the latest radial velocity data on υ And. We also thank E. Chiang, M. H. Lee and S. Peale for discussions. This work was supported by an NSF grant to F.A.R. at Northwestern University and by a Miller Research Fellowship to E.B.F. V.L. acknowledges support from the NASA Undergraduate Summer Research Program at Northwestern. F.A.R. and E.B.F. thank the Kavli Institute for Theoretical Physics for hospitality and support.

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Correspondence to Frederic A. Rasio.

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Supplementary information

Supplementary Methods

This provides technical details related to the algorithm for sampling from the posterior probability distribution for the masses and orbital parameters. (PDF 21 kb)

Supplementary Discussion

This explains the choice of initial conditions used for the calculation presented in Fig. 4. It also discusses the timescale constraints which rule out the possbility that the impulsive perturbation to Ups And d was delivered by a massive exterior primordial disk with a large viscosity. (PDF 24 kb)

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Ford, E., Lystad, V. & Rasio, F. Planet–planet scattering in the upsilon Andromedae system. Nature 434, 873–876 (2005).

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