Planet–Planet Scattering in Upsilon Andromedae

Doppler spectroscopy has detected 136 planets around nearby stars. A major puzzle is why their orbits are highly eccentric, while all planets in our Solar System are on nearly circular orbits, as 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 one. Here we report that the current orbital configuration of the three giant planets around Upsilon Andromedae (υ And) provides evidence for a close dynamical interaction with another planet, 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 9000 years. Our analysis shows that strong planet–planet scattering, one of several mechanisms previously discussed for increasing orbital eccentricities, must have operated in this system. The innermost planet around υ And has a very small orbital eccentricity, as expected from tidal circularization, but the two outer planets have large eccentricities, both around 0.3. It was quickly recognized after their discovery that the gravitational interaction between these two planets causes significant eccentricity evolution on secular timescales (∼ 10 years). In particular, in some early solutions, the middle planet appeared to have its eccentricity varying periodically with a large amplitude, from a maximum near the present value to a minimum near zero. Later fits to the data suggested that the outer two orbits had arguments of pericenter nearly equal to within about 10. This could indicate an “apsidal resonance,” in which two elliptical orbits oscillate about an aligned configuration with a small libration amplitude.