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J.E. Chambers (Armagh Observatory)
The orbits of Earth and Venus have very low eccentricities (time-averaged values e~0.03)---significantly lower than Mercury, Mars and most asteroids. N-body simulations of the final stage of planet formation by Chambers and Wetherill (Icarus in press) yield Earth-like planets with large eccentricities (e~0.15). More recent simulations using a larger number of planetary embryos have failed to overcome this problem, despite producing systems of terrestrial planets that are like our own in other respects. Here, I suggest that large eccentricities are the norm, while almost-circular orbits like those of Earth and Venus are the exception. In the simulations, e increases due to secular resonances involving neighbouring terrestrial planets, or a terrestrial planet plus Jupiter or Saturn. This situation frequently occurs when accretion is almost complete, at a stage when mechanisms that might circularize the orbits (such as gas drag or dynamical friction with planetesimals) are no longer effective. It appears that Earth managed to avoid these resonances, or left its last resonance when e was at a minimum. Given that climate stability may depend appreciably on e, it could be no coincidence that we inhabit a planet with an unusually circular orbit.