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Orbital eccentricity is the amount by which an orbit deviates from a circle. Mathematically it's defined as the distance between the two foci of an elliptical orbit divided by the major axis. A circle has an ellipticity, denoted by the little symbol "e", of zero. In the solar system most of the planets have small eccentricity and are close to circular. Eccentricity of the Earth's orbit is 0.017, one and a half percent. The only two planets where the orbital eccentricity is above ten percent are Pluto, with an eccentricity of 0.25 and Mercury with an eccentricity of 0.21. These large numbers probably indicate interactions during the history of the solar system with large bodies or perhaps in the case of Pluto capture from a distant region of space.

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