Eccentricity (e) is a number indicating the degree to which an orbit is non-circular, 0 meaning circular, between 0 and 1 being Elliptical, 1 being parabolic, and greater than 1 being hyperbolic.
The eccentricity of an elliptical orbit is:
ra - rp e = ——————— ra + rp
Eccentricity can aid in the study of Atmospheres of Extra Solar Planets, i.e., bodies orbiting stars, because the changes in EM Radiation from the host star allow observation of the atmosphere at multiple Temperatures using Light Curve Spectroscopic observations.
HD 80606 b
Laplace Radius (rL)
Semi-Major Axis (a)