Short answers, pulled from the story.
Orbital eccentricity is a dimensionless parameter that describes the shape of an orbit around another body. A value of 0 is a perfect circle, values between 0 and 1 form an ellipse, exactly 1 is a parabolic escape trajectory, and values above 1 indicate a hyperbolic path.
Mercury has the greatest orbital eccentricity of any planet in the Solar System, at 0.2056. This is large enough that Mercury receives twice as much solar radiation at perihelion as at aphelion.
Earth's current orbital eccentricity is about 0.0167. Because Earth moves faster near perihelion, Northern Hemisphere autumn and winter are shorter than spring and summer; summer is currently about 4.5 days longer than winter.
Oumuamua had an orbital eccentricity of 1.20, confirming it was never gravitationally bound to the Sun. It was the first interstellar object confirmed passing through the Solar System, discovered 0.2 AU from Earth and estimated at roughly 200 metres in diameter.
The exoplanet HD 20782 b holds the record with an orbital eccentricity of 0.97, plus or minus 0.01. It is followed by TIC 241249530b at 0.94 and HD 80606 b at approximately 0.93226.
Variations in Earth's orbital eccentricity, combined with changes in axial tilt and apsidal precession, form the basis of Milankovitch cycles. These orbital variations alter the seasonal distribution of solar radiation and are linked to the pacing of glacial and interglacial periods over the past several hundred thousand years.