- Not Applicable ()
The planet with the least circular orbit is Mercury, which has an orbital eccentricity of 0.2056. Its orbit brings it to within 46,001,200 km (0.307 AU) of the sun at its closest point (perihelion) and out as far as 69,816,900 km (0.466 AU) from the sun as its most distant (aphelion).
Orbital eccentricity is a dimensionless ratio that describes the shape of a celestial body's orbit. In this system – which takes into account the object's mass, angular momentum, orbital energy and the effect of gravity – a perfectly circular orbit would be described as having an eccentricity of 0, while a parabolic trajectory (the path of an object that is diverted, but not captured by the gravity of a larger body) would be described with a number higher than 1.
Earth (which has a near-circular orbit) has an eccentricity of 0.0167, while the elliptical orbits of dwarf planets Pluto and Eris mean they have eccentricities of 0.248 and 0.440 respectively. Halley's comet, which takes 76 years to make it looping pass around the sun, has an eccentricity of 0.967. The only object so far catalogued with an eccentricity greater than 1 is the interstellar comet 'Oumuamua, which was found to have a eccentricity of 1.201 following its 2017 slingshot through the solar system.