Which planet has the most eccentric orbit?

Mercury has the most eccentric orbit of any planet in the Solar System.

Orbital eccentricity measures how much an orbit differs from a perfect circle. Mercury’s eccentricity is about 0.206, giving it the most elongated planetary orbit. Its distance from the Sun changes from roughly 46 million kilometers at perihelion to about 70 million kilometers at aphelion.

That changing distance produces a substantial variation in sunlight. Mercury receives roughly twice as much solar energy near its closest approach to the Sun as it does near its farthest point. The planet completes one revolution in about 88 Earth days and moves fastest when it is nearest the Sun, as required by Kepler’s laws of planetary motion.

Pluto is often remembered as the answer because its orbit is more eccentric than Mercury’s, but Pluto is classified as a dwarf planet rather than a planet. Among the eight planets, Mercury is the clear answer. Mars ranks second, while Venus has one of the most nearly circular planetary orbits, with an eccentricity of only about 0.007.