An Eccentric Orbit
Mercury is the planet with the most unusual orbit in the solar system. What is an eccentric orbit, you ask? The eccentricity of an astronomical object is a parameter that defines the measure by which its orbit about another body diverges from a perfect circle.
Interestingly, its eccentricity is not all that remarkable contrasted to those of dwarf planets and some recognized extra-solar planets. Pluto, for instance, has a larger eccentric orbit than Mercury, but it’s not recognized as a “real” planet anymore.