Here's why:

* Oblique: This refers to the angle of a planet's orbital plane relative to the plane of the solar system (the ecliptic). Mercury's orbital plane is tilted by about 7 degrees relative to the ecliptic.

* Elliptical: All planets in our solar system have elliptical orbits, meaning their path around the sun is not a perfect circle. Mercury's orbit is the most eccentric (meaning the most elliptical) of all the planets, with a significant difference between its closest and farthest points from the sun.

While other planets also have some degree of orbital inclination and eccentricity, Mercury's combination of both makes it the most extreme case in our solar system.