Here's why:

* Orbital Speed: Mercury's average orbital speed is about 47.87 kilometers per second (29.78 miles per second). This is the highest of any planet in our solar system.

* Orbital Period: Because of its high speed, Mercury completes a full orbit around the Sun in just 88 Earth days.

The closer a planet is to the Sun, the stronger the Sun's gravitational pull and the faster the planet needs to travel to maintain its orbit.