Here's why:

* Orbital Speed: A planet's orbital speed is determined by its distance from the Sun. The closer a planet is to the Sun, the stronger the gravitational pull and the faster it needs to travel to maintain its orbit.

* Mercury's Proximity: Mercury is the closest planet to the Sun, making it the fastest orbiting planet.

* Orbital Period: Mercury completes one orbit around the Sun in just 88 Earth days.

Let me know if you'd like to learn more about orbital mechanics!