Here's why:

* Kepler's Laws of Planetary Motion: Kepler's laws describe how planets move around the sun. The third law states that the square of a planet's orbital period (the time it takes to complete one orbit) is proportional to the cube of the semi-major axis of its orbit (the average distance from the sun).

* Closer to the Sun, Faster the Speed: This means that planets closer to the sun have shorter orbital periods and therefore move faster.

* Mercury's Orbit: Mercury is the closest planet to the sun, making it the fastest planet in our solar system.