Here's why:

* Orbital Speed: A planet's orbital speed is determined by its distance from the Sun. Mercury is the closest planet to the Sun, meaning it has to move much faster to stay in its orbit.

* Mercury's Speed: Mercury orbits the Sun at an average speed of 47.87 kilometers per second (29.78 miles per second).

While it might seem like the closer a planet is to the Sun, the faster it should go, the orbital speed is actually a balance between the planet's gravitational pull towards the Sun and the inertia that wants to send it flying off in a straight line.