Here's why:

* Orbital speed is related to distance from the Sun: Planets closer to the Sun experience a stronger gravitational pull and have to travel faster to maintain their orbit.

* Mercury is the closest planet to the Sun: It orbits the Sun at an average speed of about 47.9 kilometers per second (29.8 miles per second).

While other planets like Jupiter and Saturn are massive, their farther distances from the Sun mean they have slower orbital speeds.