5.43 km per second.
Mercury has the slowest orbital velocity around the sun compared to the other planets in our solar system. Its average orbital speed is about 47.87 km/s.
Neptune spins at a equatorial rate of 2.68 km/sec, or a velocity of 9,660 km/hr (6000 mph).
It is the orbital velocity (speed and direction) or orbital speed (rate of motion). It is usually stated as "average orbital speed" but is actually "mean orbital speed."
According to http://solarsystem.nasa.gov/planets/, Venus has an orbital velocity of 78,341 mph, and Neptune's is 12,253 mph...
Mercury travels around the sun at the lowest speed compared to other planets in our solar system. Its average orbital speed is about 47.87 km/s.
Orbital velocity refers to the speed at which a planet travels in its orbit.
If you still consider Pluto to be a planet then Pluto would be the slowest to orbit the sun. If you don't believe Pluto is a planet anymore then Neptune would be the slowest planet to orbit our sun.
The orbital speeds are higher when closer to the sun, but slower and more similar as you move out. The two that are closest in speed are Uranus and Neptune. Uranus at 6.8352 km/sec and Neptune at 5.4778 km/sec.
The planet with an orbital speed of around 30 kilometers per second is Earth. Its average orbital speed around the Sun is approximately 29.78 kilometers per second.
The orbital speed of a planet is the time it takes to cycle around the sun. The spinning speed of a planet is the time it takes for the planet to rotate on it's axis.
Saturn has a slower orbital speed than Jupiter. Saturn's average orbital speed is about 9.69 km/s, while Jupiter's average orbital speed is about 13.07 km/s, making Jupiter about 3.38 km/s faster than Saturn.
Of the major planets, Neptune. The speed of planets in their orbits is directly related to their distance from the sun. The farther a planet is from the Sun, the slower its orbital speed.