Fastest planet

The fastest planet in the Solar System is Mercury, which orbits the sun at an average velocity of 170,496 km/h (105,941 mph). Mercury completes a full orbit of the Sun every 87 days 21 hours.

Mercury has the most eccentric orbit for a Solar System planet, approaching to within 46 million km (28.58 million mi) of the Sun at its closest point (perihelion) before swinging out to 69.81 million km (48.38 million mi) at its most distant (aphelion).

This eccentric orbit means that Mercury's orbital velocity varies significantly over the course of its year. Towards aphelion, when it is moving away from the Sun, Mercury is travelling at just 139,896 km/h (86,927 mph). At perihelion, when it is being pulled deeper into the Sun's gravity well, its speed rises to 212,328 km/h (131,934 mph).

The speed of Mercury's close orbit around the sun, when combined with its extremely long rotation period (it spins on its axis once every 1,406 hours, or 58 Earth days), means that Mercury's rotation and its day-night cycle don't line up. A full day-night cycle on Mercury takes 176 Earth days.