Why Mercury races around the Sun
Mercury is the closest planet to the Sun, and that proximity is the key to its speed. The Sun’s gravity is much stronger nearby, so Mercury must travel rapidly to keep moving around the Sun rather than falling inward. Its average orbital speed is about 47 kilometers per second—faster than that of any other planet.
The comparison is between planets’ orbits, not every object in space. Comets and other small bodies can move faster at points in their paths, especially when they swing close to the Sun.
A changing pace, not a steady cruise
Mercury’s orbit is noticeably stretched rather than perfectly circular. As it approaches the Sun, gravity pulls it into a faster pace; as it moves farther away, it slows. So “fastest” describes its average speed around the orbit, while its moment-to-moment speed changes.
The solar wind is a separate phenomenon: a stream of charged particles flowing outward from the Sun. It passes through the space around Mercury and interacts with the planet’s magnetic field and thin outer atmosphere. The planet’s rapid orbit and the solar wind’s flow are distinct motions, even though both take place in the same harsh neighborhood close to the Sun.
