Here's why:
* Everything is moving: Everything in the solar system is constantly in motion. Planets orbit the sun, moons orbit planets, and even the sun itself is moving through the Milky Way galaxy.
* Frames of reference: We can only talk about objects being stationary relative to a specific frame of reference. For example, we might say the Earth is stationary relative to an observer on Earth, but that's just because the observer is moving along with the Earth.
* Center of mass: The closest we can get to a stationary object is the barycenter of the solar system. This is the center of mass of all the objects in the solar system combined. While the Sun is the most massive object, its barycenter is slightly offset from its center due to the gravitational pull of the planets. The Sun actually "wobbles" slightly as it orbits this barycenter.
So, while there's no truly stationary object, the barycenter of the solar system is the point around which everything else is moving.
