All objects have gravitational fields surrounding them. So, just as the moon has a force exerted on it due to Earth’s gravitational field, Earth has a force on it due to the moon’s field. As a result, the moon does not orbit around Earth’s center, but the moon and Earth orbit around a point roughly 2,900 kilometers from Earth’s center. The sun and other planets also exert gravitational forces on Earth, and Earth does on them.

The gravitational fields of the planets, in particular Jupiter, cause the sun to orbit around a point near its surface. Thus, if an observer on another planetary system studied the motion of the sun carefully he or she could determine that planets were orbiting the sun. This method has been used to detect well over 200 planets (called exoplanets) or planetary systems and other stars.