The size of the solar system is defined by the volume of space over which the Sun’s influence exceeds those of other nearby stars in the Milky Way galaxy. This influence derives from two fundamental forces of nature: gravity and magnetism.
Let’s tackle gravity first. Every object in the solar system experiences a gravitational pull from the Sun; the farther away one is from the Sun, the weaker the pull. Provided, however, that the Sun’s gravity is still stronger at your position in space than the gravity from any other stars, then your motion through space will be subject to an acceleration which pulls you towards the Sun.
