The extent of the Solar System is commonly defined by the region influenced by the Sun's gravitational field. The outer boundary of the Solar System is not precisely defined, as the gravitational influence of the Sun weakens as distance increases. However, scientists typically consider the heliopause as the outer boundary. The heliopause is the point where the Sun's solar wind, a stream of charged particles emitted by the Sun, meets the interstellar medium. It is estimated to be located around 120 astronomical units (AU) from the Sun. One astronomical unit is the average distance from the Earth to the Sun, approximately 93 million miles or 150 million kilometers.
Regarding other star/planet systems, the extent of their influence depends on the mass of the star and the distribution of planets and other celestial bodies within the system. The influence of a star's gravity typically extends to several times the distance between the star and its farthest orbiting planet. For example, in the case of the Alpha Centauri system, which is the closest star system to our Solar System, the outer boundary is estimated to be around 1.34 light-years from the central binary stars, Alpha Centauri A and B. That's roughly 25 trillion miles or 40 trillion kilometers.
For other star systems, the outer boundary can vary significantly depending on the specific configuration and characteristics of the system. In general, however, the influence of a star's gravity decreases with distance, and the majority of a star system's mass and influence is concentrated closer to its central star or stars.