A celestial system formed by the gravitational pull of the sun, which causes surrounding celestial bodies to maintain a certain regularity of motion. As of October 2019, the solar system includes the Sun, 8 planets, nearly 500 satellites, and at least 1.2 million asteroids. There are also some dwarf planets and comets. If the orbit of Neptune is used as the boundary of the solar system, the diameter of the solar system is 60 astronomical units, which is approximately 9 billion kilometers. If the heliosphere is used as the boundary, the distance between the Sun and the boundary of the solar system can reach up to 100 astronomical units (the thinnest point). If the Oort Cloud is used as a boundary, the diameter of the solar system may be 200000 astronomical units.