A solar system is defined as the collection of a star and all the objects that revolve around it. The solar system that Earth belongs to is composed of the sun, eight planets, dwarf planets, asteroids and comets.
While the existence of other planets was known for centuries, the idea of a solar system was not considered until the 17th century. Prior to this, Earth was considered the center of the universe. Nicolaus Copernicus, Galileo Galilei, Johannes Kepler and Isaac Newton all contributed to developing the definition of the solar system.
At the center of the system is the sun, a ball of gas so massive that its gravitational pull governs the orbits of bodies 3.6 billion miles away. The bodies closest to the sun are the planets Mercury, Venus, Earth and Mars. The other planets in the solar system are Jupiter, Saturn, Uranus and Neptune. Some of these planets are orbited by natural satellites, as occurs when the moon orbits the Earth. These satellites are also considered part of the solar system.
Asteroids, particularly the asteroid belt that occurs between the orbits of Mars and Jupiter, are considered part of the solar system. Dwarf planets that orbit the sun, such as Pluto and Charon, are also part of the solar system. The International Astronomical Union defines which bodies are defined as planets or dwarf planets.