The solar system we call home is located in an outer spiral arm of the vast Milky Way galaxy. It consists of the Sun (our star) and everything that orbits around it. This includes the eight planets and their natural satellites (such as our Moon). Also, it includes the dwarf planets and their satellites, as well as asteroids, comets, and countless particles of smaller debris.      The order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed. Nearest the Sun, only rocky material could withstand the heat when the solar system was young. For this reason, the first four planets—Mercury, Venus, Earth, and Mars—are terrestrial planets. They're small with solid, rocky surfaces and are known to take the shortest amount of time to revolve around the Sun as compared to the other planets.
     Meanwhile, materials we are used to seeing as ice, liquid, or gas settled in the outer regions of the young solar system. Gravity pulled these materials together. This is where we find the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune.
     The solar system extends much farther than the eight planets that orbit the Sun. The solar system also includes the Kuiper Belt that lies past Neptune's orbit. This is a sparsely occupied ring of icy bodies. Here, almost all bodies are smaller than the most popular Kuiper Belt object, the dwarf planet Pluto.
     In the early 17th century, Galileo Galilei's discoveries using the newly invented telescope strongly supported the picture of a solar system in which all the planets, including Earth, revolve around a central star, the Sun. At the time, this was called the Copernican heliocentric theory. It was a revolutionary idea, as most people then thought Earth was the center of the universe.
     Since then, we have learned much about our solar system and what lies beyond it using ground-based telescopes, spacecraft, and mathematical models.​