Solar system

The Solar System consists of the Sun, eight planets, dwarf planets, and many moons, asteroids, comets and meteoroids. The Sun is by far the most massive object in the Solar System, containing 99.8% of the system's mass and 99.9% of the system's volume. The four smaller inner planets, Mercury, Venus, Earth, and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being composed mainly of hydrogen and helium. The two largest planets, Jupiter and Saturn, are gas giants. Uranus and Neptune are ice giants, being composed mainly of ices such as water, ammonia, and methane. Small objects in the Solar System include the asteroids, which orbit the Sun between the orbits of Mars and Jupiter, and the comets, which orbit the Sun in highly elliptical orbits. Comets have highly volatile ices that vaporize when they approach the Sun, producing a visible "tail". Meteoroids are small rocky or metallic objects that orbit the Sun, but are too small to be considered planets or satellites. When a meteoroid enters Earth's atmosphere, it becomes a meteor and may produce a visible streak of light as it vaporizes. If a meteoroid survives passage through the atmosphere and reaches the ground, it is called a meteorite. The Solar System formed about 4.5 billion years ago from the gravitational collapse of a giant molecular cloud. Most of the mass was concentrated in the center, forming the Sun, while the rest flattened into a disk that became the Solar System. The planets formed out of this disk, with the inner planets forming from rocky material and the outer planets forming from gaseous material. The moons of the planets formed by either accretion from the disks around the planets or by capture from asteroids or comets. The Solar System is located in the Milky Way galaxy, which is about 100,000 light-years across. The Sun is located about 27,000 light-years from the center of the Milky Way.