A solar system is a group of heavenly bodies consisting of a star and the planets and other objects orbiting around it. We are most familiar with our own solar system, which includes Earth, seven other major planets, and the sun. Our solar system also includes many smaller objects that revolve around the sun, such as dwarf planets, meteoroids, and comets; and a thin cloud of gas and dust known as the interplanetary medium. More than 100 moons, also called satellites, orbit the planets.
The Solar System is also home to two main belts of small bodies. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. The Kuiper belt (and its subpopulation, the scattered disc), which lies beyond Neptune's orbit, is composed mostly of ices such as water, ammonia and methane. Within these belts, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times beyond these regions.
Within the Solar System, various populations of small bodies, such as comets, centaurs and interplanetary dust, freely travel between these regions, while the solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc.
Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
The sun is the largest and most important object in our solar system. It contains 99.8 percent of the solar system's mass (quantity of matter). The sun provides most of the heat, light, and other energy that makes life possible. The sun's outer layers are hot and stormy. The hot gases and electrically charged particles in those layers continually stream into space and often burst out in solar eruptions. This flow of gases and particles forms the solar wind, which bathes everything in the solar system. Planets orbit the sun in oval-shaped paths called ellipses. The sun is slightly off to the side of the center of each ellipse at a point called a focus.
Inner Solar System
The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanos.
Outer Solar System
The outer region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the centaurs, also orbit in this region. The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn consist overwhelmingly of hydrogen and helium; Uranus and Neptune possess a greater proportion of ices in their makeup. All four gas giants have rings.
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids, minor planets, or planetoids. Some have elliptical orbits that pass inside the orbit of Earth or even that of Mercury. Others travel on a circular path among the outer planets. Most asteroids circle the sun in the region of the Asteroid Belt.
Meteoroids are chunks of metal or rock smaller than asteroids. When meteoroids plunge into Earth's atmosphere, they form bright streaks of light called meteors, or commonly a shooting star or falling star, as they disintegrate. Some meteoroids reach the ground, and then they become known as meteorites. Many meteors are part of a meteor shower. Most meteoroids are broken chunks of asteroids that resulted from collisions in the asteroid belt.
Comets are small Solar System bodies, usually only a few kilometers across, composed largely of volatile ices. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye. Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud.
The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. It appears to consist overwhelmingly of small worlds composed mainly of rock and ice.
The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It is composed mainly of small Solar System bodies, but many of the largest Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may be reclassified as dwarf planets. Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.
Pluto, a dwarf planet, is the largest known object in the Kuiper belt. Haumea and Makemake are the largest known objects in the classical Kuiper belt. Haumea is an egg-shaped object with two moons. Makemake is the brightest object in the Kuiper belt after Pluto.
The scattered disc overlaps the Kuiper belt but extends much further outwards. This region is thought to be the source of short-period comets.