Our planet, Earth, is the only planet known to support life, but our solar system may not be that unusual. Extrasolar planetary systems—planets orbiting stars other than our Sun—are not uncommon. Research has found that, on average, there is at least one planet for every star. The Sun is just one of hundreds of billions of stars contained in the Milky Way galaxy and there are hundreds of billions of galaxies in the observable universe. Our solar system is located in the disk of the Milky Way galaxy, about two-thirds out from the center in one of the spiral arms.

In our solar system, all of the planets have nearly circular orbits around the Sun in more or less the same plane (as defined by Earth's orbit, known as the ecliptic plane) and in the same direction (counterclockwise, as viewed from above). The Sun contains the vast majority—more than 99 percent—of the mass of the solar system. The four planets of the inner solar system—Mercury, Venus, Earth, Mars—are relatively small and rocky and are known as the terrestrial planets. The four planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune—are primarily made of gases and ices and are known as gas giants; they contain about 99 percent of the mass orbiting the Sun.

Our solar system also contains other objects such as asteroids, comets, and dwarf planets. Asteroids are small, rocky, irregularly shaped objects that are leftovers from the formation of the planets. The majority of asteroids are located in the asteroid belt, a region between the orbits of Mars and Jupiter that contains millions of objects. There are large distances between asteroids and the two largest objects in the asteroid belt, Vesta and Ceres, contain nearly half of the mass of the entire asteroid belt.

Comets are like dirty snowballs and are made of ice, dust, and rock; they are also remnants from the formation of the planets. Many comets have highly elliptical orbits that take them relatively close to the Sun for a part of their orbit, where the Sun's energy causes them to release gas and dust. Some comets originate in the Kuiper belt, a region in the outer solar system beyond the planets that contains more than a trillion icy bodies (although, like the asteroid belt, it is mostly empty space). Comets and Kuiper belt objects frequently have orbits that do no lie in the ecliptic plane. The Kuiper belt is also home to larger objects such as Pluto, which has been classified a dwarf planet. Some comets may originate from the Oort cloud, a hypothesized spherical cloud of icy objects in the outermost regions of the solar system.

There is still much to learn about our solar system and scientists continue to explore it. For example, NASA's Dawn mission was designed to study the two largest objects in the asteroid belt to learn more about the conditions and processes of the early solar system; Dawn has already collected data from Vesta and will reach Ceres in 2015. Another NASA project, the New Horizons mission, will be the first to visit Pluto and other Kuiper belt objects; it will reach Pluto in 2015.

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