The Solar System is the Sun and the extraterrestrial bodies orbiting the Sun, as well as the nine planets and their satellites; the asteroids and comets; and interplanetary dust and gas. The word may moreover refer to a cluster of celestial bodies orbiting an additional star. Solar system refers to the system that includes Earth and the Sun. The magnitude of the solar systems are specified in terms of the mean distance from Earth to the Sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant known planet, Pluto, orbits about 39 AU from the Sun. The boundary between the solar system and interstellar space called the heliopause is estimated to occur near 100 AU. Comets, on the other hand, accomplish the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more.
The solar system was the solitary planetary system recognized to subsist around a star comparable to the Sun until 1995, when astronomers discovered a planet about 0.6 times the mass of Jupiter orbiting the star 51 Pegasi. Soon after, astronomers found a planet about 8.1 times the mass of Jupiter orbiting the star 70 Virginis, and a planet about 3.5 times the mass of Jupiter orbiting the star 47 Ursa Majoris. Since then, astronomers have found planets and disks of dust in the process of forming planets around many other stars. Most astronomers think it likely that solar systems of some sort are numerous throughout the universe. Nine major planets are currently known. They are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune).......
