Solar System, the sun and all the objects orbiting it, including the planets, asteroids, comets, and meteors. There are 9 known planets in the Solar System. Mercury, Venus, Earth, Mars, and Pluto make up the terrestrial (Earthlike) planets. Jupiter, Saturn, Uranus, and Neptune are giants largely made of gas. All but 3 planets have their own moons orbiting them. There are at least 66 known moons in the Solar System. Astronomers estimate that the Solar System contains at least 100,000 asteroids and about 100 billion comets. But the combined mass of all these bodies is little more than 1/1000th the mass of the Sun.
The Solar System is vast. Pluto, usually the outermost planet (sometimes Neptune is), moves as far as 4 1/2 billion mi (7.2 billion km) from the Sun, while comets move even farther than this.
Origin of the Solar System
From the dating of meteorites (pieces of rock from the Solar System that have hit the earth), scientists believe that the Solar System formed about 4 1/2 billion years ago. No one knows exactly how the planets originated. The first scientific theories were developed in the 18th century by the philosopher Immanuel Kant and the mathematician Pierre Simon Laplace. They believed that the Sun and planets formed together from a cloud of gas or dust. Early in the 20th century a rival theory gained favor. This suggested that a star passed close to the Sun and pulled out some of its material. Part of this material fell into orbit around the Sun, forming planets. Modern theories have returned to Laplace's idea. Astronomers now believe that stars form when giant, diffuse clouds of gas condense. As the cloud contracts, a star forms at its center. Around the rapidly spinning star is a disk of material that slowly spirals away. Most of the disk is lost into space, but at certain points the material of the disk starts to collide and build into lumps. These lumps sweep up more of the surrounding material, eventually forming planets.
Other Solar Systems
The above theory suggests that Solar Systems form naturally around most stars. Although planets are far too faint to be seen going around other stars, they will exert a slight gravitational tug on those stars. By looking at the slight movements of nearby stars, observers have found that some stars have dark companions revolving around them. Astronomers now think that Solar Systems are common in the universe and that perhaps half the stars in the sky have planets orbiting them.