SOLAR SYSTEM

Solar System, the Sun and everything that orbits the Sun, including the planets and their satellites; the dwarf planets, asteroids, Kuiper Belt Objects, and comets; and interplanetary dust and gas. The term may also refer to a group of celestial bodies orbiting another star (see Extrasolar Planets). In this article, solar system refers to the system that includes Earth and the Sun.

The dimensions of the solar system 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). Estimates for the boundary where the Sun’s magnetic field ends and interstellar space begins—called the heliopause—range from 86 to 100 AU from the Sun.

The most distant known body orbiting the Sun is the dwarf planet Eris, whose discovery was reported in July 2005. Eris is currently about 97 AU from the Sun. Another planetlike object in the outer solar system named Sedna is currently at 90 AU but will reach about 900 AU at the farthest point in its orbit thousands of years from now. Comets known as long-period comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more. (A comet’s period is how long it takes it to complete one revolution about the Sun.) They are members of the Oort cloud, a spherical shell of comet nuclei that surrounds the flat plane of planetary orbits at this enormous distance.

The solar system was the only planetary system known to exist around a star similar to the Sun until 1995, when astronomers discovered a planet about 0.6 times the mass of Jupiter orbiting the star 51 Pegasi. Jupiter is the most massive planet in our solar system. 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. See Astronomy; Galaxy; Star.